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    Thinking in Java Fourth Edition Bruce Eckel President, MindView, Inc. Comments from readers: Thinking In Java should be read cover to cover by every Java programmer, then kept close at hand for frequent reference. The exercises are challenging, and the chapter on Collections is superb! Not only did this book help me to pass the Sun Certified Java Programmer exam; it’s also the first book I turn to whenever I have a Java question. Jim Pleger, Loudoun County (Virginia) Government Much better than any other Java book I’ve seen. Make that “by an order of magnitude”... very complete, with excellent right-to-the-point examples and intelligent, not dumbed-down, explanations ... In contrast to many other Java books I found it to be unusually mature, consistent, intellectually honest, well-written and precise. IMHO, an ideal book for studying Java. Anatoly Vorobey, Technion University, Haifa, Israel One of the absolutely best programming tutorials I’ve seen for any language. Joakim Ziegler, FIX sysop Thank you for your wonderful, wonderful book on Java. Dr. Gavin Pillay, Registrar, King Edward VIII Hospital, South Africa Thank you again for your awesome book. I was really floundering (being a non-C programmer), but your book has brought me up to speed as fast as I could read it. It’s really cool to be able to understand the underlying principles and concepts from the start, rather than having to try to build that conceptual model through trial and error. Hopefully I will be able to attend your seminar in the not-too-distant future. Randall R. Hawley, Automation Technician, Eli Lilly & Co. The best computer book writing I have seen. Tom Holland This is one of the best books I’ve read about a programming language… The best book ever written on Java. Ravindra Pai, Oracle Corporation, SUNOS product line This is the best book on Java that I have ever found! You have done a great job. Your depth is amazing. I will be purchasing the book when it is published. I have been learning Java since October 96. I have read a few books, and consider yours a “MUST READ.” These past few months we have been focused on a product written entirely in Java. Your book has helped solidify topics I was shaky on and has expanded my knowledge base. I have even used some of your explanations as information in interviewing contractors to help our team. I have found how much Java knowledge they have by asking them about things I have learned from reading your book (e.g., the difference between arrays and Vectors). Your book is great! Steve Wilkinson, Senior Staff Specialist, MCI Telecommunications Great book. Best book on Java I have seen so far. Jeff Sinclair, Software Engineer, Kestral Computing Thank you for Thinking in Java. It’s time someone went beyond mere language description to a thoughtful, penetrating analytic tutorial that doesn’t kowtow to The Manufacturers. I’ve read almost all the others—only yours and Patrick Winston’s have found a place in my heart. I’m already recommending it to customers. Thanks again. Richard Brooks, Java Consultant, Sun Professional Services, Dallas Bruce, your book is wonderful! Your explanations are clear and direct. Through your fantastic book I have gained a tremendous amount of Java knowledge. The exercises are also FANTASTIC and do an excellent job reinforcing the ideas explained throughout the chapters. I look forward to reading more books written by you. Thank you for the tremendous service that you are providing by writing such great books. My code will be much better after reading Thinking in Java. I thank you and I’m sure any programmers who will have to maintain my code are also grateful to you. Yvonne Watkins, Java Artisan, Discover Technologies, Inc. Other books cover the WHAT of Java (describing the syntax and the libraries) or the HOW of Java (practical programming examples). Thinking in Java is the only book I know that explains the WHY of Java; why it was designed the way it was, why it works the way it does, why it sometimes doesn’t work, why it’s better than C++, why it’s not. Although it also does a good job of teaching the what and how of the language, Thinking in Java is definitely the thinking person’s choice in a Java book. Robert S. Stephenson Thanks for writing a great book. The more I read it the better I like it. My students like it, too. Chuck Iverson I just want to commend you for your work on Thinking in Java. It is people like you that dignify the future of the Internet and I just want to thank you for your effort. It is very much appreciated. Patrick Barrell, Network Officer Mamco, QAF Mfg. Inc. I really, really appreciate your enthusiasm and your work. I download every revision of your online books and am looking into languages and exploring what I would never have dared (C#, C++, Python, and Ruby, as a side effect). I have at least 15 other Java books (I needed 3 to make both JavaScript and PHP viable!) and subscriptions to Dr. Dobbs, JavaPro, JDJ, JavaWorld, etc., as a result of my pursuit of Java (and Enterprise Java) and certification but I still keep your book in higher esteem. It truly is a thinking man’s book. I subscribe to your newsletter and hope to one day sit down and solve some of the problems you extend for the solutions guides for you (I’ll buy the guides!) in appreciation. But in the meantime, thanks a lot. Joshua Long, www.starbuxman.com Most of the Java books out there are fine for a start, and most just have beginning stuff and a lot of the same examples. Yours is by far the best advanced thinking book I’ve seen. Please publish it soon! ... I also bought Thinking in C++ just because I was so impressed with Thinking in Java. George Laframboise, LightWorx Technology Consulting, Inc. I wrote to you earlier about my favorable impressions regarding your Thinking in C++ (a book that stands prominently on my shelf here at work). And today I’ve been able to delve into Java with your e-book in my virtual hand, and I must say (in my best Chevy Chase from Modern Problems), “I like it!” Very informative and explanatory, without reading like a dry textbook. You cover the most important yet the least covered concepts of Java development: the whys. Sean Brady I develop in both Java and C++, and both of your books have been lifesavers for me. If I am stumped about a particular concept, I know that I can count on your books to a) explain the thought to me clearly and b) have solid examples that pertain to what I am trying to accomplish. I have yet to find another author that I continually whole-heartedly recommend to anyone who is willing to listen. Josh Asbury, A^3 Software Consulting, Cincinnati, Ohio Your examples are clear and easy to understand. You took care of many important details of Java that can’t be found easily in the weak Java documentation. And you don’t waste the reader’s time with the basic facts a programmer already knows. Kai Engert, Innovative Software, Germany I’m a great fan of your Thinking in C++ and have recommended it to associates. As I go through the electronic version of your Java book, I’m finding that you’ve retained the same high level of writing. Thank you! Peter R. Neuwald VERY well-written Java book...I think you’ve done a GREAT job on it. As the leader of a Chicagoarea Java special interest group, I’ve favorably mentioned your book and Web site several times at our recent meetings. I would like to use Thinking in Java as the basis for a part of each monthly SIG meeting, in which we review and discuss each chapter in succession. Mark Ertes By the way, printed TIJ2 in Russian is still selling great, and remains bestseller. Learning Java became synonym of reading TIJ2, isn’t that nice? Ivan Porty, translator and publisher of Thinking in Java 2nd Edition in Russian I really appreciate your work and your book is good. I recommend it here to our users and Ph.D. students. Hugues Leroy // Irisa-Inria Rennes France, Head of Scientific Computing and Industrial Tranfert OK, I’ve only read about 40 pages of Thinking in Java, but I’ve already found it to be the most clearly written and presented programming book I’ve come across...and I’m a writer, myself, so I am probably a little critical. I have Thinking in C++ on order and can’t wait to crack it—I’m fairly new to programming and am hitting learning curves head-on everywhere. So this is just a quick note to say thanks for your excellent work. I had begun to burn a little low on enthusiasm from slogging through the mucky, murky prose of most computer books— even ones that came with glowing recommendations. I feel a whole lot better now. Glenn Becker, Educational Theatre Association Thank you for making your wonderful book available. I have found it immensely useful in finally understanding what I experienced as confusing in Java and C++. Reading your book has been very satisfying. Felix Bizaoui, Twin Oaks Industries, Louisa, Va. I must congratulate you on an excellent book. I decided to have a look at Thinking in Java based on my experience with Thinking in C++, and I was not disappointed. Jaco van der Merwe, Software Specialist, DataFusion Systems Ltd, Stellenbosch, South Africa This has to be one of the best Java books I’ve seen. E.F. Pritchard, Senior Software Engineer, Cambridge Animation Systems Ltd., United Kingdom Your book makes all the other Java books I’ve read or flipped through seem doubly useless and insulting. Brett Porter, Senior Programmer, Art & Logic I have been reading your book for a week or two and compared to the books I have read earlier on Java, your book seems to have given me a great start. I have recommended this book to a lot of my friends and they have rated it excellent. Please accept my congratulations for coming out with an excellent book. Rama Krishna Bhupathi, Software Engineer, TCSI Corporation, San Jose Just wanted to say what a “brilliant” piece of work your book is. I’ve been using it as a major reference for in-house Java work. I find that the table of contents is just right for quickly locating the section that is required. It’s also nice to see a book that is not just a rehash of the API nor treats the programmer like a dummy. Grant Sayer, Java Components Group Leader, Ceedata Systems Pty Ltd, Australia Wow! A readable, in-depth Java book. There are a lot of poor (and admittedly a couple of good) Java books out there, but from what I’ve seen yours is definitely one of the best. John Root, Web Developer, Department of Social Security, London I’ve just started Thinking in Java. I expect it to be very good because I really liked Thinking in C++ (which I read as an experienced C++ programmer, trying to stay ahead of the curve) … You are a wonderful author. Kevin K. Lewis, Technologist, ObjectSpace, Inc. I think it’s a great book. I learned all I know about Java from this book. Thank you for making it available for free over the Internet. If you wouldn’t have I’d know nothing about Java at all. But the best thing is that your book isn’t a commercial brochure for Java. It also shows the bad sides of Java. YOU have done a great job here. Frederik Fix, Belgium I have been hooked to your books all the time. A couple of years ago, when I wanted to start with C++, it was C++ Inside & Out which took me around the fascinating world of C++. It helped me in getting better opportunities in life. Now, in pursuit of more knowledge and when I wanted to learn Java, I bumped into Thinking in Java—no doubts in my mind as to whether I need some other book. Just fantastic. It is more like rediscovering myself as I get along with the book. It is just a month since I started with Java, and heartfelt thanks to you, I am understanding it better now. Anand Kumar S., Software Engineer, Computervision, India Your book stands out as an excellent general introduction. Peter Robinson, University of Cambridge Computer Laboratory It’s by far the best material I have come across to help me learn Java and I just want you to know how lucky I feel to have found it. THANKS! Chuck Peterson, Product Leader, Internet Product Line, IVIS International The book is great. It’s the third book on Java I’ve started and I’m about two-thirds of the way through it now. I plan to finish this one. I found out about it because it is used in some internal classes at Lucent Technologies and a friend told me the book was on the Net. Good work. Jerry Nowlin, MTS, Lucent Technologies Of the six or so Java books I’ve accumulated to date, your Thinking in Java is by far the best and clearest. Michael Van Waas, Ph.D., President, TMR Associates I just want to say thanks for Thinking in Java. What a wonderful book you’ve made here! Not to mention downloadable for free! As a student I find your books invaluable (I have a copy of C++ Inside Out, another great book about C++), because they not only teach me the how-to, but also the whys, which are of course very important in building a strong foundation in languages such as C++ or Java. I have quite a lot of friends here who love programming just as I do, and I’ve told them about your books. They think it’s great! Thanks again! By the way, I’m Indonesian and I live in Java. Ray Frederick Djajadinata, Student at Trisakti University, Jakarta The mere fact that you have made this work free over the Net puts me into shock. I thought I’d let you know how much I appreciate and respect what you’re doing. Shane LeBouthillier, Computer Engineering student, University of Alberta, Canada I have to tell you how much I look forward to reading your monthly column. As a newbie to the world of object oriented programming, I appreciate the time and thoughtfulness that you give to even the most elementary topic. I have downloaded your book, but you can bet that I will purchase the hard copy when it is published. Thanks for all of your help. Dan Cashmer, B. C. Ziegler & Co. Just want to congratulate you on a job well done. First I stumbled upon the PDF version of Thinking in Java. Even before I finished reading it, I ran to the store and found Thinking in C++. Now, I have been in the computer business for over eight years, as a consultant, software engineer, teacher/trainer, and recently as self-employed, so I’d like to think that I have seen enough (not “have seen it all,” mind you, but enough). However, these books cause my girlfriend to call me a ”geek.” Not that I have anything against the concept—it is just that I thought this phase was well beyond me. But I find myself truly enjoying both books, like no other computer book I have touched or bought so far. Excellent writing style, very nice introduction of every new topic, and lots of wisdom in the books. Well done. Simon Goland, simonsez@smartt.com, Simon Says Consulting, Inc. I must say that your Thinking in Java is great! That is exactly the kind of documentation I was looking for. Especially the sections about good and poor software design using Java. Dirk Duehr, Lexikon Verlag, Bertelsmann AG, Germany Thank you for writing two great books (Thinking in C++, Thinking in Java). You have helped me immensely in my progression to object oriented programming. Donald Lawson, DCL Enterprises Thank you for taking the time to write a really helpful book on Java. If teaching makes you understand something, by now you must be pretty pleased with yourself. Dominic Turner, GEAC Support It’s the best Java book I have ever read—and I read some. Jean-Yves MENGANT, Chief Software Architect NAT-SYSTEM, Paris, France Thinking in Java gives the best coverage and explanation. Very easy to read, and I mean the code fragments as well. Ron Chan, Ph.D., Expert Choice, Inc., Pittsburgh, Pa. Your book is great. I have read lots of programming books and your book still adds insights to programming in my mind. Ningjian Wang, Information System Engineer, The Vanguard Group Thinking in Java is an excellent and readable book. I recommend it to all my students. Dr. Paul Gorman, Department of Computer Science, University of Otago, Dunedin, New Zealand With your book, I have now understood what object oriented programming means. ... I believe that Java is much more straightforward and often even easier than Perl. Torsten Römer, Orange Denmark You make it possible for the proverbial free lunch to exist, not just a soup kitchen type of lunch but a gourmet delight for those who appreciate good software and books about it. Jose Suriol, Scylax Corporation Thanks for the opportunity of watching this book grow into a masterpiece! IT IS THE BEST book on the subject that I’ve read or browsed. Jeff Lapchinsky, Programmer, Net Results Technologies Your book is concise, accessible and a joy to read. Keith Ritchie, Java Research & Development Team, KL Group Inc. It truly is the best book I’ve read on Java! Daniel Eng The best book I have seen on Java! Rich Hoffarth, Senior Architect, West Group Thank you for a wonderful book. I’m having a lot of fun going through the chapters. Fred Trimble, Actium Corporation You have mastered the art of slowly and successfully making us grasp the details. You make learning VERY easy and satisfying. Thank you for a truly wonderful tutorial. Rajesh Rau, Software Consultant Thinking in Java rocks the free world! Miko O’Sullivan, President, Idocs Inc. About Thinking in C++: Winner of the 1995 Software Development Magazine Jolt Award for Best Book of the Year “This book is a tremendous achievement. You owe it to yourself to have a copy on your shelf. The chapter on iostreams is the most comprehensive and understandable treatment of that subject I’ve seen to date.” Al Stevens Contributing Editor, Doctor Dobbs Journal “Eckel’s book is the only one to so clearly explain how to rethink program construction for object orientation. That the book is also an excellent tutorial on the ins and outs of C++ is an added bonus.” Andrew Binstock Editor, Unix Review “Bruce continues to amaze me with his insight into C++, and Thinking in C++ is his best collection of ideas yet. If you want clear answers to difficult questions about C++, buy this outstanding book.” Gary Entsminger Author, The Tao of Objects “Thinking in C++ patiently and methodically explores the issues of when and how to use inlines, references, operator overloading, inheritance, and dynamic objects, as well as advanced topics such as the proper use of templates, exceptions and multiple inheritance. The entire effort is woven in a fabric that includes Eckel’s own philosophy of object and program design. A must for every C++ developer’s bookshelf, Thinking in C++ is the one C++ book you must have if you’re doing serious development with C++.” Richard Hale Shaw Contributing Editor, PC Magazine Thinking in Java Fourth Edition Bruce Eckel President, MindView, Inc. Upper Saddle River, NJ ● Boston ● Indianapolis ● San Francisco New York ● Toronto ● Montreal ● London ● Munich ● Paris Madrid ● Capetown ● Sydney ● Tokyo ● Singapore ● Mexico City Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed with initial capital letters or in all capitals. Java is a trademark of Sun Microsystems, Inc. Windows 95, Windows NT, Windows 2000, and Windows XP are trademarks of Microsoft Corporation. All other product names and company names mentioned herein are the property of their respective owners. The author and publisher have taken care in the preparation of this book, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information or programs contained herein. The publisher offers excellent discounts on this book when ordered in quantity for bulk purchases or special sales, which may include custom covers and/or content particular to your business, training goals, marketing focus, and branding interests. For more information, please contact: U.S. Corporate and Government Sales (800) 382-3419 corpsales@pearsontechgroup.com For sales outside the U.S., please contact: International Sales international@pearsoned.com Visit us on the Web: www.prenhallprofessional.com Cover design and interior design by Daniel Will-Harris, www.Will-Harris.com Library of Congress Cataloging-in-Publication Data: Eckel, Bruce. Thinking in Java / Bruce Eckel.—4th ed. p. cm. Includes bibliographical references and index. ISBN 0-13-187248-6 (pbk. : alk. paper) 1. Java (Computer program language) I. Title. QA76.73.J38E25 2006 005.13’3—dc22 2005036339 Copyright © 2006 by Bruce Eckel, President, MindView, Inc. All rights reserved. Printed in the United States of America. This publication is protected by copyright, and permission must be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permissions, write to: Pearson Education, Inc. Rights and Contracts Department One Lake Street Upper Saddle River, NJ 07458 Fax: (201) 236-3290 ISBN 0-13-187248-6 Text printed in the United States on recycled paper at Courier in Stoughton, Massachusetts. First printing, January 2006 Dedication To Dawn Overview Preface Introduction Introduction to Objects Everything Is an Object Operators Controlling Execution Initialization & Cleanup Access Control Reusing Classes Polymorphism Interfaces Inner Classes Holding Your Objects Error Handling with Exceptions Strings Type Information Generics Arrays Containers in Depth I/O Enumerated Types Annotations Concurrency Graphical User Interfaces A: Supplements B: Resources Index 1 9 15 41 63 93 107 145 165 193 219 243 275 313 355 393 439 535 567 647 725 761 797 933 1035 1039 1045 What’s Inside Preface 1 Java SE5 and SE6 .................. 2 Java SE6.........................................2 The 4th edition........................ 2 Changes ..........................................3 Note on the cover design ....... 4 Acknowledgements................ 4 Introduction 9 Prerequisites .......................... 9 Learning Java .......................10 Goals .....................................10 Teaching from this book....... 11 JDK HTML documentation...................... 11 Exercises ............................... 12 Foundations for Java............12 Source code ........................... 12 Coding standards ......................... 14 Errors .................................... 14 Introduction to Objects 15 The progress of abstraction ........................ 15 An object has an interface ........................... 17 An object provides services...................18 The hidden implementation .................... 19 Reusing the implementation ................... 20 Inheritance............................ 21 Is-a vs. is-like-a relationships......24 Interchangeable objects with polymorphism ............. 25 The singly rooted hierarchy .............................. 28 Containers............................ 28 Parameterized types (Generics) ..29 Object creation & lifetime ... 30 Exception handling: dealing with errors ............... 31 Concurrent programming ... 32 Java and the Internet .......... 33 What is the Web? .........................33 Client-side programming ............34 Server-side programming............38 Summary .............................. 38 Everything Is an Object 41 You manipulate objects with references..................... 41 You must create all the objects ....................... 42 Where storage lives......................42 Special case: primitive types .......43 Arrays in Java ..............................44 You never need to destroy an object .................. 45 Scoping ........................................ 45 Scope of objects ...........................46 Creating new data types: class..................................... 46 Fields and methods ..................... 47 Methods, arguments, and return values ................. 48 The argument list.........................49 Building a Java program...... 50 Name visibility.............................50 Using other components .............50 The static keyword ..................... 51 Your first Java program ....... 52 Compiling and running ...............54 Comments and embedded documentation ..................... 55 Comment documentation............ 55 Syntax .......................................... 56 Embedded HTML ........................ 56 Some example tags ...................... 57 Documentation example .............59 Coding style..........................60 Summary .............................. 60 Exercises .............................. 60 Operators 63 Simpler print statements ..... 63 Using Java operators ........... 64 Precedence ........................... 64 Assignment .......................... 65 Aliasing during method calls.......66 Mathematical operators....... 67 Unary minus and plus operators .......................68 Auto increment and decrement ............................ 69 Relational operators ............ 70 Testing object equivalence...........70 Logical operators .................. 71 Short-circuiting ............................ 72 Literals ..................................73 Exponential notation ...................74 Bitwise operators ..................75 Shift operators ......................76 Ternary if-else operator ......79 String operator + and += .............................. 80 Common pitfalls when using operators ...........81 Casting operators..................81 Truncation and rounding ........... 82 Promotion ................................... 83 Java has no “sizeof” ............. 83 A compendium of operators.......................... 84 Summary............................... 91 Controlling Execution 93 true and false..................... 93 if-else .................................. 93 Iteration ............................... 94 do-while .....................................95 for ................................................95 The comma operator................... 96 Foreach syntax......................97 return ................................. 99 break and continue.......... 99 The infamous “goto” ........... 101 switch ................................104 Summary............................ 106 Initialization & Cleanup 107 Guaranteed initialization with the constructor ...........107 Method overloading .......... 109 Distinguishing overloaded methods .................. 110 Overloading with primitives ....... 111 Overloading on return values .... 114 Default constructors ........... 114 The this keyword ............... 116 Calling constructors from constructors ...................... 118 The meaning of static ............... 119 Cleanup: finalization and garbage collection........ 119 What is finalize() for? ............. 120 You must perform cleanup .........121 The termination condition .........121 How a garbage collector works.. 122 Member initialization .........125 Specifying initialization ............. 126 Constructor initialization ...127 Order of initialization ................ 127 static data initialization ........... 128 Explicit static initialization ...... 130 Non-static instance initialization ................ 132 Array initialization ............. 133 Variable argument lists ............. 137 Enumerated types ............... 141 Summary ............................ 143 Access Control 145 package: the library unit ................... 146 Code organization...................... 147 Creating unique package names........................... 148 A custom tool library ..................151 Using imports to change behavior..................... 152 Package caveat ........................... 153 Java access specifiers..........153 Package access ........................... 153 public: interface access ............ 154 private: you can’t touch that! .. 155 protected: inheritance access . 156 Interface and implementation .......... 158 Class access ........................ 159 Summary ............................ 162 Reusing Classes 165 Composition syntax ........... 165 Inheritance syntax ............. 168 Initializing the base class........... 169 Delegation ........................... 171 Combining composition and inheritance ...................173 Guaranteeing proper cleanup.... 174 Name hiding ...............................177 Choosing composition vs. inheritance .................... 178 protected ......................... 180 Upcasting ............................181 Why “upcasting”? ...................... 181 Composition vs. inheritance revisited ..................................... 182 The final keyword ............. 182 final data................................... 183 final methods............................186 final classes............................... 187 final caution..............................188 Initialization and class loading ................189 Initialization with inheritance...189 Summary............................. 191 Polymorphism 193 Upcasting revisited .............193 Forgetting the object type.......... 194 The twist .............................196 Method-call binding .................. 196 Producing the right behavior..... 196 Extensibility ............................... 199 Pitfall: “overriding” private methods ...................... 202 Pitfall: fields and static methods .................. 203 Constructors and polymorphism ................... 204 Order of constructor calls ......... 204 Inheritance and cleanup ........... 206 Behavior of polymorphic methods inside constructors ....210 Covariant return types........ 211 Designing with inheritance..................212 Substitution vs. extension ......... 213 Downcasting and runtime type information ......... 215 Summary............................. 217 Interfaces 219 Abstract classes and methods .......................219 Interfaces ........................... 222 Complete decoupling ......... 225 “Multiple inheritance” in Java ................................ 230 Extending an interface with inheritance .......................... 231 Name collisions when combining Interfaces ................233 Adapting to an interface .... 234 Fields in interfaces ............ 235 Initializing fields in interfaces .. 236 Nesting interfaces .............. 237 Interfaces and factories ..... 239 Summary ............................. 241 Inner Classes 243 Creating inner classes........ 243 The link to the outer class .................... 244 Using .this and .new ........ 246 Inner classes and upcasting ..................... 247 Inner classes in methods and scopes...........249 Anonymous inner classes ........................251 Factory Method revisited ..........254 Nested classes .................... 256 Classes inside interfaces ............ 257 Reaching outward from a multiplynested class ...............259 Why inner classes?............. 259 Closures & callbacks .................. 261 Inner classes & control frameworks ...................263 Inheriting from inner classes ....................... 269 Can inner classes be overridden? ................... 269 Local inner classes ..............271 Inner-class identifiers........ 272 Summary ............................ 273 Holding Your Objects 275 Generics and type-safe containers ........... 276 Basic concepts .................... 278 Adding groups of elements ......................... 279 Printing containers ............ 281 List..................................... 283 Iterator ............................. 286 ListIterator ............................ 288 LinkedList .......................289 Stack ................................. 291 Set ...................................... 292 Map ................................... 295 Queue ................................ 298 PriorityQueue ........................299 Collection vs. Iterator ... 301 Foreach and iterators.........304 The Adapter Method idiom...... 306 Summary ............................308 Error Handling with Exceptions 313 Concepts ............................. 313 Basic exceptions..................314 Exception arguments ................. 315 Catching an exception ........ 315 The try block ............................. 316 Exception handlers .................... 316 Creating your own exceptions ................... 317 Exceptions and logging.............. 319 The exception specification ....................... 322 Catching any exception ..... 323 The stack trace .......................... 324 Rethrowing an exception...........325 Exception chaining ....................327 Standard Java exceptions .......................... 330 Special case: RuntimeException ............... 330 Performing cleanup with finally ....................... 332 What’s finally for? ....................333 Using finally during return....335 Pitfall: the lost exception .......... 336 Exception restrictions ....... 338 Constructors ...................... 340 Exception matching........... 344 Alternative approaches...... 345 History ...................................... 346 Perspectives ...............................347 Passing exceptions to the console ............................ 349 Converting checked to unchecked exceptions........... 350 Exception guidelines ......... 352 Summary............................ 352 Strings 355 Immutable Strings ............355 Overloading ‘+’ vs. StringBuilder ................. 356 Unintended recursion ....... 359 Operations on Strings.......361 Formatting output ............. 362 printf() .................................... 363 System.out.format()............ 363 The Formatter class ............... 363 Format specifiers ...................... 364 Formatter conversions........... 366 String.format() ..................... 368 Regular expressions........... 370 Basics .........................................370 Creating regular expressions .....372 Quantifiers .................................374 Pattern and Matcher ............. 375 split() ........................................382 Replace operations ....................383 reset() .......................................384 Regular expressions and Java I/O ..............................385 Scanning input ...................386 Scanner delimiters ................. 388 Scanning with regular expressions................... 389 StringTokenizer ............. 389 Summary ............................ 391 Type Information 393 The need for RTTI.............. 393 The Class object ................ 395 Class literals...............................399 Generic class references ............ 401 New cast syntax ........................ 403 Checking before a cast .......404 Using class literals .................... 409 A dynamic instanceof ..............411 Counting recursively.................. 412 Registered factories ........... 413 instanceof vs. Class equivalence......................... 416 Reflection: runtime class information ................417 A class method extractor ........... 418 Dynamic proxies ................420 Null Objects........................ 424 Mock Objects & Stubs................429 Interfaces and type information ................430 Summary ............................ 436 Generics 439 Comparison with C++........440 Simple generics ..................440 A tuple library ............................442 A stack class ...............................444 RandomList ............................445 Generic interfaces .............. 446 Generic methods ................ 449 Leveraging type argument inference ...................450 Varargs and generic methods....452 A generic method to use with Generators............453 A general-purpose Generator.453 Simplifying tuple use ................. 455 A Set utility................................456 Anonymous inner classes ....................... 459 Building complex models ................. 460 The mystery of erasure ...... 462 The C++ approach .................... 464 Migration compatibility............ 466 The problem with erasure .........467 The action at the boundaries .... 468 Compensating for erasure........................... 471 Creating instances of types ........472 Arrays of generics ......................475 Bounds ............................... 479 Wildcards ........................... 482 How smart is the compiler?...... 484 Contravariance.......................... 485 Unbounded wildcards............... 488 Capture conversion ................... 492 Issues ................................. 493 No primitives as type parameters .................... 493 Implementing parameterized interfaces ...........495 Casting and warnings ............... 496 Overloading............................... 498 Base class hijacks an interface.. 498 Self-bounded types ............ 500 Curiously recurring generics .... 500 Self-bounding ............................ 501 Argument covariance................ 503 Dynamic type safety .......... 506 Exceptions ......................... 507 Mixins ................................ 509 Mixins in C++ ........................... 509 Mixing with interfaces ............... 510 Using the Decorator pattern.......511 Mixins with dynamic proxies .... 512 Latent typing.......................514 Compensating for the lack of latent typing ......518 Reflection ................................... 518 Applying a method to a sequence.............................. 519 When you don’t happen to have the right interface .......... 521 Simulating latent typing with adapters .............................523 Using function objects as strategies ....................... 526 Summary: Is casting really so bad?......................531 Further reading..........................533 Arrays 535 Why arrays are special........535 Arrays are first-class objects ............... 536 Returning an array............. 539 Multidimensional arrays .................................. 540 Arrays and generics ........... 543 Creating test data ............... 546 Arrays.fill() ............................. 546 Data Generators...................... 547 Creating arrays from Generators ..................... 551 Arrays utilities.................. 555 Copying an array........................ 555 Comparing arrays ...................... 556 Array element comparisons ...... 557 Sorting an array .........................560 Searching a sorted array ............ 561 Summary ............................ 564 Containers in Depth 567 Full container taxonomy.... 567 Filling containers ............... 568 A Generator solution ..............569 Map generators.........................570 Using Abstract classes............. 573 Collection functionality .......................580 Optional operations ........... 582 Unsupported operations............583 List functionality ............... 586 Sets and storage order ...... 589 SortedSet ................................. 591 Queues................................ 594 Priority queues...........................594 Deques ....................................... 595 Understanding Maps ........ 598 Performance ..............................599 SortedMap ............................. 602 LinkedHashMap................... 603 Hashing and hash codes .... 605 Understanding hashCodeQ ....607 Hashing for speed......................610 Overriding hashCode() ........... 613 Choosing an implementation..............617 A performance test framework........................... 618 Choosing between Lists ............ 621 Microbenchmarking dangers ....626 Choosing between Sets ............. 627 Choosing between Maps...........629 Utilities............................... 632 Sorting and searching Lists ......635 I/O Making a Collection or Map unmodifiable ............... 636 Synchronizing a Collection or Map...................637 Holding references ............ 639 The WeakHashMap .............. 640 Java 1.0/1.1 containers ...... 642 Vector & Enumeration ........ 642 Hashtable ............................... 643 Stack ........................................ 643 BitSet ....................................... 644 Summary............................ 646 647 The File class .................... 647 A directory lister ........................647 Directory utilities ...................... 650 Checking for and creating directories .............654 Input and output ............... 656 Types of InputStream.............657 Types of OutputStream......... 658 Adding attributes and useful interfaces.......... 659 Reading from an InputStream with FilterlnputStream ........ 660 Writing to an OutputStream with FilterOutputStream...... 661 Readers & Writers ......... 662 Sources and sinks of data ......... 662 Modifying stream behavior ...... 663 Unchanged classes .................... 664 Off by itself: RandomAccessFile ....... 665 Typical uses of I/O streams.................... 665 Buffered input file......................665 Input from memory .................. 666 Formatted memory input ..........667 Basic file output ........................ 668 Storing and recovering data ..... 669 Reading and writing random-access files .................. 670 Piped streams ............................672 File reading & writing utilities ............... 672 Reading binary files ...................674 Standard I/O.......................675 Reading from standard input ....675 Changing System.out to a PrintWriter ......................676 Redirecting standard I/O ..........676 Process control ...................677 New I/O ............................. 679 Converting data..........................681 Fetching primitives................... 684 View buffers ...............................685 Data manipulation with buffers ............................... 688 Buffer details............................. 689 Memory-mapped files ...............692 File locking.................................695 Compression ......................698 Simple compression with GZIP .................................. 698 Multifile storage with Zip ..........699 Java ARchives (JARs)................ 701 Object serialization ............ 703 Finding the class ........................706 Controlling serialization ............ 707 Using persistence....................... 713 XML.................................... 718 Preferences .......................... 721 Summary ............................ 722 Enumerated Types 725 Basic enum features ......... 725 Using static imports with enums ............................... 726 Adding methods to an enum........................ 727 Overriding enum methods.......728 enums in switch statements............. 728 The mystery of values() ........................ 729 Implements, not inherits......................... 732 Random selection .............. 732 Using interfaces for organization.................. 734 Using EnumSet instead of flags ................... 737 Using EnumMap ............. 739 Constant-specific methods.............................. 740 Chain of Responsibility with enums ............................... 743 State machines with enums .....746 Multiple dispatching...........751 Dispatching with enums .......... 753 Using constant-specific methods......... 755 Dispatching with EnumMaps ...................... 756 Using a 2-D array....................... 757 Summary ............................ 759 Annotations 761 Basic syntax ....................... 762 Defining annotations .................762 Meta-annotations ......................763 Writing annotation processors ........765 Annotation elements .................765 Default value constraints...........766 Generating external files............766 Annotations don’t support inheritance ...................769 Implementing the processor......769 Using apt to process annotations............772 Using the Visitor pattern with apt .............................. 775 Annotation-based unit testing..........................778 Using @Unit with generics.......785 No “suites” necessary.................786 Implementing @Unit ...............787 Removing test code....................792 Summary ............................. 795 Concurrency 797 The many faces of concurrency ....................... 798 Faster execution.........................798 Improving code design ............. 800 Basic threading .................. 801 Defining tasks ............................801 The Thread class ..................... 802 Using Executors ..................... 804 Producing return values from tasks ................................. 806 Sleeping..................................... 808 Priority ...................................... 809 Yielding ......................................810 Daemon threads.........................810 Coding variations.......................814 Terminology ............................... 819 Joining a thread ......................... 819 Creating responsive user interfaces............................821 Thread groups........................... 822 Catching exceptions .................. 822 Sharing resources .............. 824 Improperly accessing resources................... 825 Resolving shared resource contention ...................827 Atomicity and volatility ............. 831 Atomic classes........................... 836 Critical sections..........................837 Synchronizing on other objects .............................. 841 Thread local storage ..................843 Terminating tasks ..............844 The ornamental garden ............ 844 Terminating when blocked........847 Interruption .............................. 848 Checking for an interrupt ..........854 Cooperation between tasks ..................... 856 wait() and notifyAll() ............ 857 notify() vs. notifyAll() ........... 861 Producers and consumers ........ 863 Producer-consumers and queues ................................ 868 Using pipes for I/O between tasks.............................872 Deadlock............................. 874 New library components........................ 879 CountDownLatch ..................879 CyclicBarrier .......................... 881 DelayQueue ........................... 883 PriorityBlockingQueue....... 885 The greenhouse controller with ScheduledExecutor ......887 Semaphore............................. 890 Exchanger .............................. 893 Simulation .......................... 896 Bank teller simulation .............. 896 The restaurant simulation ........ 900 Distributing work ..................... 904 Performance tuning ...........909 Comparing mutex technologies................... 909 Lock-free containers .................. 916 Optimistic locking......................922 ReadWriteLocks ....................923 Active objects ..................... 925 Summary ............................ 929 Further reading.......................... 931 Graphical User Interfaces 933 Applets ............................... 935 Swing basics ....................... 935 A display framework..................937 Making a button.................938 Capturing an event............. 939 Text areas ........................... 941 Controlling layout .............. 942 BorderLayout .........................942 FlowLayout .............................943 GridLayout .............................. 944 GridBagLayout....................... 944 Absolute positioning..................945 BoxLayout ...............................945 The best approach?....................945 The Swing event model ..... 945 Event and listener types ........... 946 Tracking multiple events ........... 951 A selection of Swing components ............ 953 Buttons .......................................953 Icons .......................................... 955 Tool tips .....................................957 Text fields...................................957 Borders ....................................... 959 A mini-editor..............................959 Check boxes .............................. 960 Radio buttons............................. 961 Combo boxes (drop-down lists) ...................... 962 List boxes .................................. 963 Tabbed panes .............................965 Message boxes ...........................965 Menus ......................................... 967 Pop-up menus............................972 Drawing ...................................... 973 Dialog boxes...............................975 File dialogs .................................978 HTML on Swing components.................... 980 Sliders and progress bars ......... 980 Selecting look & feel...................981 Trees, tables & clipboard .......... 983 JNLP and Java Web Start................... 983 Concurrency & Swing ........ 988 Long-running tasks................... 988 Visual threading........................ 994 Visual programming and JavaBeans ................... 996 What is a JavaBean? ................. 996 Extracting Beanlnfo with the Introspector ............ 998 A more sophisticated Bean ..... 1002 JavaBeans and synchronization .......................1005 Packaging a Bean .................... 1008 More complex Bean support .. 1009 More to Beans .......................... 1010 Alternatives to Swing........1010 Building Flash Web clients with Flex................ 1011 Hello, Flex .................................1011 Compiling MXML .................... 1012 MXML and ActionScript.......... 1013 Containers and controls........... 1013 Effects and styles ..................... 1015 Events....................................... 1016 Connecting to Java .................. 1016 Data models and data binding...................... 1018 Building and deploying............ 1019 Creating SWT applications ...................... 1020 Installing SWT .........................1020 Hello, SWT............................... 1021 Eliminating redundant code.... 1023 Menus ...................................... 1024 Tabbed panes, buttons, and events ................................ 1025 Graphics ................................... 1028 Concurrency in SWT................1030 SWT vs. Swing?........................1032 Summary .......................... 1033 Resources ................................. 1033 A: Supplements 1035 Downloadable supplements ..................... 1035 Thinking in C: Foundations for Java ....... 1035 Thinking in Java seminar............................. 1035 Hands-On Java seminar-on-CD ................ 1036 Thinking in Objects seminar............................. 1036 Thinking in Enterprise Java ................ 1036 Thinking in Patterns (with Java) ....................... 1037 Thinking in Patterns seminar............................. 1037 Design consulting and reviews ...................... 1038 B: Resources 1039 Software ........................... 1039 Editors & IDEs ................. 1039 Books ................................ 1039 Analysis & design.....................1040 Python ...................................... 1042 My own list of books ................ 1042 Index 1045 Preface I originally approached Java as “just another programming language,” which in many senses it is. But as time passed and I studied it more deeply, I began to see that the fundamental intent of this language was different from other languages I had seen up to that point. Programming is about managing complexity: the complexity of the problem you want to solve, laid upon the complexity of the machine in which it is solved. Because of this complexity, most of our programming projects fail. And yet, of all the programming languages of which I am aware, almost none have gone all out and decided that their main design goal would be to conquer the complexity of developing and maintaining programs.1 Of course, many language design decisions were made with complexity in mind, but at some point there were always other issues that were considered essential to be added into the mix. Inevitably, those other issues are what cause programmers to eventually “hit the wall” with that language. For example, C++ had to be backwards-compatible with C (to allow easy migration for C programmers), as well as efficient. Those are both very useful goals and account for much of the success of C++, but they also expose extra complexity that prevents some projects from being finished (certainly, you can blame programmers and management, but if a language can help by catching your mistakes, why shouldn’t it?). As another example, Visual BASIC (VB) was tied to BASIC, which wasn’t really designed to be an extensible language, so all the extensions piled upon VB have produced some truly unmaintainable syntax. Perl is backwards-compatible with awk, sed, grep, and other Unix tools it was meant to replace, and as a result it is often accused of producing “write-only code” (that is, after a while you can’t read it). On the other hand, C++, VB, Perl, and other languages like Smalltalk had some of their design efforts focused on the issue of complexity and as a result are remarkably successful in solving certain types of problems. What has impressed me most as I have come to understand Java is that somewhere in the mix of Sun’s design objectives, it seems that there was a goal of reducing complexity for the programmer. As if to say, “We care about reducing the time and difficulty of producing robust code.” In the early days, this goal resulted in code that didn’t run very fast (although this has improved over time), but it has indeed produced amazing reductions in development time—half or less of the time that it takes to create an equivalent C++ program. This result alone can save incredible amounts of time and money, but Java doesn’t stop there. It goes on to wrap many of the complex tasks that have become important, such as multithreading and network programming, in language features or libraries that can at times make those tasks easy. And finally, it tackles some really big complexity problems: cross-platform programs, dynamic code changes, and even security, each of which can fit on your complexity spectrum anywhere from “impediment” to “show-stopper.” So despite the performance problems that we’ve seen, the promise of Java is tremendous: It can make us significantly more productive programmers. In all ways—creating the programs, working in teams, building user interfaces to communicate with the user, running the programs on different types of machines, and easily writing programs that communicate across the Internet—Java increases the communication bandwidth between people. I think that the results of the communication revolution may not be seen from the effects of moving large quantities of bits around. We shall see the true revolution because we will all communicate with each other more easily: one-on-one, but also in groups and as a planet. 1 However, I believe that the Python language comes closest to doing exactly that. See www.Python.org. I’ve heard it suggested that the next revolution is the formation of a kind of global mind that results from enough people and enough interconnectedness. Java may or may not be the tool that foments that revolution, but at least the possibility has made me feel like I’m doing something meaningful by attempting to teach the language. Java SE5 and SE6 This edition of the book benefits greatly from the improvements made to the Java language in what Sun originally called JDK 1.5, and then later changed to JDK5 or J2SE5, then finally they dropped the outdated “2” and changed it to Java SE5. Many of the Java SE5 language changes were designed to improve the experience of the programmer. As you shall see, the Java language designers did not completely succeed at this task, but in general they made large steps in the right direction. One of the important goals of this edition is to completely absorb the improvements of Java SE5/6, and to introduce and use them throughout this book. This means that this edition takes the somewhat bold step of being “Java SE5/6-only,” and much of the code in the book will not compile with earlier versions of Java; the build system will complain and stop if you try. However, I think the benefits are worth the risk. If you are somehow fettered to earlier versions of Java, I have covered the bases by providing free downloads of previous editions of this book via www.MindView.net. For various reasons, I have decided not to provide the current edition of the book in free electronic form, but only the prior editions. Java SE6 This book was a monumental, time-consuming project, and before it was published, Java SE6 (code-named mustang) appeared in beta form. Although there were a few minor changes in Java SE6 that improved some of the examples in the book, for the most part the focus of Java SE6 did not affect the content of this book; the features were primarily speed improvements and library features that were outside the purview of this text. The code in this book was successfully tested with a release candidate of Java SE6, so I do not expect any changes that will affect the content of this book. If there are any important changes by the time Java SE6 is officially released, these will be reflected in the book’s source code, which is downloadable from www.MindView.net. The cover indicates that this book is for “Java SE5/6,” which means “written for Java SE5 and the very significant changes that version introduced into the language, but is equally applicable to Java SE6.” The 4th edition The satisfaction of doing a new edition of a book is in getting things “right,” according to what I have learned since the last edition came out. Often these insights are in the nature of the saying “A learning experience is what you get when you don’t get what you want,” and my opportunity is to fix something embarrassing or simply tedious. Just as often, creating the next edition produces fascinating new ideas, and the embarrassment is far outweighed by the delight of discovery and the ability to express ideas in a better form than what I have previously achieved. There is also the challenge that whispers in the back of my brain, that of making the book something that owners of previous editions will want to buy. This presses me to improve, 2 Thinking in Java Bruce Eckel rewrite and reorganize everything that I can, to make the book a new and valuable experience for dedicated readers. Changes The CD-ROM that has traditionally been packaged as part of this book is not part of this edition. The essential part of that CD, the Thinking in C multimedia seminar (created for MindView by Chuck Allison), is now available as a downloadable Flash presentation. The goal of that seminar is to prepare those who are not familiar enough with C syntax to understand the material presented in this book. Although two of the chapters in this book give decent introductory syntax coverage, they may not be enough for people without an adequate background, and Thinking in C is intended to help those people get to the necessary level. The Concurrency chapter (formerly called “Multithreading”) has been completely rewritten to match the major changes in the Java SE5 concurrency libraries, but it still gives you a basic foundation in the core ideas of concurrency. Without that core, it’s hard to understand more complex issues of threading. I spent many months working on this, immersed in that netherworld called “concurrency,” and in the end the chapter is something that not only provides a basic foundation but also ventures into more advanced territory. There is a new chapter on every significant new Java SE5 language feature, and the other new features have been woven into modifications made to the existing material. Because of my continuing study of design patterns, more patterns have been introduced throughout the book as well. The book has undergone significant reorganization. Much of this has come from the teaching process together with a realization that, perhaps, my perception of what a “chapter” was could stand some rethought. I have tended towards an unconsidered belief that a topic had to be “big enough” to justify being a chapter. But especially while teaching design patterns, I find that seminar attendees do best if I introduce a single pattern and then we immediately do an exercise, even if it means I only speak for a brief time (I discovered that this pace was also more enjoyable for me as a teacher). So in this version of the book I’ve tried to break chapters up by topic, and not worry about the resulting length of the chapters. I think it has been an improvement. I have also come to realize the importance of code testing. Without a built-in test framework with tests that are run every time you do a build of your system, you have no way of knowing if your code is reliable or not. To accomplish this in the book, I created a test framework to display and validate the output of each program. (The framework was written in Python; you can find it in the downloadable code for this book at www.MindView.net.) Testing in general is covered in the supplement you will find at http://MindView.net/Books/BetterJava, which introduces what I now believe are fundamental skills that all programmers should have in their basic toolkit. In addition, I’ve gone over every single example in the book and asked myself, “Why did I do it this way?” In most cases I have done some modification and improvement, both to make the examples more consistent within themselves and also to demonstrate what I consider to be best practices in Java coding (at least, within the limitations of an introductory text). Many of the existing examples have had very significant redesign and reimplementation. Examples that no longer made sense to me were removed, and new examples have been added. Readers have made many, many wonderful comments about the first three editions of this book, which has naturally been very pleasant for me. However, every now and then, someone will have complaints, and for some reason one complaint that comes up periodically is “The book is too big.” In my mind it is faint damnation indeed if “too many pages” is your only Preface 3 gripe. (One is reminded of the Emperor of Austria’s complaint about Mozart’s work: “Too many notes!” Not that I am in any way trying to compare myself to Mozart.) In addition, I can only assume that such a complaint comes from someone who is yet to be acquainted with the vastness of the Java language itself and has not seen the rest of the books on the subject. Despite this, one of the things I have attempted to do in this edition is trim out the portions that have become obsolete, or at least nonessential. In general, I’ve tried to go over everything, remove what is no longer necessary, include changes, and improve everything I could. I feel comfortable removing portions because the original material remains on the Web site (www.MindView.net), in the form of the freely downloadable 1st through 3rd editions of the book, and in the downloadable supplements for this book. For those of you who still can’t stand the size of the book, I do apologize. Believe it or not, I have worked hard to keep the size down. Note on the cover design The cover of Thinking in Java is inspired by the American Arts & Crafts Movement that began near the turn of the century and reached its zenith between 1900 and 1920. It began in England as a reaction to both the machine production of the Industrial Revolution and the highly ornamental style of the Victorian era. Arts & Crafts emphasized spare design, the forms of nature as seen in the art nouveau movement, hand-crafting, and the importance of the individual craftsperson, and yet it did not eschew the use of modern tools. There are many echoes with the situation we have today: the turn of the century, the evolution from the raw beginnings of the computer revolution to something more refined and meaningful, and the emphasis on software craftsmanship rather than just manufacturing code. I see Java in this same way: as an attempt to elevate the programmer away from an operating system mechanic and toward being a “software craftsman.” Both the author and the book/cover designer (who have been friends since childhood) find inspiration in this movement, and both own furniture, lamps, and other pieces that are either original or inspired by this period. The other theme in this cover suggests a collection box that a naturalist might use to display the insect specimens that he or she has preserved. These insects are objects that are placed within the box objects. The box objects are themselves placed within the “cover object,” which illustrates the fundamental concept of aggregation in object-oriented programming. Of course, a programmer cannot help but make the association with “bugs,” and here the bugs have been captured and presumably killed in a specimen jar, and finally confined within a small display box, as if to imply Java’s ability to find, display, and subdue bugs (which is truly one of its most powerful attributes). In this edition, I created the watercolor painting that you see as the cover background. Acknowledgements First, thanks to associates who have worked with me to give seminars, provide consulting, and develop teaching projects: Dave Bartlett, Bill Venners, Chuck Allison, Jeremy Meyer, and Jamie King. I appreciate your patience as I continue to try to develop the best model for independent folks like us to work together. Recently, no doubt because of the Internet, I have become associated with a surprisingly large number of people who assist me in my endeavors, usually working from their own home offices. In the past, I would have had to pay for a pretty big office space to accommodate all these folks, but because of the Net, FedEx, and the telephone, I’m able to benefit from their help without the extra costs. In my attempts to learn to “play well with 4 Thinking in Java Bruce Eckel others,” you have all been very helpful, and I hope to continue learning how to make my own work better through the efforts of others. Paula Steuer has been invaluable in taking over my haphazard business practices and making them sane (thanks for prodding me when I don’t want to do something, Paula). Jonathan Wilcox, Esq., has sifted through my corporate structure and turned over every possible rock that might hide scorpions, and frog-marched us through the process of putting everything straight, legally. Thanks for your care and persistence. Sharlynn Cobaugh has made herself an expert in sound processing and an essential part of creating the multimedia training experiences, as well as tackling other problems. Thanks for your perseverance when faced with intractable computer problems. The folks at Amaio in Prague have helped me out with several projects. Daniel Will-Harris was the original work-by-Internet inspiration, and he is of course fundamental to all my graphic design solutions. Over the years, through his conferences and workshops, Gerald Weinberg has become my unofficial coach and mentor, for which I thank him. Ervin Varga was exceptionally helpful with technical corrections on the 4th edition—although other people helped on various chapters and examples, Ervin was my primary technical reviewer for the book, and he also took on the task of rewriting the solution guide for the 4th edition. Ervin found errors and made improvements to the book that were invaluable additions to this text. His thoroughness and attention to detail are amazing, and he’s far and away the best technical reader I’ve ever had. Thanks, Ervin. My weblog on Bill Venners’ www.Artima.com has been a source of assistance when I’ve needed to bounce ideas around. Thanks to the readers that have helped me clarify concepts by submitting comments, including James Watson, Howard Lovatt, Michael Barker, and others, in particular those who helped with generics. Thanks to Mark Welsh for his continuing assistance. Evan Cofsky continues to be very supportive by knowing off the top of his head all the arcane details of setting up and maintaining Linux-based Web servers, and keeping the MindView server tuned and secure. A special thanks to my new friend, coffee, who generated nearly boundless enthusiasm for this project. Camp4 Coffee in Crested Butte, Colorado, has become the standard hangout when people have come up to take MindView seminars, and during seminar breaks it is the best catering I’ve ever had. Thanks to my buddy Al Smith for creating it and making it such a great place, and for being such an interesting and entertaining part of the Crested Butte experience. And to all the Camp4 barristas who so cheerfully dole out beverages. Thanks to the folks at Prentice Hall for continuing to give me what I want, putting up with all my special requirements, and for going out of their way to make things run smoothly for me. Certain tools have proved invaluable during my development process and I am very grateful to the creators every time I use these. Cygwin (www.cygwin.com) has solved innumerable problems for me that Windows can’t/won’t and I become more attached to it each day (if I only had this 15 years ago when my brain was still hard-wired with Gnu Emacs). IBM’s Eclipse (www.eclipse.org) is a truly wonderful contribution to the development community, and I expect to see great things from it as it continues to evolve (how did IBM become hip? I must have missed a memo). JetBrains IntelliJ Idea continues to forge creative new paths in development tools. I began using Enterprise Architect from Sparxsystems on this book, and it has rapidly become my UML tool of choice. Marco Hunsicker’s Jalopy code formatter (www.triemax.com) came in handy on numerous occasions, and Marco was very helpful in Preface 5 configuring it to my particular needs. I’ve also found Slava Pestov’s JEdit and plug-ins to be helpful at times (www.jedit.org) and it’s quite a reasonable beginner’s editor for seminars. And of course, if I don’t say it enough everywhere else, I use Python (www.Python.org) constantly to solve problems, the brainchild of my buddy Guido Van Rossum and the gang of goofy geniuses with whom I spent a few great days sprinting (Tim Peters, I’ve now framed that mouse you borrowed, officially named the “TimBotMouse”). You guys need to find healthier places to eat lunch. (Also, thanks to the entire Python community, an amazing bunch of people.) Lots of people sent in corrections and I am indebted to them all, but particular thanks go to (for the 1st edition): Kevin Raulerson (found tons of great bugs), Bob Resendes (simply incredible), John Pinto, Joe Dante, Joe Sharp (all three were fabulous), David Combs (many grammar and clarification corrections), Dr. Robert Stephenson, John Cook, Franklin Chen, Zev Griner, David Karr, Leander A. Stroschein, Steve Clark, Charles A. Lee, Austin Maher, Dennis P. Roth, Roque Oliveira, Douglas Dunn, Dejan Ristic, Neil Galarneau, David B. Malkovsky, Steve Wilkinson, and a host of others. Prof. Ir. Marc Meurrens put in a great deal of effort to publicize and make the electronic version of the 1st edition of the book available in Europe. Thanks to those who helped me rewrite the examples to use the Swing library (for the 2nd edition), and for other assistance: Jon Shvarts, Thomas Kirsch, Rahim Adatia, Rajesh Jain, Ravi Manthena, Banu Rajamani, Jens Brandt, Nitin Shivaram, Malcolm Davis, and everyone who expressed support. In the 4th edition, Chris Grindstaff was very helpful during the development of the SWT section, and Sean Neville wrote the first draft of the Flex section for me. Kraig Brockschmidt and Gen Kiyooka have been some of the smart technical people in my life who have become friends and have also been both influential and unusual in that they do yoga and practice other forms of spiritual enhancement, which I find quite inspirational and instructional. It’s not that much of a surprise to me that understanding Delphi helped me understand Java, since there are many concepts and language design decisions in common. My Delphi friends provided assistance by helping me gain insight into that marvelous programming environment. They are Marco Cantu (another Italian—perhaps being steeped in Latin gives one aptitude for programming languages?), Neil Rubenking (who used to do the yoga/vegetarian/Zen thing until he discovered computers), and of course Zack Urlocker (the original Delphi product manager), a long-time pal whom I’ve traveled the world with. We’re all indebted to the brilliance of Anders Hejlsberg, who continues to toil away at C# (which, as you’ll learn in this book, was a major inspiration for Java SE5). My friend Richard Hale Shaw’s insights and support have been very helpful (and Kim’s, too). Richard and I spent many months giving seminars together and trying to work out the perfect learning experience for the attendees. The book design, cover design, and cover photo were created by my friend Daniel WillHarris, noted author and designer (www.Will-Harris.com), who used to play with rub-on letters in junior high school while he awaited the invention of computers and desktop publishing, and complained of me mumbling over my algebra problems. However, I produced the cameraready pages myself, so the typesetting errors are mine. Microsoft® Word XP for Windows was used to write the book and to create camera-ready pages in Adobe Acrobat; the book was created directly from the Acrobat PDF files. As a tribute to the electronic age, I happened to be overseas when I produced the final versions of the 1st and 2nd editions of the book—the 1st edition was sent from Cape Town, South Africa, and the 2nd edition was posted from Prague. 6 Thinking in Java Bruce Eckel The 3rd and 4th came from Crested Butte, Colorado. The body typeface is Georgia and the headlines are in Verdana. The cover typeface is ITC Rennie Mackintosh. A special thanks to all my teachers and all my students (who are my teachers as well). Molly the cat often sat in my lap while I worked on this edition, and thus offered her own kind of warm, furry support. The supporting cast of friends includes, but is not limited to: Patty Gast (Masseuse extraordinaire), Andrew Binstock, Steve Sinofsky, JD Hildebrandt, Tom Keffer, Brian McElhinney, Brinkley Barr, Bill Gates at Midnight Engineering Magazine, Larry Constantine and Lucy Lockwood, Gene Wang, Dave Mayer, David Intersimone, Chris and Laura Strand, the Almquists, Brad Jerbic, Marilyn Cvitanic, Mark Mabry, the Robbins families, the Moelter families (and the McMillans), Michael Wilk, Dave Stoner, the Cranstons, Larry Fogg, Mike Sequeira, Gary Entsminger, Kevin and Sonda Donovan, Joe Lordi, Dave and Brenda Bartlett, Patti Gast, Blake, Annette & Jade, the Rentschlers, the Sudeks, Dick, Patty, and Lee Eckel, Lynn and Todd, and their families. And of course, Mom and Dad. Preface 7 Introduction “He gave man speech, and speech created thought, Which is the measure of the Universe”—Prometheus Unbound, Shelley Human beings ... are very much at the mercy of the particular language which has become the medium of expression for their society. It is quite an illusion to imagine that one adjusts to reality essentially without the use of language and that language is merely an incidental means of solving specific problems of communication and reflection. The fact of the matter is that the “real world” is to a large extent unconsciously built up on the language habits of the group. The Status of Linguistics as a Science, 1929, Edward Sapir Like any human language, Java provides a way to express concepts. If successful, this medium of expression will be significantly easier and more flexible than the alternatives as problems grow larger and more complex. You can’t look at Java as just a collection of features—some of the features make no sense in isolation. You can use the sum of the parts only if you are thinking about design, not simply coding. And to understand Java in this way, you must understand the problems with the language and with programming in general. This book discusses programming problems, why they are problems, and the approach Java has taken to solve them. Thus, the set of features that I explain in each chapter are based on the way I see a particular type of problem being solved with the language. In this way I hope to move you, a little at a time, to the point where the Java mindset becomes your native tongue. Throughout, I’ll be taking the attitude that you want to build a model in your head that allows you to develop a deep understanding of the language; if you encounter a puzzle, you’ll feed it to your model and deduce the answer. Prerequisites This book assumes that you have some programming familiarity: You understand that a program is a collection of statements, the idea of a subroutine/function/macro, control statements such as “if” and looping constructs such as “while,” etc. However, you might have learned this in many places, such as programming with a macro language or working with a tool like Perl. As long as you’ve programmed to the point where you feel comfortable with the basic ideas of programming, you’ll be able to work through this book. Of course, the book will be easier for C programmers and more so for C++ programmers, but don’t count yourself out if you’re not experienced with those languages—however, come willing to work hard. Also, the Thinking in C multimedia seminar that you can download from www.MindView.net will bring you up to speed in the fundamentals necessary to learn Java. However, I will be introducing the concepts of object-oriented programming (OOP) and Java’s basic control mechanisms. Although references may be made to C and C++ language features, these are not intended to be insider comments, but instead to help all programmers put Java in perspective with those languages, from which, after all, Java is descended. I will attempt to make these references simple and to explain anything that I think a non-C/C++ programmer would not be familiar with. Learning Java At about the same time that my first book, Using C++ (Osborne/McGraw-Hill, 1989), came out, I began teaching that language. Teaching programming ideas has become my profession; I’ve seen nodding heads, blank faces, and puzzled expressions in audiences all over the world since 1987. As I began giving in-house training with smaller groups of people, I discovered something during the exercises. Even those people who were smiling and nodding were confused about many issues. I found out, by creating and chairing the C++ track at the Software Development Conference for a number of years (and later creating and chairing the Java track), that I and other speakers tended to give the typical audience too many topics too quickly. So eventually, through both variety in the audience level and the way that I presented the material, I would end up losing some portion of the audience. Maybe it’s asking too much, but because I am one of those people resistant to traditional lecturing (and for most people, I believe, such resistance results from boredom), I wanted to try to keep everyone up to speed. For a time, I was creating a number of different presentations in fairly short order. Thus, I ended up learning by experiment and iteration (a technique that also works well in program design). Eventually, I developed a course using everything I had learned from my teaching experience. My company, MindView, Inc., now gives this as the public and in-house Thinking in Java seminar; this is our main introductory seminar that provides the foundation for our more advanced seminars. You can find details at www.MindView.net. (The introductory seminar is also available as the Hands-On Java CD ROM. Information is available at the same Web site.) The feedback that I get from each seminar helps me change and refocus the material until I think it works well as a teaching medium. But this book isn’t just seminar notes; I tried to pack as much information as I could within these pages, and structured it to draw you through into the next subject. More than anything, the book is designed to serve the solitary reader who is struggling with a new programming language. Goals Like my previous book, Thinking in C++, this book was designed with one thing in mind: the way people learn a language. When I think of a chapter in the book, I think in terms of what makes a good lesson during a seminar. Seminar audience feedback helped me understand the difficult parts that needed illumination. In the areas where I got ambitious and included too many features all at once, I came to know—through the process of presenting the material— that if you include a lot of new features, you need to explain them all, and this easily compounds the student’s confusion. Each chapter tries to teach a single feature, or a small group of associated features, without relying on concepts that haven’t been introduced yet. That way you can digest each piece in the context of your current knowledge before moving on. My goals in this book are to: 1. Present the material one simple step at a time so that you can easily digest each idea before moving on. Carefully sequence the presentation of features so that you’re exposed to a topic before you see it in use. Of course, this isn’t always possible; in those situations, a brief introductory description is given. 2. Use examples that are as simple and short as possible. This sometimes prevents me from tackling “real world” problems, but I’ve found that beginners are usually happier when they can understand every detail of an example rather than being impressed by 10 Thinking in Java Bruce Eckel the scope of the problem it solves. Also, there’s a severe limit to the amount of code that can be absorbed in a classroom situation. For this I will no doubt receive criticism for using “toy examples,” but I’m willing to accept that in favor of producing something pedagogically useful. 3. Give you what I think is important for you to understand about the language, rather than everything that I know. I believe there is an information importance hierarchy, and that there are some facts that 95 percent of programmers will never need to know—details that just confuse people and increase their perception of the complexity of the language. To take an example from C, if you memorize the operator precedence table (I never did), you can write clever code. But if you need to think about it, it will also confuse the reader/maintainer of that code. So forget about precedence, and use parentheses when things aren’t clear. 4. Keep each section focused enough so that the lecture time—and the time between exercise periods—is small. Not only does this keep the audience’s minds more active and involved during a hands-on seminar, but it gives the reader a greater sense of accomplishment. 5. Provide you with a solid foundation so that you can understand the issues well enough to move on to more difficult coursework and books. Teaching from this book The original edition of this book evolved from a one-week seminar which was, when Java was in its infancy, enough time to cover the language. As Java grew and continued to encompass more and more features and libraries, I stubbornly tried to teach it all in one week. At one point, a customer asked me to teach “just the fundamentals,” and in doing so I discovered that trying to cram everything into a single week had become painful for both myself and for seminarians. Java was no longer a “simple” language that could be taught in a week. That experience and realization drove much of the reorganization of this book, which is now designed to support a two-week seminar or a two-term college course. The introductory portion ends with the Error Handling with Exceptions chapter, but you may also want to supplement this with an introduction to JDBC, Servlets and JSPs. This provides a foundation course, and is the core of the Hands-On Java CD ROM. The remainder of the book comprises an intermediatelevel course, and is the material covered in the Intermediate Thinking in Java CD ROM. Both of these CD ROMs are for sale at www.MindView.net. Contact Prentice-Hall at www.prenhallprofessional.com for information about professor support materials for this book. JDK HTML documentation The Java language and libraries from Sun Microsystems (a free download from http://java.sun.com) come with documentation in electronic form, readable using a Web browser. Many books published on Java have duplicated this documentation. So you either already have it or you can download it, and unless necessary, this book will not repeat that documentation, because it’s usually much faster if you find the class descriptions with your Web browser than if you look them up in a book (and the online documentation is probably more upto-date). You’ll simply be referred to “the JDK documentation.” This book will provide extra descriptions of the classes only when it’s necessary to supplement that documentation so you can understand a particular example. Introduction 11 Exercises I’ve discovered that simple exercises are exceptionally useful to complete a student’s understanding during a seminar, so you’ll find a set at the end of each chapter. Most exercises are designed to be easy enough that they can be finished in a reasonable amount of time in a classroom situation while the instructor observes, making sure that all the students are absorbing the material. Some are more challenging, but none present major challenges. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Foundations for Java Another bonus with this edition is the free multimedia seminar that you can download from www.MindView.net. This is the Thinking in C seminar that gives you an introduction to the C syntax, operators, and functions that Java syntax is based upon. In previous editions of the book this was in the Foundations for Java CD that was packaged with the book, but now the seminar may be freely downloaded. I originally commissioned Chuck Allison to create Thinking in C as a standalone product, but decided to include it with the 2nd edition of Thinking in C++ and 2nd and 3rd editions of Thinking in Java because of the consistent experience of having people come to seminars without an adequate background in basic C syntax. The thinking apparently goes “I’m a smart programmer and I don’t want to learn C, but rather C++ or Java, so I’ll just skip C and go directly to C++/Java.” After arriving at the seminar, it slowly dawns on folks that the prerequisite of understanding C syntax is there for a very good reason. Technologies have changed, and it made more sense to rework Thinking in C as a downloadable Flash presentation rather than including it as a CD. By providing this seminar online, I can ensure that everyone can begin with adequate preparation. The Thinking in C seminar also allows the book to appeal to a wider audience. Even though the Operators and Controlling Execution chapters do cover the fundamental parts of Java that come from C, the online seminar is a gentler introduction, and assumes even less about the student’s programming background than does the book. Source code All the source code for this book is available as copyrighted freeware, distributed as a single package, by visiting the Web site www.MindView.net. To make sure that you get the most current version, this is the official code distribution site. You may distribute the code in classroom and other educational situations. The primary goal of the copyright is to ensure that the source of the code is properly cited, and to prevent you from republishing the code in print media without permission. (As long as the source is cited, using examples from the book in most media is generally not a problem.) In each source-code file you will find a reference to the following copyright notice: //:! Copyright.txt This computer source code is Copyright ©2006 MindView, Inc. All Rights Reserved. 12 Thinking in Java Bruce Eckel Permission to use, copy, modify, and distribute this computer source code (Source Code) and its documentation without fee and without a written agreement for the purposes set forth below is hereby granted, provided that the above copyright notice, this paragraph and the following five numbered paragraphs appear in all copies. 1. Permission is granted to compile the Source Code and to include the compiled code, in executable format only, in personal and commercial software programs. 2. Permission is granted to use the Source Code without modification in classroom situations, including in presentation materials, provided that the book "Thinking in Java" is cited as the origin. 3. Permission to incorporate the Source Code into printed media may be obtained by contacting: MindView, Inc. 5343 Valle Vista La Mesa, California 91941 Wayne@MindView.net 4. The Source Code and documentation are copyrighted by MindView, Inc. The Source code is provided without express or implied warranty of any kind, including any implied warranty of merchantability, fitness for a particular purpose or non-infringement. MindView, Inc. does not warrant that the operation of any program that includes the Source Code will be uninterrupted or error-free. MindView, Inc. makes no representation about the suitability of the Source Code or of any software that includes the Source Code for any purpose. The entire risk as to the quality and performance of any program that includes the Source Code is with the user of the Source Code. The user understands that the Source Code was developed for research and instructional purposes and is advised not to rely exclusively for any reason on the Source Code or any program that includes the Source Code. Should the Source Code or any resulting software prove defective, the user assumes the cost of all necessary servicing, repair, or correction. 5. IN NO EVENT SHALL MINDVIEW, INC., OR ITS PUBLISHER BE LIABLE TO ANY PARTY UNDER ANY LEGAL THEORY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, BUSINESS INTERRUPTION, LOSS OF BUSINESS INFORMATION, OR ANY OTHER PECUNIARY LOSS, OR FOR PERSONAL INJURIES, ARISING OUT OF THE USE OF THIS SOURCE CODE AND ITS DOCUMENTATION, OR ARISING OUT OF THE INABILITY TO USE ANY RESULTING PROGRAM, EVEN IF MINDVIEW, INC., OR ITS PUBLISHER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. MINDVIEW, INC. SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOURCE CODE AND DOCUMENTATION PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, WITHOUT ANY ACCOMPANYING SERVICES FROM MINDVIEW, INC., AND MINDVIEW, INC. HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS. Please note that MindView, Inc. maintains a Web site which is the sole distribution point for electronic copies of the Source Code, http://www.MindView.net (and official mirror sites), where it is freely available under the terms stated above. If you think you’ve found an error in the Source Code, please submit a correction using the feedback system that you will find at http://www.MindView.net. ///:~ You may use the code in your projects and in the classroom (including your presentation materials) as long as the copyright notice that appears in each source file is retained. Introduction 13 Coding standards In the text of this book, identifiers (methods, variables, and class names) are set in bold. Most keywords are also set in bold, except for those keywords that are used so much that the bolding can become tedious, such as “class.” I use a particular coding style for the examples in this book. As much as possible, this follows the style that Sun itself uses in virtually all of the code you will find at its site (see http://java.sun.com/docs/codeconv/index.html), and seems to be supported by most Java development environments. If you’ve read my other works, you’ll also notice that Sun’s coding style coincides with mine—this pleases me, although I had nothing (that I know of) to do with it. The subject of formatting style is good for hours of hot debate, so I’ll just say I’m not trying to dictate correct style via my examples; I have my own motivation for using the style that I do. Because Java is a free-form programming language, you can continue to use whatever style you’re comfortable with. One solution to the coding style issue is to use a tool like Jalopy (www.triemax.com), which assisted me in developing this book, to change formatting to that which suits you. The code files printed in the book are tested with an automated system, and should all work without compiler errors. This book focuses on and is tested with Java SE5/6. If you need to learn about earlier releases of the language that are not covered in this edition, the 1st through 3rd editions of the book are freely downloadable at www.MindView.net. Errors No matter how many tools a writer uses to detect errors, some always creep in and these often leap off the page for a fresh reader. If you discover anything you believe to be an error, please use the link you will find for this book at www.MindView.net to submit the error along with your suggested correction. Your help is appreciated. 14 Thinking in Java Bruce Eckel Introduction to Objects “We cut nature up, organize it into concepts, and ascribe significances as we do, largely because we are parties to an agreement that holds throughout our speech community and is codified in the patterns of our language … we cannot talk at all except by subscribing to the organization and classification of data which the agreement decrees.” Benjamin Lee Whorf (1897-1941) The genesis of the computer revolution was in a machine. The genesis of our programming languages thus tends to look like that machine. But computers are not so much machines as they are mind amplification tools (“bicycles for the mind,” as Steve Jobs is fond of saying) and a different kind of expressive medium. As a result, the tools are beginning to look less like machines and more like parts of our minds, and also like other forms of expression such as writing, painting, sculpture, animation, and filmmaking. Object-oriented programming (OOP) is part of this movement toward using the computer as an expressive medium. This chapter will introduce you to the basic concepts of OOP, including an overview of development methods. This chapter, and this book, assumes that you have some programming experience, although not necessarily in C. If you think you need more preparation in programming before tackling this book, you should work through the Thinking in C multimedia seminar, downloadable from www.MindView.net. This chapter is background and supplementary material. Many people do not feel comfortable wading into object-oriented programming without understanding the big picture first. Thus, there are many concepts that are introduced here to give you a solid overview of OOP. However, other people may not get the big picture concepts until they’ve seen some of the mechanics first; these people may become bogged down and lost without some code to get their hands on. If you’re part of this latter group and are eager to get to the specifics of the language, feel free to jump past this chapter—skipping it at this point will not prevent you from writing programs or learning the language. However, you will want to come back here eventually to fill in your knowledge so you can understand why objects are important and how to design with them. The progress of abstraction All programming languages provide abstractions. It can be argued that the complexity of the problems you’re able to solve is directly related to the kind and quality of abstraction. By “kind” I mean, “What is it that you are abstracting?” Assembly language is a small abstraction of the underlying machine. Many so-called “imperative” languages that followed (such as FORTRAN, BASIC, and C) were abstractions of assembly language. These languages are big improvements over assembly language, but their primary abstraction still requires you to think in terms of the structure of the computer rather than the structure of the problem you are trying to solve. The programmer must establish the association between the machine model (in the “solution space,” which is the place where you’re implementing that solution, such as a computer) and the model of the problem that is actually being solved (in the “problem space,” which is the place where the problem exists, such as a business). The effort required to perform this mapping, and the fact that it is extrinsic to the programming language, produces programs that are difficult to write and expensive to maintain, and as a side effect created the entire “programming methods” industry. The alternative to modeling the machine is to model the problem you’re trying to solve. Early languages such as LISP and APL chose particular views of the world (“All problems are ultimately lists” or “All problems are algorithmic,” respectively). Prolog casts all problems into chains of decisions. Languages have been created for constraint-based programming and for programming exclusively by manipulating graphical symbols. (The latter proved to be too restrictive.) Each of these approaches may be a good solution to the particular class of problem they’re designed to solve, but when you step outside of that domain they become awkward. The object-oriented approach goes a step further by providing tools for the programmer to represent elements in the problem space. This representation is general enough that the programmer is not constrained to any particular type of problem. We refer to the elements in the problem space and their representations in the solution space as “objects.” (You will also need other objects that don’t have problem-space analogs.) The idea is that the program is allowed to adapt itself to the lingo of the problem by adding new types of objects, so when you read the code describing the solution, you’re reading words that also express the problem. This is a more flexible and powerful language abstraction than what we’ve had before.1 Thus, OOP allows you to describe the problem in terms of the problem, rather than in terms of the computer where the solution will run. There’s still a connection back to the computer: Each object looks quite a bit like a little computer—it has a state, and it has operations that you can ask it to perform. However, this doesn’t seem like such a bad analogy to objects in the real world—they all have characteristics and behaviors. Alan Kay summarized five basic characteristics of Smalltalk, the first successful objectoriented language and one of the languages upon which Java is based. These characteristics represent a pure approach to object-oriented programming: 1. Everything is an object. Think of an object as a fancy variable; it stores data, but you can “make requests” to that object, asking it to perform operations on itself. In theory, you can take any conceptual component in the problem you’re trying to solve (dogs, buildings, services, etc.) and represent it as an object in your program. 2. A program is a bunch of objects telling each other what to do by sending messages. To make a request of an object, you “send a message” to that object. More concretely, you can think of a message as a request to call a method that belongs to a particular object. 3. Each object has its own memory made up of other objects. Put another way, you create a new kind of object by making a package containing existing objects. Thus, you can build complexity into a program while hiding it behind the simplicity of objects. 4. Every object has a type. Using the parlance, each object is an instance of a class, in which “class” is synonymous with “type.” The most important distinguishing characteristic of a class is “What messages can you send to it?” 5. All objects of a particular type can receive the same messages. This is actually a loaded statement, as you will see later. Because an object of type “circle” is also an object of type “shape,” a circle is guaranteed to accept shape messages. This 1 Some language designers have decided that object-oriented programming by itself is not adequate to easily solve all programming problems, and advocate the combination of various approaches into multiparadigm programming languages. See Multiparadigm Programming in Leda by Timothy Budd (Addison-Wesley, 1995). 16 Thinking in Java Bruce Eckel means you can write code that talks to shapes and automatically handle anything that fits the description of a shape. This substitutability is one of the powerful concepts in OOP. Booch offers an even more succinct description of an object: An object has state, behavior and identity. This means that an object can have internal data (which gives it state), methods (to produce behavior), and each object can be uniquely distinguished from every other object—to put this in a concrete sense, each object has a unique address in memory.2 An object has an interface Aristotle was probably the first to begin a careful study of the concept of type; he spoke of “the class of fishes and the class of birds.” The idea that all objects, while being unique, are also part of a class of objects that have characteristics and behaviors in common was used directly in the first object-oriented language, Simula-67, with its fundamental keyword class that introduces a new type into a program. Simula, as its name implies, was created for developing simulations such as the classic “bank teller problem.” In this, you have numerous tellers, customers, accounts, transactions, and units of money—a lot of “objects.” Objects that are identical except for their state during a program’s execution are grouped together into “classes of objects,” and that’s where the keyword class came from. Creating abstract data types (classes) is a fundamental concept in object-oriented programming. Abstract data types work almost exactly like built-in types: You can create variables of a type (called objects or instances in object-oriented parlance) and manipulate those variables (called sending messages or requests; you send a message and the object figures out what to do with it). The members (elements) of each class share some commonality: Every account has a balance, every teller can accept a deposit, etc. At the same time, each member has its own state: Each account has a different balance, each teller has a name. Thus, the tellers, customers, accounts, transactions, etc., can each be represented with a unique entity in the computer program. This entity is the object, and each object belongs to a particular class that defines its characteristics and behaviors. So, although what we really do in object-oriented programming is create new data types, virtually all object-oriented programming languages use the “class” keyword. When you see the word “type” think “class” and vice versa.3 Since a class describes a set of objects that have identical characteristics (data elements) and behaviors (functionality), a class is really a data type because a floating point number, for example, also has a set of characteristics and behaviors. The difference is that a programmer defines a class to fit a problem rather than being forced to use an existing data type that was designed to represent a unit of storage in a machine. You extend the programming language by adding new data types specific to your needs. The programming system welcomes the new classes and gives them all the care and type checking that it gives to built-in types. The object-oriented approach is not limited to building simulations. Whether or not you agree that any program is a simulation of the system you’re designing, the use of OOP techniques can easily reduce a large set of problems to a simple solution. 2 This is actually a bit restrictive, since objects can conceivably exist in different machines and address spaces, and they can also be stored on disk. In these cases, the identity of the object must be determined by something other than memory address. 3 Some people make a distinction, stating that type determines the interface while class is a particular implementation of that interface. Introduction to Objects 17 Once a class is established, you can make as many objects of that class as you like, and then manipulate those objects as if they are the elements that exist in the problem you are trying to solve. Indeed, one of the challenges of object-oriented programming is to create a one-toone mapping between the elements in the problem space and objects in the solution space. But how do you get an object to do useful work for you? There needs to be a way to make a request of the object so that it will do something, such as complete a transaction, draw something on the screen, or turn on a switch. And each object can satisfy only certain requests. The requests you can make of an object are defined by its interface, and the type is what determines the interface. A simple example might be a representation of a light bulb: Light lt = new Light(); lt.on(); The interface determines the requests that you can make for a particular object. However, there must be code somewhere to satisfy that request. This, along with the hidden data, comprises the implementation. From a procedural programming standpoint, it’s not that complicated. A type has a method associated with each possible request, and when you make a particular request to an object, that method is called. This process is usually summarized by saying that you “send a message” (make a request) to an object, and the object figures out what to do with that message (it executes code). Here, the name of the type/class is Light, the name of this particular Light object is lt, and the requests that you can make of a Light object are to turn it on, turn it off, make it brighter, or make it dimmer. You create a Light object by defining a “reference” (lt) for that object and calling new to request a new object of that type. To send a message to the object, you state the name of the object and connect it to the message request with a period (dot). From the standpoint of the user of a predefined class, that’s pretty much all there is to programming with objects. The preceding diagram follows the format of the Unified Modeling Language (UML). Each class is represented by a box, with the type name in the top portion of the box, any data members that you care to describe in the middle portion of the box, and the methods (the functions that belong to this object, which receive any messages you send to that object) in the bottom portion of the box. Often, only the name of the class and the public methods are shown in UML design diagrams, so the middle portion is not shown, as in this case. If you’re interested only in the class name, then the bottom portion doesn’t need to be shown, either. An object provides services While you’re trying to develop or understand a program design, one of the best ways to think about objects is as “service providers.” Your program itself will provide services to the user, and it will accomplish this by using the services offered by other objects. Your goal is to 18 Thinking in Java Bruce Eckel produce (or even better, locate in existing code libraries) a set of objects that provide the ideal services to solve your problem. A way to start doing this is to ask, “If I could magically pull them out of a hat, what objects would solve my problem right away?” For example, suppose you are creating a bookkeeping program. You might imagine some objects that contain pre-defined bookkeeping input screens, another set of objects that perform bookkeeping calculations, and an object that handles printing of checks and invoices on all different kinds of printers. Maybe some of these objects already exist, and for the ones that don’t, what would they look like? What services would those objects provide, and what objects would they need to fulfill their obligations? If you keep doing this, you will eventually reach a point where you can say either, “That object seems simple enough to sit down and write” or “I’m sure that object must exist already.” This is a reasonable way to decompose a problem into a set of objects. Thinking of an object as a service provider has an additional benefit: It helps to improve the cohesiveness of the object. High cohesion is a fundamental quality of software design: It means that the various aspects of a software component (such as an object, although this could also apply to a method or a library of objects) “fit together” well. One problem people have when designing objects is cramming too much functionality into one object. For example, in your check printing module, you may decide you need an object that knows all about formatting and printing. You’ll probably discover that this is too much for one object, and that what you need is three or more objects. One object might be a catalog of all the possible check layouts, which can be queried for information about how to print a check. One object or set of objects can be a generic printing interface that knows all about different kinds of printers (but nothing about bookkeeping—this one is a candidate for buying rather than writing yourself). And a third object could use the services of the other two to accomplish the task. Thus, each object has a cohesive set of services it offers. In a good object-oriented design, each object does one thing well, but doesn’t try to do too much. This not only allows the discovery of objects that might be purchased (the printer interface object), but it also produces new objects that might be reused somewhere else (the catalog of check layouts). Treating objects as service providers is a great simplifying tool. This is useful not only during the design process, but also when someone else is trying to understand your code or reuse an object. If they can see the value of the object based on what service it provides, it makes it much easier to fit it into the design. The hidden implementation It is helpful to break up the playing field into class creators (those who create new data types) and client programmers4 (the class consumers who use the data types in their applications). The goal of the client programmer is to collect a toolbox full of classes to use for rapid application development. The goal of the class creator is to build a class that exposes only what’s necessary to the client programmer and keeps everything else hidden. Why? Because if it’s hidden, the client programmer can’t access it, which means that the class creator can change the hidden portion at will without worrying about the impact on anyone else. The hidden portion usually represents the tender insides of an object that could easily be corrupted by a careless or uninformed client programmer, so hiding the implementation reduces program bugs. In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is also a programmer, but one who is putting together an application by using your library, possibly to build a bigger library. If all the members of a class are available to everyone, then the client programmer can do anything with that class and there’s no way to enforce rules. Even though you might really prefer that the client programmer not directly 4 I’m indebted to my friend Scott Meyers for this term. Introduction to Objects 19 manipulate some of the members of your class, without access control there’s no way to prevent it. Everything’s naked to the world. So the first reason for access control is to keep client programmers’ hands off portions they shouldn’t touch—parts that are necessary for the internal operation of the data type but not part of the interface that users need in order to solve their particular problems. This is actually a service to client programmers because they can easily see what’s important to them and what they can ignore. The second reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. For example, you might implement a particular class in a simple fashion to ease development, and then later discover that you need to rewrite it in order to make it run faster. If the interface and implementation are clearly separated and protected, you can accomplish this easily. Java uses three explicit keywords to set the boundaries in a class: public, private, and protected. These access specifiers determine who can use the definitions that follow. public means the following element is available to everyone. The private keyword, on the other hand, means that no one can access that element except you, the creator of the type, inside methods of that type. private is a brick wall between you and the client programmer. Someone who tries to access a private member will get a compile-time error. The protected keyword acts like private, with the exception that an inheriting class has access to protected members, but not private members. Inheritance will be introduced shortly. Java also has a “default” access, which comes into play if you don’t use one of the aforementioned specifiers. This is usually called package access because classes can access the members of other classes in the same package (library component), but outside of the package those same members appear to be private. Reusing the implementation Once a class has been created and tested, it should (ideally) represent a useful unit of code. It turns out that this reusability is not nearly so easy to achieve as many would hope; it takes experience and insight to produce a reusable object design. But once you have such a design, it begs to be reused. Code reuse is one of the greatest advantages that object-oriented programming languages provide. The simplest way to reuse a class is to just use an object of that class directly, but you can also place an object of that class inside a new class. We call this “creating a member object.” Your new class can be made up of any number and type of other objects, in any combination that you need to achieve the functionality desired in your new class. Because you are composing a new class from existing classes, this concept is called composition (if the composition happens dynamically, it’s usually called aggregation). Composition is often referred to as a “has-a” relationship, as in “A car has an engine.” 20 Thinking in Java Bruce Eckel (This UML diagram indicates composition with the filled diamond, which states there is one car. I will typically use a simpler form: just a line, without the diamond, to indicate an association.5) Composition comes with a great deal of flexibility. The member objects of your new class are typically private, making them inaccessible to the client programmers who are using the class. This allows you to change those members without disturbing existing client code. You can also change the member objects at run time, to dynamically change the behavior of your program. Inheritance, which is described next, does not have this flexibility since the compiler must place compile-time restrictions on classes created with inheritance. Because inheritance is so important in object-oriented programming, it is often highly emphasized, and the new programmer can get the idea that inheritance should be used everywhere. This can result in awkward and overly complicated designs. Instead, you should first look to composition when creating new classes, since it is simpler and more flexible. If you take this approach, your designs will be cleaner. Once you’ve had some experience, it will be reasonably obvious when you need inheritance. Inheritance By itself, the idea of an object is a convenient tool. It allows you to package data and functionality together by concept, so you can represent an appropriate problem-space idea rather than being forced to use the idioms of the underlying machine. These concepts are expressed as fundamental units in the programming language by using the class keyword. It seems a pity, however, to go to all the trouble to create a class and then be forced to create a brand new one that might have similar functionality. It’s nicer if we can take the existing class, clone it, and then make additions and modifications to the clone. This is effectively what you get with inheritance, with the exception that if the original class (called the base class or superclass or parent class) is changed, the modified “clone” (called the derived class or inherited class or subclass or child class) also reflects those changes. (The arrow in this UML diagram points from the derived class to the base class. As you will see, there is commonly more than one derived class.) A type does more than describe the constraints on a set of objects; it also has a relationship with other types. Two types can have characteristics and behaviors in common, but one type may contain more characteristics than another and may also handle more messages (or handle them differently). Inheritance expresses this similarity between types by using the concept of base types and derived types. A base type contains all of the characteristics and behaviors that are shared among the types derived from it. You create a base type to 5 This is usually enough detail for most diagrams, and you don’t need to get specific about whether you’re using aggregation or composition. Introduction to Objects 21 represent the core of your ideas about some objects in your system. From the base type, you derive other types to express the different ways that this core can be realized. For example, a trash-recycling machine sorts pieces of trash. The base type is “trash”, and each piece of trash has a weight, a value, and so on, and can be shredded, melted, or decomposed. From this, more specific types of trash are derived that may have additional characteristics (a bottle has a color) or behaviors (an aluminum can may be crushed, a steel can is magnetic). In addition, some behaviors may be different (the value of paper depends on its type and condition). Using inheritance, you can build a type hierarchy that expresses the problem you’re trying to solve in terms of its types. A second example is the classic “shape” example, perhaps used in a computer-aided design system or game simulation. The base type is “shape,” and each shape has a size, a color, a position, and so on. Each shape can be drawn, erased, moved, colored, etc. From this, specific types of shapes are derived (inherited)—circle, square, triangle, and so on—each of which may have additional characteristics and behaviors. Certain shapes can be flipped, for example. Some behaviors may be different, such as when you want to calculate the area of a shape. The type hierarchy embodies both the similarities and differences between the shapes. Casting the solution in the same terms as the problem is very useful because you don’t need a lot of intermediate models to get from a description of the problem to a description of the solution. With objects, the type hierarchy is the primary model, so you go directly from the description of the system in the real world to the description of the system in code. Indeed, one of the difficulties people have with object-oriented design is that it’s too simple to get from the beginning to the end. A mind trained to look for complex solutions can initially be stumped by this simplicity. When you inherit from an existing type, you create a new type. This new type contains not only all the members of the existing type (although the private ones are hidden away and inaccessible), but more importantly it duplicates the interface of the base class. That is, all the messages you can send to objects of the base class you can also send to objects of the derived class. Since we know the type of a class by the messages we can send to it, this means that the derived class is the same type as the base class. In the previous example, “A circle is a shape.” This type equivalence via inheritance is one of the fundamental gateways in understanding the meaning of object-oriented programming. Since both the base class and derived class have the same fundamental interface, there must be some implementation to go along with that interface. That is, there must be some code to execute when an object receives a particular message. If you simply inherit a class and don’t 22 Thinking in Java Bruce Eckel do anything else, the methods from the base-class interface come right along into the derived class. That means objects of the derived class have not only the same type, they also have the same behavior, which isn’t particularly interesting. You have two ways to differentiate your new derived class from the original base class. The first is quite straightforward: You simply add brand new methods to the derived class. These new methods are not part of the base-class interface. This means that the base class simply didn’t do as much as you wanted it to, so you added more methods. This simple and primitive use for inheritance is, at times, the perfect solution to your problem. However, you should look closely for the possibility that your base class might also need these additional methods. This process of discovery and iteration of your design happens regularly in object-oriented programming. Although inheritance may sometimes imply (especially in Java, where the keyword for inheritance is extends) that you are going to add new methods to the interface, that’s not necessarily true. The second and more important way to differentiate your new class is to change the behavior of an existing base-class method. This is referred to as overriding that method. Introduction to Objects 23 To override a method, you simply create a new definition for the method in the derived class. You’re saying, “I’m using the same interface method here, but I want it to do something different for my new type.” Is-a vs. is-like-a relationships There’s a certain debate that can occur about inheritance: Should inheritance override only baseclass methods (and not add new methods that aren’t in the base class)? This would mean that the derived class is exactly the same type as the base class since it has exactly the same interface. As a result, you can exactly substitute an object of the derived class for an object of the base class. This can be thought of as pure substitution, and it’s often referred to as the substitution principle. In a sense, this is the ideal way to treat inheritance. We often refer to the relationship between the base class and derived classes in this case as an is-a relationship, because you can say, “A circle is a shape.” A test for inheritance is to determine whether you can state the is-a relationship about the classes and have it make sense. There are times when you must add new interface elements to a derived type, thus extending the interface. The new type can still be substituted for the base type, but the substitution isn’t perfect because your new methods are not accessible from the base type. This can be described as an islike-a relationship (my term). The new type has the interface of the old type but it also contains other methods, so you can’t really say it’s exactly the same. For example, consider an air conditioner. Suppose your house is wired with all the controls for cooling; that is, it has an interface that allows you to control cooling. Imagine that the air conditioner breaks down and you replace it with a heat pump, which can both heat and cool. The heat pump is-like-an air conditioner, but it can do more. Because the control system of your house is designed only to control cooling, it is restricted to communication with the cooling part of the new object. The interface of the new object has been extended, and the existing system doesn’t know about anything except the original interface. 24 Thinking in Java Bruce Eckel Of course, once you see this design it becomes clear that the base class “cooling system” is not general enough, and should be renamed to “temperature control system” so that it can also include heating—at which point the substitution principle will work. However, this diagram is an example of what can happen with design in the real world. When you see the substitution principle it’s easy to feel like this approach (pure substitution) is the only way to do things, and in fact it is nice if your design works out that way. But you’ll find that there are times when it’s equally clear that you must add new methods to the interface of a derived class. With inspection both cases should be reasonably obvious. Interchangeable objects with polymorphism When dealing with type hierarchies, you often want to treat an object not as the specific type that it is, but instead as its base type. This allows you to write code that doesn’t depend on specific types. In the shape example, methods manipulate generic shapes, unconcerned about whether they’re circles, squares, triangles, or some shape that hasn’t even been defined yet. All shapes can be drawn, erased, and moved, so these methods simply send a message to a shape object; they don’t worry about how the object copes with the message. Such code is unaffected by the addition of new types, and adding new types is the most common way to extend an object-oriented program to handle new situations. For example, you can derive a new subtype of shape called pentagon without modifying the methods that deal only with generic shapes. This ability to easily extend a design by deriving new subtypes is one of the essential ways to encapsulate change. This greatly improves designs while reducing the cost of software maintenance. There’s a problem, however, with attempting to treat derived-type objects as their generic base types (circles as shapes, bicycles as vehicles, cormorants as birds, etc.). If a method is going to tell a generic shape to draw itself, or a generic vehicle to steer, or a generic bird to move, the compiler cannot know at compile time precisely what piece of code will be executed. That’s the whole point—when the message is sent, the programmer doesn’t want to know what piece of code will be executed; the draw method can be applied equally to a circle, a square, or a triangle, and the object will execute the proper code depending on its specific type. If you don’t have to know what piece of code will be executed, then when you add a new subtype, the code it executes can be different without requiring changes to the method that Introduction to Objects 25 calls it. Therefore, the compiler cannot know precisely what piece of code is executed, so what does it do? For example, in the following diagram the BirdController object just works with generic Bird objects and does not know what exact type they are. This is convenient from BirdController’s perspective because it doesn’t have to write special code to determine the exact type of Bird it’s working with or that Bird’s behavior. So how does it happen that, when move( ) is called while ignoring the specific type of Bird, the right behavior will occur (a Goose walks, flies, or swims, and a Penguin walks or swims)? The answer is the primary twist in object-oriented programming: The compiler cannot make a function call in the traditional sense. The function call generated by a non-OOP compiler causes what is called early binding, a term you may not have heard before because you’ve never thought about it any other way. It means the compiler generates a call to a specific function name, and the runtime system resolves this call to the absolute address of the code to be executed. In OOP, the program cannot determine the address of the code until run time, so some other scheme is necessary when a message is sent to a generic object. To solve the problem, object-oriented languages use the concept of late binding. When you send a message to an object, the code being called isn’t determined until run time. The compiler does ensure that the method exists and performs type checking on the arguments and return value, but it doesn’t know the exact code to execute. To perform late binding, Java uses a special bit of code in lieu of the absolute call. This code calculates the address of the method body, using information stored in the object (this process is covered in great detail in the Polymorphism chapter). Thus, each object can behave differently according to the contents of that special bit of code. When you send a message to an object, the object actually does figure out what to do with that message. In some languages you must explicitly state that you want a method to have the flexibility of latebinding properties (C++ uses the virtual keyword to do this). In these languages, by default, methods are not dynamically bound. In Java, dynamic binding is the default behavior and you don’t need to remember to add any extra keywords in order to get polymorphism. Consider the shape example. The family of classes (all based on the same uniform interface) was diagrammed earlier in this chapter. To demonstrate polymorphism, we want to write a single piece of code that ignores the specific details of type and talks only to the base class. That code is decoupled from type-specific information and thus is simpler to write and easier to understand. And, if a new type—a Hexagon, for example—is added through inheritance, the code you write will work just as well for the new type of Shape as it did on the existing types. Thus, the program is extensible. 26 Thinking in Java Bruce Eckel If you write a method in Java (as you will soon learn how to do): void doSomething(Shape shape) { shape.erase(); // ... shape.draw(); } This method speaks to any Shape, so it is independent of the specific type of object that it’s drawing and erasing. If some other part of the program uses the doSomething( ) method: Circle circle = new Circle(); Triangle triangle = new Triangle(); Line line= new Line(); doSomething(circle); doSomething(triangle); doSomething(line); The calls to doSomething( ) automatically work correctly, regardless of the exact type of the object. This is a rather amazing trick. Consider the line: doSomething(circle); What’s happening here is that a Circle is being passed into a method that’s expecting a Shape. Since a Circle is a Shape it can be treated as one by doSomething( ). That is, any message that doSomething( ) can send to a Shape, a Circle can accept. So it is a completely safe and logical thing to do. We call this process of treating a derived type as though it were its base type upcasting. The name cast is used in the sense of casting into a mold and the up comes from the way the inheritance diagram is typically arranged, with the base type at the top and the derived classes fanning out downward. Thus, casting to a base type is moving up the inheritance diagram: “upcasting.” An object-oriented program contains some upcasting somewhere, because that’s how you decouple yourself from knowing about the exact type you’re working with. Look at the code in doSomething( ): shape.erase(); // ... shape.draw(); Introduction to Objects 27 Notice that it doesn’t say, “If you’re a Circle, do this, if you’re a Square, do that, etc.” If you write that kind of code, which checks for all the possible types that a Shape can actually be, it’s messy and you need to change it every time you add a new kind of Shape. Here, you just say, “You’re a shape, I know you can erase( ) and draw( ) yourself, do it, and take care of the details correctly.” What’s impressive about the code in doSomething( ) is that, somehow, the right thing happens. Calling draw( ) for Circle causes different code to be executed than when calling draw( ) for a Square or a Line, but when the draw( ) message is sent to an anonymous Shape, the correct behavior occurs based on the actual type of the Shape. This is amazing because, as mentioned earlier, when the Java compiler is compiling the code for doSomething( ), it cannot know exactly what types it is dealing with. So ordinarily, you’d expect it to end up calling the version of erase( ) and draw( ) for the base class Shape, and not for the specific Circle, Square, or Line. And yet the right thing happens because of polymorphism. The compiler and runtime system handle the details; all you need to know right now is that it does happen, and more importantly, how to design with it. When you send a message to an object, the object will do the right thing, even when upcasting is involved. The singly rooted hierarchy One of the issues in OOP that has become especially prominent since the introduction of C++ is whether all classes should ultimately be inherited from a single base class. In Java (as with virtually all other OOP languages except for C++) the answer is yes, and the name of this ultimate base class is simply Object. It turns out that the benefits of the singly rooted hierarchy are many. All objects in a singly rooted hierarchy have an interface in common, so they are all ultimately the same fundamental type. The alternative (provided by C++) is that you don’t know that everything is the same basic type. From a backward-compatibility standpoint this fits the model of C better and can be thought of as less restrictive, but when you want to do full-on objectoriented programming you must then build your own hierarchy to provide the same convenience that’s built into other OOP languages. And in any new class library you acquire, some other incompatible interface will be used. It requires effort (and possibly multiple inheritance) to work the new interface into your design. Is the extra “flexibility” of C++ worth it? If you need it—if you have a large investment in C—it’s quite valuable. If you’re starting from scratch, other alternatives such as Java can often be more productive. All objects in a singly rooted hierarchy can be guaranteed to have certain functionality. You know you can perform certain basic operations on every object in your system. All objects can easily be created on the heap, and argument passing is greatly simplified. A singly rooted hierarchy makes it much easier to implement a garbage collector, which is one of the fundamental improvements of Java over C++. And since information about the type of an object is guaranteed to be in all objects, you’ll never end up with an object whose type you cannot determine. This is especially important with system-level operations, such as exception handling, and to allow greater flexibility in programming. Containers In general, you don’t know how many objects you’re going to need to solve a particular problem, or how long they will last. You also don’t know how to store those objects. How can you know how much space to create if that information isn’t known until run time? 28 Thinking in Java Bruce Eckel The solution to most problems in object-oriented design seems flippant: You create another type of object. The new type of object that solves this particular problem holds references to other objects. Of course, you can do the same thing with an array, which is available in most languages. But this new object, generally called a container (also called a collection, but the Java library uses that term in a different sense so this book will use “container”), will expand itself whenever necessary to accommodate everything you place inside it. So you don’t need to know how many objects you’re going to hold in a container. Just create a container object and let it take care of the details. Fortunately, a good OOP language comes with a set of containers as part of the package. In C++, it’s part of the Standard C++ Library and is often called the Standard Template Library (STL). Smalltalk has a very complete set of containers. Java also has numerous containers in its standard library. In some libraries, one or two generic containers is considered good enough for all needs, and in others (Java, for example) the library has different types of containers for different needs: several different kinds of List classes (to hold sequences), Maps (also known as associative arrays, to associate objects with other objects), Sets (to hold one of each type of object), and more components such as queues, trees, stacks, etc. From a design standpoint, all you really want is a container that can be manipulated to solve your problem. If a single type of container satisfied all of your needs, there’d be no reason to have different kinds. There are two reasons that you need a choice of containers. First, containers provide different types of interfaces and external behavior. A stack has a different interface and behavior than a queue, which is different from a set or a list. One of these might provide a more flexible solution to your problem than the other. Second, different containers have different efficiencies for certain operations. For example, there are two basic types of List: ArrayList and LinkedList. Both are simple sequences that can have identical interfaces and external behaviors. But certain operations can have significantly different costs. Randomly accessing elements in an ArrayList is a constant-time operation; it takes the same amount of time regardless of the element you select. However, in a LinkedList it is expensive to move through the list to randomly select an element, and it takes longer to find an element that is farther down the list. On the other hand, if you want to insert an element in the middle of a sequence, it’s cheaper in a LinkedList than in an ArrayList. These and other operations have different efficiencies depending on the underlying structure of the sequence. You might start building your program with a LinkedList and, when tuning for performance, change to an ArrayList. Because of the abstraction via the interface List, you can change from one to the other with minimal impact on your code. Parameterized types (generics) Before Java SE5, containers held the one universal type in Java: Object. The singly rooted hierarchy means that everything is an Object, so a container that holds Objects can hold anything.6 This made containers easy to reuse. To use such a container, you simply add object references to it and later ask for them back. But, since the container held only Objects, when you added an object reference into the container it was upcast to Object, thus losing its character. When fetching it back, you got an Object reference, and not a reference to the type that you put in. So how do you turn it back into something that has the specific type of the object that you put into the container? Here, the cast is used again, but this time you’re not casting up the inheritance hierarchy to a more general type. Instead, you cast down the hierarchy to a more specific type. This manner of casting is called downcasting. With upcasting, you know, for example, that a Circle is a type of Shape so it’s safe to upcast, but you don’t know that an Object is necessarily a 6 They do not hold primitives, but Java SE5 autoboxing makes this restriction almost a non-issue. This is discussed in detail later in the book. Introduction to Objects 29 Circle or a Shape so it’s hardly safe to downcast unless you know exactly what you’re dealing with. It’s not completely dangerous, however, because if you downcast to the wrong thing you’ll get a runtime error called an exception, which will be described shortly. When you fetch object references from a container, though, you must have some way to remember exactly what they are so you can perform a proper downcast. Downcasting and the runtime checks require extra time for the running program and extra effort from the programmer. Wouldn’t it make sense to somehow create the container so that it knows the types that it holds, eliminating the need for the downcast and a possible mistake? The solution is called a parameterized type mechanism. A parameterized type is a class that the compiler can automatically customize to work with particular types. For example, with a parameterized container, the compiler could customize that container so that it would accept only Shapes and fetch only Shapes. One of the big changes in Java SE5 is the addition of parameterized types, called generics in Java. You’ll recognize the use of generics by the angle brackets with types inside; for example, an ArrayList that holds Shape can be created like this: ArrayList shapes = new ArrayList(); There have also been changes to many of the standard library components in order to take advantage of generics. As you will see, generics have an impact on much of the code in this book. Object creation & lifetime One critical issue when working with objects is the way they are created and destroyed. Each object requires resources, most notably memory, in order to exist. When an object is no longer needed it must be cleaned up so that these resources are released for reuse. In simple programming situations the question of how an object is cleaned up doesn’t seem too challenging: You create the object, use it for as long as it’s needed, and then it should be destroyed. However, it’s not hard to encounter situations that are more complex. Suppose, for example, you are designing a system to manage air traffic for an airport. (The same model might also work for managing crates in a warehouse, or a video rental system, or a kennel for boarding pets.) At first it seems simple: Make a container to hold airplanes, then create a new airplane and place it in the container for each airplane that enters the air-trafficcontrol zone. For cleanup, simply clean up the appropriate airplane object when a plane leaves the zone. But perhaps you have some other system to record data about the planes; perhaps data that doesn’t require such immediate attention as the main controller function. Maybe it’s a record of the flight plans of all the small planes that leave the airport. So you have a second container of small planes, and whenever you create a plane object you also put it in this second container if it’s a small plane. Then some background process performs operations on the objects in this container during idle moments. Now the problem is more difficult: How can you possibly know when to destroy the objects? When you’re done with the object, some other part of the system might not be. This same problem can arise in a number of other situations, and in programming systems (such as C++) in which you must explicitly delete an object when you’re done with it this can become quite complex. 30 Thinking in Java Bruce Eckel Where is the data for an object and how is the lifetime of the object controlled? C++ takes the approach that control of efficiency is the most important issue, so it gives the programmer a choice. For maximum runtime speed, the storage and lifetime can be determined while the program is being written, by placing the objects on the stack (these are sometimes called automatic or scoped variables) or in the static storage area. This places a priority on the speed of storage allocation and release, and this control can be very valuable in some situations. However, you sacrifice flexibility because you must know the exact quantity, lifetime, and type of objects while you’re writing the program. If you are trying to solve a more general problem such as computer-aided design, warehouse management, or air-traffic control, this is too restrictive. The second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you don’t know until run time how many objects you need, what their lifetime is, or what their exact type is. Those are determined at the spur of the moment while the program is running. If you need a new object, you simply make it on the heap at the point that you need it. Because the storage is managed dynamically, at run time, the amount of time required to allocate storage on the heap can be noticeably longer than the time to create storage on the stack. Creating storage on the stack is often a single assembly instruction to move the stack pointer down and another to move it back up. The time to create heap storage depends on the design of the storage mechanism. The dynamic approach makes the generally logical assumption that objects tend to be complicated, so the extra overhead of finding storage and releasing that storage will not have an important impact on the creation of an object. In addition, the greater flexibility is essential to solve the general programming problem. Java uses dynamic memory allocation, exclusively.7 Every time you want to create an object, you use the new operator to build a dynamic instance of that object. There’s another issue, however, and that’s the lifetime of an object. With languages that allow objects to be created on the stack, the compiler determines how long the object lasts and can automatically destroy it. However, if you create it on the heap the compiler has no knowledge of its lifetime. In a language like C++, you must determine programmatically when to destroy the object, which can lead to memory leaks if you don’t do it correctly (and this is a common problem in C++ programs). Java provides a feature called a garbage collector that automatically discovers when an object is no longer in use and destroys it. A garbage collector is much more convenient because it reduces the number of issues that you must track and the code you must write. More importantly, the garbage collector provides a much higher level of insurance against the insidious problem of memory leaks, which has brought many a C++ project to its knees. With Java, the garbage collector is designed to take care of the problem of releasing the memory (although this doesn’t include other aspects of cleaning up an object). The garbage collector “knows” when an object is no longer in use, and it then automatically releases the memory for that object. This, combined with the fact that all objects are inherited from the single root class Object and that you can create objects only one way—on the heap—makes the process of programming in Java much simpler than programming in C++. You have far fewer decisions to make and hurdles to overcome. Exception handling: dealing with errors Ever since the beginning of programming languages, error handling has been a particularly difficult issue. Because it’s so hard to design a good error-handling scheme, many languages simply ignore the issue, passing the problem on to library designers who come up with 7 Primitive types, which you’ll learn about later, are a special case. Introduction to Objects 31 halfway measures that work in many situations but that can easily be circumvented, generally by just ignoring them. A major problem with most error-handling schemes is that they rely on programmer vigilance in following an agreed-upon convention that is not enforced by the language. If the programmer is not vigilant—often the case if they are in a hurry—these schemes can easily be forgotten. Exception handling wires error handling directly into the programming language and sometimes even the operating system. An exception is an object that is “thrown” from the site of the error and can be “caught” by an appropriate exception handler designed to handle that particular type of error. It’s as if exception handling is a different, parallel path of execution that can be taken when things go wrong. And because it uses a separate execution path, it doesn’t need to interfere with your normally executing code. This tends to make that code simpler to write because you aren’t constantly forced to check for errors. In addition, a thrown exception is unlike an error value that’s returned from a method or a flag that’s set by a method in order to indicate an error condition—these can be ignored. An exception cannot be ignored, so it’s guaranteed to be dealt with at some point. Finally, exceptions provide a way to reliably recover from a bad situation. Instead of just exiting the program, you are often able to set things right and restore execution, which produces much more robust programs. Java’s exception handling stands out among programming languages, because in Java, exception handling was wired in from the beginning and you’re forced to use it. It is the single acceptable way to report errors. If you don’t write your code to properly handle exceptions, you’ll get a compile-time error message. This guaranteed consistency can sometimes make error handling much easier. It’s worth noting that exception handling isn’t an object-oriented feature, although in objectoriented languages the exception is normally represented by an object. Exception handling existed before object-oriented languages. Concurrent programming A fundamental concept in computer programming is the idea of handling more than one task at a time. Many programming problems require that the program stop what it’s doing, deal with some other problem, and then return to the main process. The solution has been approached in many ways. Initially, programmers with low-level knowledge of the machine wrote interrupt service routines, and the suspension of the main process was initiated through a hardware interrupt. Although this worked well, it was difficult and non-portable, so it made moving a program to a new type of machine slow and expensive. Sometimes, interrupts are necessary for handling time-critical tasks, but there’s a large class of problems in which you’re simply trying to partition the problem into separately running pieces (tasks) so that the whole program can be more responsive. Within a program, these separately running pieces are called threads, and the general concept is called concurrency. A common example of concurrency is the user interface. By using tasks, a user can press a button and get a quick response rather than being forced to wait until the program finishes its current task. Ordinarily, tasks are just a way to allocate the time of a single processor. But if the operating system supports multiple processors, each task can be assigned to a different processor, and they can truly run in parallel. One of the convenient features of concurrency at the language level is that the programmer doesn’t need to worry about whether there are many processors or just one. The program is logically divided into tasks, and if the machine has more than one processor, then the program runs faster, without any special adjustments. All this makes concurrency sound pretty simple. There is a catch: shared resources. If you have more than one task running that’s expecting to access the same resource, you have a 32 Thinking in Java Bruce Eckel problem. For example, two processes can’t simultaneously send information to a printer. To solve the problem, resources that can be shared, such as the printer, must be locked while they are being used. So a task locks a resource, completes its task, and then releases the lock so that someone else can use the resource. Java’s concurrency is built into the language, and Java SE5 has added significant additional library support. Java and the Internet If Java is, in fact, yet another computer programming language, you may question why it is so important and why it is being promoted as a revolutionary step in computer programming. The answer isn’t immediately obvious if you’re coming from a traditional programming perspective. Although Java is very useful for solving traditional standalone programming problems, it is also important because it solves programming problems for the World Wide Web. What is the Web? The Web can seem a bit of a mystery at first, with all this talk of “surfing,” “presence,” and “home pages.” It’s helpful to step back and see what it really is, but to do this you must understand client/server systems, another aspect of computing that’s full of confusing issues. Client/server computing The primary idea of a client/server system is that you have a central repository of information— some kind of data, usually in a database—that you want to distribute on demand to some set of people or machines. A key to the client/server concept is that the repository of information is centrally located so that it can be changed and so that those changes will propagate out to the information consumers. Taken together, the information repository, the software that distributes the information, and the machine(s) where the information and software reside are called “the server.” The software that resides on the consumer machine, communicates with the server, fetches the information, processes it, and then displays it on the consumer machine is called the client. The basic concept of client/server computing, then, is not so complicated. The problems arise because you have a single server trying to serve many clients at once. Generally, a database management system is involved, so the designer “balances” the layout of data into tables for optimal use. In addition, systems often allow a client to insert new information into a server. This means you must ensure that one client’s new data doesn’t walk over another client’s new data, or that data isn’t lost in the process of adding it to the database (this is called transaction processing). As client software changes, it must be built, debugged, and installed on the client machines, which turns out to be more complicated and expensive than you might think. It’s especially problematic to support multiple types of computers and operating systems. Finally, there’s the all-important performance issue: You might have hundreds of clients making requests of your server at any moment, so a small delay can be critical. To minimize latency, programmers work hard to offload processing tasks, often to the client machine, but sometimes to other machines at the server site, using so-called middleware. (Middleware is also used to improve maintainability.) The simple idea of distributing information has so many layers of complexity that the whole problem can seem hopelessly enigmatic. And yet it’s crucial: Client/server computing accounts for roughly half of all programming activities. It’s responsible for everything from taking orders and credit-card transactions to the distribution of any kind of data—stock market, scientific, government, you name it. What we’ve come up with in the past is Introduction to Objects 33 individual solutions to individual problems, inventing a new solution each time. These were hard to create and hard to use, and the user had to learn a new interface for each one. The entire client/server problem needed to be solved in a big way. The Web as a giant server The Web is actually one giant client/server system. It’s a bit worse than that, since you have all the servers and clients coexisting on a single network at once. You don’t need to know that, because all you care about is connecting to and interacting with one server at a time (even though you might be hopping around the world in your search for the correct server). Initially it was a simple one-way process. You made a request of a server and it handed you a file, which your machine’s browser software (i.e., the client) would interpret by formatting onto your local machine. But in short order people began wanting to do more than just deliver pages from a server. They wanted full client/server capability so that the client could feed information back to the server, for example, to do database lookups on the server, to add new information to the server, or to place an order (which requires special security measures). These are the changes we’ve been seeing in the development of the Web. The Web browser was a big step forward: the concept that one piece of information can be displayed on any type of computer without change. However, the original browsers were still rather primitive and rapidly bogged down by the demands placed on them. They weren’t particularly interactive, and tended to clog up both the server and the Internet because whenever you needed to do something that required programming you had to send information back to the server to be processed. It could take many seconds or minutes to find out you had misspelled something in your request. Since the browser was just a viewer it couldn’t perform even the simplest computing tasks. (On the other hand, it was safe, because it couldn’t execute any programs on your local machine that might contain bugs or viruses.) To solve this problem, different approaches have been taken. To begin with, graphics standards have been enhanced to allow better animation and video within browsers. The remainder of the problem can be solved only by incorporating the ability to run programs on the client end, under the browser. This is called client-side programming. Client-side programming The Web’s initial server-browser design provided for interactive content, but the interactivity was completely provided by the server. The server produced static pages for the client browser, which would simply interpret and display them. Basic HyperText Markup Language (HTML) contains simple mechanisms for data gathering: text-entry boxes, check boxes, radio boxes, lists and dropdown lists, as well as a button that could only be programmed to reset the data on the form or “submit” the data on the form back to the server. This submission passes through the Common Gateway Interface (CGI) provided on all Web servers. The text within the submission tells CGI what to do with it. The most common action is to run a program located on the server in a directory that’s typically called “cgi-bin.” (If you watch the address window at the top of your browser when you push a button on a Web page, you can sometimes see “cgi-bin” within all the gobbledygook there.) These programs can be written in most languages. Perl has been a common choice because it is designed for text manipulation and is interpreted, so it can be installed on any server regardless of processor or operating system. However, Python (www.Python.org) has been making inroads because of its greater power and simplicity. Many powerful Web sites today are built strictly on CGI, and you can in fact do nearly anything with CGI. However, Web sites built on CGI programs can rapidly become overly complicated to maintain, and there is also the problem of response time. The response of a CGI program depends on how much data must be sent, as well as the load on both the server and the Internet. (On top of this, starting a CGI program tends to be slow.) The initial 34 Thinking in Java Bruce Eckel designers of the Web did not foresee how rapidly this bandwidth would be exhausted for the kinds of applications people developed. For example, any sort of dynamic graphing is nearly impossible to perform with consistency because a Graphics Interchange Format (GIF) file must be created and moved from the server to the client for each version of the graph. In addition, you’ve no doubt experienced the process of data validation for a Web input form. You press the submit button on a page; the data is shipped back to the server; the server starts a CGI program that discovers an error, formats an HTML page informing you of the error, and then sends the page back to you; you must then back up a page and try again. Not only is this slow, it’s inelegant. The solution is client-side programming. Most desktop computers that run Web browsers are powerful engines capable of doing vast work, and with the original static HTML approach they are sitting there, just idly waiting for the server to dish up the next page. Client-side programming means that the Web browser is harnessed to do whatever work it can, and the result for the user is a much speedier and more interactive experience at your Web site. The problem with discussions of client-side programming is that they aren’t very different from discussions of programming in general. The parameters are almost the same, but the platform is different; a Web browser is like a limited operating system. In the end, you must still program, and this accounts for the dizzying array of problems and solutions produced by client-side programming. The rest of this section provides an overview of the issues and approaches in client-side programming. Plug-ins One of the most significant steps forward in client-side programming is the development of the plug-in. This is a way for a programmer to add new functionality to the browser by downloading a piece of code that plugs itself into the appropriate spot in the browser. It tells the browser, “From now on you can perform this new activity.” (You need to download the plug-in only once.) Some fast and powerful behavior is added to browsers via plug-ins, but writing a plug-in is not a trivial task, and isn’t something you’d want to do as part of the process of building a particular site. The value of the plug-in for client-side programming is that it allows an expert programmer to develop extensions and add those extensions to a browser without the permission of the browser manufacturer. Thus, plug-ins provide a “back door” that allows the creation of new client-side programming languages (although not all languages are implemented as plug-ins). Scripting languages Plug-ins resulted in the development of browser scripting languages. With a scripting language, you embed the source code for your client-side program directly into the HTML page, and the plug-in that interprets that language is automatically activated while the HTML page is being displayed. Scripting languages tend to be reasonably easy to understand and, because they are simply text that is part of an HTML page, they load very quickly as part of the single server hit required to procure that page. The trade-off is that your code is exposed for everyone to see (and steal). Generally, however, you aren’t doing amazingly sophisticated things with scripting languages, so this is not too much of a hardship. One scripting language that you can expect a Web browser to support without a plug-in is JavaScript (this has only a passing resemblance to Java and you’ll have to climb an additional learning curve to use it. It was named that way just to grab some of Java’s marketing momentum). Unfortunately, most Web browsers originally implemented JavaScript in a different way from the other Web browsers, and even from other versions of themselves. The standardization of JavaScript in the form of ECMAScript has helped, but it has taken a long time for the various browsers to catch up (and it didn’t help that Microsoft was pushing its own agenda in the form of VBScript, which also had vague similarities to JavaScript). In general, you must program in a kind of least-common-denominator form of JavaScript in Introduction to Objects 35 order to be able to run on all browsers. Dealing with errors and debugging JavaScript can only be described as a mess. As proof of its difficulty, only recently has anyone created a truly complex piece of JavaScript (Google, in GMail), and that required excessive dedication and expertise. This points out that the scripting languages used inside Web browsers are really intended to solve specific types of problems, primarily the creation of richer and more interactive graphical user interfaces (GUIs). However, a scripting language might solve 80 percent of the problems encountered in client-side programming. Your problems might very well fit completely within that 80 percent, and since scripting languages can allow easier and faster development, you should probably consider a scripting language before looking at a more involved solution such as Java programming. Java If a scripting language can solve 80 percent of the client-side programming problems, what about the other 20 percent—the “really hard stuff”? Java is a popular solution for this. Not only is it a powerful programming language built to be secure, cross-platform, and international, but Java is being continually extended to provide language features and libraries that elegantly handle problems that are difficult in traditional programming languages, such as concurrency, database access, network programming, and distributed computing. Java allows client-side programming via the applet and with Java Web Start. An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page (just as, for example, a graphic is automatically downloaded). When the applet is activated, it executes a program. This is part of its beauty—it provides you with a way to automatically distribute the client software from the server at the time the user needs the client software, and no sooner. The user gets the latest version of the client software without fail and without difficult reinstallation. Because of the way Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters. (This safely includes the vast majority of machines.) Since Java is a full-fledged programming language, you can do as much work as possible on the client before and after making requests of the server. For example, you won’t need to send a request form across the Internet to discover that you’ve gotten a date or some other parameter wrong, and your client computer can quickly do the work of plotting data instead of waiting for the server to make a plot and ship a graphic image back to you. Not only do you get the immediate win of speed and responsiveness, but the general network traffic and load on servers can be reduced, preventing the entire Internet from slowing down. Alternatives To be honest, Java applets have not particularly lived up to their initial fanfare. When Java first appeared, what everyone seemed most excited about was applets, because these would finally allow serious client-side programmability, to increase responsiveness and decrease bandwidth requirements for Internet-based applications. People envisioned vast possibilities. Indeed, you can find some very clever applets on the Web. But the overwhelming move to applets never happened. The biggest problem was probably that the 10 MB download necessary to install the Java Runtime Environment (JRE) was too scary for the average user. The fact that Microsoft chose not to include the JRE with Internet Explorer may have sealed its fate. In any event, Java applets didn’t happen on a large scale. Nonetheless, applets and Java Web Start applications are still valuable in some situations. Anytime you have control over user machines, for example within a corporation, it is 36 Thinking in Java Bruce Eckel reasonable to distribute and update client applications using these technologies, and this can save considerable time, effort, and money, especially if you need to do frequent updates. In the Graphical User Interfaces chapter, we will look at one promising new technology, Macromedia’s Flex, which allows you to create Flash-based applet-equivalents. Because the Flash Player is available on upwards of 98 percent of all Web browsers (including Windows, Linux and the Mac) it can be considered an accepted standard. Installing or upgrading the Flash Player is quick and easy. The ActionScript language is based on ECMAScript so it is reasonably familiar, but Flex allows you to program without worrying about browser specifics—thus it is far more attractive than JavaScript. For client-side programming, this is an alternative worth considering. .NET and C# For a while, the main competitor to Java applets was Microsoft’s ActiveX, although that required that the client be running Windows. Since then, Microsoft has produced a full competitor to Java in the form of the .NET platform and the C# programming language. The .NET platform is roughly the same as the Java Virtual Machine (JVM; the software platform on which Java programs execute) and Java libraries, and C# bears unmistakable similarities to Java. This is certainly the best work that Microsoft has done in the arena of programming languages and programming environments. Of course, they had the considerable advantage of being able to see what worked well and what didn’t work so well in Java, and build upon that, but build they have. This is the first time since its inception that Java has had any real competition. As a result, the Java designers at Sun have taken a hard look at C# and why programmers might want to move to it, and have responded by making fundamental improvements to Java in Java SE5. Currently, the main vulnerability and important question concerning .NET is whether Microsoft will allow it to be completely ported to other platforms. They claim there’s no problem doing this, and the Mono project (www.go-mono.com) has a partial implementation of .NET working on Linux, but until the implementation is complete and Microsoft has not decided to squash any part of it, .NET as a cross-platform solution is still a risky bet. Internet vs. intranet The Web is the most general solution to the client/server problem, so it makes sense to use the same technology to solve a subset of the problem, in particular the classic client/server problem within a company. With traditional client/server approaches you have the problem of multiple types of client computers, as well as the difficulty of installing new client software, both of which are handily solved with Web browsers and client-side programming. When Web technology is used for an information network that is restricted to a particular company, it is referred to as an intranet. Intranets provide much greater security than the Internet, since you can physically control access to the servers within your company. In terms of training, it seems that once people understand the general concept of a browser it’s much easier for them to deal with differences in the way pages and applets look, so the learning curve for new kinds of systems seems to be reduced. The security problem brings us to one of the divisions that seems to be automatically forming in the world of client-side programming. If your program is running on the Internet, you don’t know what platform it will be working under, and you want to be extra careful that you don’t disseminate buggy code. You need something cross-platform and secure, like a scripting language or Java. If you’re running on an intranet, you might have a different set of constraints. It’s not uncommon that your machines could all be Intel/Windows platforms. On an intranet, you’re responsible for the quality of your own code and can repair bugs when they’re discovered. In Introduction to Objects 37 addition, you might already have a body of legacy code that you’ve been using in a more traditional client/server approach, whereby you must physically install client programs every time you do an upgrade. The time wasted in installing upgrades is the most compelling reason to move to browsers, because upgrades are invisible and automatic (Java Web Start is also a solution to this problem). If you are involved in such an intranet, the most sensible approach to take is the shortest path that allows you to use your existing code base, rather than trying to recode your programs in a new language. When faced with this bewildering array of solutions to the client-side programming problem, the best plan of attack is a cost-benefit analysis. Consider the constraints of your problem and what would be the shortest path to your solution. Since client-side programming is still programming, it’s always a good idea to take the fastest development approach for your particular situation. This is an aggressive stance to prepare for inevitable encounters with the problems of program development. Server-side programming This whole discussion has ignored the issue of server-side programming, which is arguably where Java has had its greatest success. What happens when you make a request of a server? Most of the time the request is simply “Send me this file.” Your browser then interprets the file in some appropriate fashion: as an HTML page, a graphic image, a Java applet, a script program, etc. A more complicated request to a server generally involves a database transaction. A common scenario involves a request for a complex database search, which the server then formats into an HTML page and sends to you as the result. (Of course, if the client has more intelligence via Java or a scripting language, the raw data can be sent and formatted at the client end, which will be faster and less load on the server.) Or you might want to register your name in a database when you join a group or place an order, which will involve changes to that database. These database requests must be processed via some code on the server side, which is generally referred to as server-side programming. Traditionally, server-side programming has been performed using Perl, Python, C++, or some other language to create CGI programs, but more sophisticated systems have since appeared. These include Java-based Web servers that allow you to perform all your server-side programming in Java by writing what are called servlets. Servlets and their offspring, JSPs, are two of the most compelling reasons that companies that develop Web sites are moving to Java, especially because they eliminate the problems of dealing with differently abled browsers. Server-side programming topics are covered in Thinking in Enterprise Java at www.MindView.net. Despite all this talk about Java on the Internet, it is a general-purpose programming language that can solve the kinds of problems that you can solve with other languages. Here, Java’s strength is not only in its portability, but also its programmability, its robustness, its large, standard library and the numerous third-party libraries that are available and that continue to be developed. Summary You know what a procedural program looks like: data definitions and function calls. To find the meaning of such a program, you must work at it, looking through the function calls and low-level concepts to create a model in your mind. This is the reason we need intermediate representations when designing procedural programs—by themselves, these programs tend to be confusing because the terms of expression are oriented more toward the computer than to the problem you’re solving. Because OOP adds many new concepts on top of what you find in a procedural language, your natural assumption may be that the resulting Java program will be far more 38 Thinking in Java Bruce Eckel complicated than the equivalent procedural program. Here, you’ll be pleasantly surprised: A well-written Java program is generally far simpler and much easier to understand than a procedural program. What you’ll see are the definitions of the objects that represent concepts in your problem space (rather than the issues of the computer representation) and messages sent to those objects to represent the activities in that space. One of the delights of objectoriented programming is that, with a well-designed program, it’s easy to understand the code by reading it. Usually, there’s a lot less code as well, because many of your problems will be solved by reusing existing library code. OOP and Java may not be for everyone. It’s important to evaluate your own needs and decide whether Java will optimally satisfy those needs, or if you might be better off with another programming system (including the one you’re currently using). If you know that your needs will be very specialized for the foreseeable future and if you have specific constraints that may not be satisfied by Java, then you owe it to yourself to investigate the alternatives (in particular, I recommend looking at Python; see www.Python.org). If you still choose Java as your language, you’ll at least understand what the options were and have a clear vision of why you took that direction. Introduction to Objects 39 Everything Is an Object “If we spoke a different language, we would perceive a somewhat different world.” Ludwig Wittgenstein (1889-1951) Although it is based on C++, Java is more of a “pure” object-oriented language. Both C++ and Java are hybrid languages, but in Java the designers felt that the hybridization was not as important as it was in C++. A hybrid language allows multiple programming styles; the reason C++ is hybrid is to support backward compatibility with the C language. Because C++ is a superset of the C language, it includes many of that language’s undesirable features, which can make some aspects of C++ overly complicated. The Java language assumes that you want to do only object-oriented programming. This means that before you can begin you must shift your mindset into an object-oriented world (unless it’s already there). The benefit of this initial effort is the ability to program in a language that is simpler to learn and to use than many other OOP languages. In this chapter you’ll see the basic components of a Java program and learn that (almost) everything in Java is an object. You manipulate objects with references Each programming language has its own means of manipulating elements in memory. Sometimes the programmer must be constantly aware of what type of manipulation is going on. Are you manipulating the element directly, or are you dealing with some kind of indirect representation (a pointer in C or C++) that must be treated with a special syntax? All this is simplified in Java. You treat everything as an object, using a single consistent syntax. Although you treat everything as an object, the identifier you manipulate is actually a “reference” to an object.1 You might imagine a television (the object) and a remote control (the reference). As long as you’re holding this reference, you have a connection to the television, but when someone says, “Change the channel” or “Lower the volume,” what you’re manipulating is the reference, which in turn modifies the object. If you want to move around 1 This can be a flashpoint. There are those who say, “Clearly, it’s a pointer,” but this presumes an underlying implementation. Also, Java references are much more akin to C++ references than to pointers in their syntax. In the 1st edition of this book, I chose to invent a new term, “handle,” because C++ references and Java references have some important differences. I was coming out of C++ and did not want to confuse the C++ programmers whom I assumed would be the largest audience for Java. In the 2nd edition, I decided that “reference” was the more commonly used term, and that anyone changing from C++ would have a lot more to cope with than the terminology of references, so they might as well jump in with both feet. However, there are people who disagree even with the term “reference.” I read in one book where it was “completely wrong to say that Java supports pass by reference,” because Java object identifiers (according to that author) are actually “object references.” And (he goes on) everything is actually pass by value. So you’re not passing by reference, you’re “passing an object reference by value.” One could argue for the precision of such convoluted explanations, but I think my approach simplifies the understanding of the concept without hurting anything (well, the language lawyers may claim that I’m lying to you, but I’ll say that I’m providing an appropriate abstraction). the room and still control the television, you take the remote/reference with you, not the television. Also, the remote control can stand on its own, with no television. That is, just because you have a reference doesn’t mean there’s necessarily an object connected to it. So if you want to hold a word or sentence, you create a String reference: String s; But here you’ve created only the reference, not an object. If you decided to send a message to s at this point, you’ll get an error because s isn’t actually attached to anything (there’s no television). A safer practice, then, is always to initialize a reference when you create it: String s = "asdf"; However, this uses a special Java feature: Strings can be initialized with quoted text. Normally, you must use a more general type of initialization for objects. You must create all the objects When you create a reference, you want to connect it with a new object. You do so, in general, with the new operator. The keyword new says, “Make me a new one of these objects.” So in the preceding example, you can say: String s = new String("asdf"); Not only does this mean “Make me a new String,” but it also gives information about how to make the String by supplying an initial character string. Of course, Java comes with a plethora of ready-made types in addition to String. What’s more important is that you can create your own types. In fact, creating new types is the fundamental activity in Java programming, and it’s what you’ll be learning about in the rest of this book. Where storage lives It’s useful to visualize some aspects of how things are laid out while the program is running— in particular how memory is arranged. There are five different places to store data: 1. Registers. This is the fastest storage because it exists in a place different from that of other storage: inside the processor. However, the number of registers is severely limited, so registers are allocated as they are needed. You don’t have direct control, nor do you see any evidence in your programs that registers even exist (C & C++, on the other hand, allow you to suggest register allocation to the compiler). 2. The stack. This lives in the general random-access memory (RAM) area, but has direct support from the processor via its stack pointer. The stack pointer is moved down to create new memory and moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java system must know, while it is creating the program, the exact lifetime of all the items that are stored on the stack. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stack—in particular, object references—Java objects themselves are not placed on the stack. 42 Thinking in Java Bruce Eckel 3. The heap. This is a general-purpose pool of memory (also in the RAM area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the compiler doesn’t need to know how long that storage must stay on the heap. Thus, there’s a great deal of flexibility in using storage on the heap. Whenever you need an object, you simply write the code to create it by using new, and the storage is allocated on the heap when that code is executed. Of course there’s a price you pay for this flexibility: It may take more time to allocate and clean up heap storage than stack storage (if you even could create objects on the stack in Java, as you can in C++). 4. Constant storage. Constant values are often placed directly in the program code, which is safe since they can never change. Sometimes constants are cordoned off by themselves so that they can be optionally placed in read-only memory (ROM), in embedded systems.2 5. Non-RAM storage. If data lives completely outside a program, it can exist while the program is not running, outside the control of the program. The two primary examples of this are streamed objects, in which objects are turned into streams of bytes, generally to be sent to another machine, and persistent objects, in which the objects are placed on disk so they will hold their state even when the program is terminated. The trick with these types of storage is turning the objects into something that can exist on the other medium, and yet can be resurrected into a regular RAMbased object when necessary. Java provides support for lightweight persistence, and mechanisms such as JDBC and Hibernate provide more sophisticated support for storing and retrieving object information in databases. Special case: primitive types One group of types, which you’ll use quite often in your programming, gets special treatment. You can think of these as “primitive” types. The reason for the special treatment is that to create an object with new—especially a small, simple variable—isn’t very efficient, because new places objects on the heap. For these types Java falls back on the approach taken by C and C++. That is, instead of creating the variable by using new, an “automatic” variable is created that is not a reference. The variable holds the value directly, and it’s placed on the stack, so it’s much more efficient. Java determines the size of each primitive type. These sizes don’t change from one machine architecture to another as they do in most languages. This size invariance is one reason Java programs are more portable than programs in most other languages. Primitive type boolean char byte short int long float double void Size Minimum Maximum — 16 bits 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits — — Unicode 0 -128 -215 -231 -263 IEEE754 IEEE754 — — Unicode 216- 1 +127 +215-1 +231-1 +263-1 IEEE754 IEEE754 — Wrapper type Boolean Character Byte Short Integer Long Float Double Void 2 An example of this is the string pool. All literal strings and string-valued constant expressions are interned automatically and put into special static storage. Everything Is an Object 43 All numeric types are signed, so don’t look for unsigned types. The size of the boolean type is not explicitly specified; it is only defined to be able to take the literal values true or false. The “wrapper” classes for the primitive data types allow you to make a non-primitive object on the heap to represent that primitive type. For example: char c = ‘x’; Character ch = new Character(c); Or you could also use: Character ch = new Character(‘x’); Java SE5 autoboxing will automatically convert from a primitive to a wrapper type: Character ch = ‘x’; and back: char c = ch; The reasons for wrapping primitives will be shown in a later chapter. High-precision numbers Java includes two classes for performing high-precision arithmetic: BigInteger and BigDecimal. Although these approximately fit into the same category as the “wrapper” classes, neither one has a primitive analogue. Both classes have methods that provide analogues for the operations that you perform on primitive types. That is, you can do anything with a BigInteger or BigDecimal that you can with an int or float, it’s just that you must use method calls instead of operators. Also, since there’s more involved, the operations will be slower. You’re exchanging speed for accuracy. BigInteger supports arbitrary-precision integers. This means that you can accurately represent integral values of any size without losing any information during operations. BigDecimal is for arbitrary-precision fixed-point numbers; you can use these for accurate monetary calculations, for example. Consult the JDK documentation for details about the constructors and methods you can call for these two classes. Arrays in Java Virtually all programming languages support some kind of arrays. Using arrays in C and C++ is perilous because those arrays are only blocks of memory. If a program accesses the array outside of its memory block or uses the memory before initialization (common programming errors), there will be unpredictable results. One of the primary goals of Java is safety, so many of the problems that plague programmers in C and C++ are not repeated in Java. A Java array is guaranteed to be initialized and cannot 44 Thinking in Java Bruce Eckel be accessed outside of its range. The range checking comes at the price of having a small amount of memory overhead on each array as well as verifying the index at run time, but the assumption is that the safety and increased productivity are worth the expense (and Java can sometimes optimize these operations). When you create an array of objects, you are really creating an array of references, and each of those references is automatically initialized to a special value with its own keyword: null. When Java sees null, it recognizes that the reference in question isn’t pointing to an object. You must assign an object to each reference before you use it, and if you try to use a reference that’s still null, the problem will be reported at run time. Thus, typical array errors are prevented in Java. You can also create an array of primitives. Again, the compiler guarantees initialization because it zeroes the memory for that array. Arrays will be covered in detail in later chapters. You never need to destroy an object In most programming languages, the concept of the lifetime of a variable occupies a significant portion of the programming effort. How long does the variable last? If you are supposed to destroy it, when should you? Confusion over variable lifetimes can lead to a lot of bugs, and this section shows how Java greatly simplifies the issue by doing all the cleanup work for you. Scoping Most procedural languages have the concept of scope. This determines both the visibility and lifetime of the names defined within that scope. In C, C++, and Java, scope is determined by the placement of curly braces {}. So for example: { int x = 12; // Only x available { int q = 96; // Both x & q available } // Only x available // q is "out of scope" } A variable defined within a scope is available only to the end of that scope. Any text after a ‘//’ to the end of a line is a comment. Indentation makes Java code easier to read. Since Java is a free-form language, the extra spaces, tabs, and carriage returns do not affect the resulting program. You cannot do the following, even though it is legal in C and C++: { int x = 12; { Everything Is an Object 45 int x = 96; // Illegal } } The compiler will announce that the variable x has already been defined. Thus the C and C++ ability to “hide” a variable in a larger scope is not allowed, because the Java designers thought that it led to confusing programs. Scope of objects Java objects do not have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Thus if you use: { String s = new String("a string"); } // End of scope the reference s vanishes at the end of the scope. However, the String object that s was pointing to is still occupying memory. In this bit of code, there is no way to access the object after the end of the scope, because the only reference to it is out of scope. In later chapters you’ll see how the reference to the object can be passed around and duplicated during the course of a program. It turns out that because objects created with new stay around for as long as you want them, a whole slew of C++ programming problems simply vanish in Java. In C++ you must not only make sure that the objects stay around for as long as you need them, you must also destroy the objects when you’re done with them. That brings up an interesting question. If Java leaves the objects lying around, what keeps them from filling up memory and halting your program? This is exactly the kind of problem that would occur in C++. This is where a bit of magic happens. Java has a garbage collector, which looks at all the objects that were created with new and figures out which ones are not being referenced anymore. Then it releases the memory for those objects, so the memory can be used for new objects. This means that you never need to worry about reclaiming memory yourself. You simply create objects, and when you no longer need them, they will go away by themselves. This eliminates a certain class of programming problem: the so-called “memory leak,” in which a programmer forgets to release memory. Creating new data types: class If everything is an object, what determines how a particular class of object looks and behaves? Put another way, what establishes the type of an object? You might expect there to be a keyword called “type,” and that certainly would have made sense. Historically, however, most objectoriented languages have used the keyword class to mean “I’m about to tell you what a new type of object looks like.” The class keyword (which is so common that it will not usually be boldfaced throughout this book) is followed by the name of the new type. For example: class ATypeName { /* Class body goes here */ } This introduces a new type, although the class body consists only of a comment (the stars and slashes and what is inside, which will be discussed later in this chapter), so there is not too much that you can do with it. However, you can create an object of this type using new: ATypeName a = new ATypeName(); 46 Thinking in Java Bruce Eckel But you cannot tell it to do much of anything (that is, you cannot send it any interesting messages) until you define some methods for it. Fields and methods When you define a class (and all you do in Java is define classes, make objects of those classes, and send messages to those objects), you can put two types of elements in your class: fields (sometimes called data members), and methods (sometimes called member functions). A field is an object of any type that you can talk to via its reference, or a primitive type. If it is a reference to an object, you must initialize that reference to connect it to an actual object (using new, as seen earlier). Each object keeps its own storage for its fields; ordinary fields are not shared among objects. Here is an example of a class with some fields: class DataOnly { int i; double d; boolean b; } This class doesn’t do anything except hold data. But you can create an object like this: DataOnly data = new DataOnly(); You can assign values to the fields, but you must first know how to refer to a member of an object. This is accomplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object: objectReference.member For example: data.i = 47; data.d = 1.1; data.b = false; It is also possible that your object might contain other objects that contain data you’d like to modify. For this, you just keep “connecting the dots.” For example: myPlane.leftTank.capacity = 100; The DataOnly class cannot do much of anything except hold data, because it has no methods. To understand how those work, you must first understand arguments and return values, which will be described shortly. Default values for primitive members When a primitive data type is a member of a class, it is guaranteed to get a default value if you do not initialize it: Primitive type boolean char Default false ‘\u0000’ (null) Everything Is an Object 47 Primitive type byte short int long float double Default (byte)0 (short)0 0 0L 0.0f 0.0d The default values are only what Java guarantees when the variable is used as a member of a class. This ensures that member variables of primitive types will always be initialized (something C++ doesn’t do), reducing a source of bugs. However, this initial value may not be correct or even legal for the program you are writing. It’s best to always explicitly initialize your variables. This guarantee doesn’t apply to local variables—those that are not fields of a class. Thus, if within a method definition you have: int x; Then x will get some arbitrary value (as in C and C++); it will not automatically be initialized to zero. You are responsible for assigning an appropriate value before you use x. If you forget, Java definitely improves on C++: You get a compile-time error telling you the variable might not have been initialized. (Many C++ compilers will warn you about uninitialized variables, but in Java these are errors.) Methods, arguments, and return values In many languages (like C and C++), the term function is used to describe a named subroutine. The term that is more commonly used in Java is method, as in “a way to do something.” If you want, you can continue thinking in terms of functions. It’s really only a syntactic difference, but this book follows the common Java usage of the term “method.” Methods in Java determine the messages an object can receive. The fundamental parts of a method are the name, the arguments, the return type, and the body. Here is the basic form: ReturnType methodName( /* Argument list */ ) { /* Method body */ } The return type describes the value that comes back from the method after you call it. The argument list gives the types and names for the information that you want to pass into the method. The method name and argument list (which is called the signature of the method) uniquely identify that method. Methods in Java can be created only as part of a class. A method can be called only for an object,3 and that object must be able to perform that method call. If you try to call the wrong method for an object, you’ll get an error message at compile time. You call a method for an object by naming the object followed by a period (dot), followed by the name of the method and its argument list, like this: 3 static methods, which you’ll learn about soon, can be called for the class, without an object. 48 Thinking in Java Bruce Eckel objectName.methodName(arg1, arg2, arg3); For example, suppose you have a method f( ) that takes no arguments and returns a value of type int. Then, if you have an object called a for which f( ) can be called, you can say this: int x = a.f(); The type of the return value must be compatible with the type of x. This act of calling a method is commonly referred to as sending a message to an object. In the preceding example, the message is f( ) and the object is a. Object-oriented programming is often summarized as simply “sending messages to objects.” The argument list The method argument list specifies what information you pass into the method. As you might guess, this information—like everything else in Java—takes the form of objects. So, what you must specify in the argument list are the types of the objects to pass in and the name to use for each one. As in any situation in Java where you seem to be handing objects around, you are actually passing references.4 The type of the reference must be correct, however. If the argument is supposed to be a String, you must pass in a String or the compiler will give an error. Consider a method that takes a String as its argument. Here is the definition, which must be placed within a class definition for it to be compiled: int storage(String s) { return s.length() * 2; } This method tells you how many bytes are required to hold the information in a particular String. (The size of each char in a String is 16 bits, or two bytes, to support Unicode characters.) The argument is of type String and is called s. Once s is passed into the method, you can treat it just like any other object. (You can send messages to it.) Here, the length( ) method is called, which is one of the methods for Strings; it returns the number of characters in a string. You can also see the use of the return keyword, which does two things. First, it means “Leave the method, I’m done.” Second, if the method produces a value, that value is placed right after the return statement. In this case, the return value is produced by evaluating the expression s.length( ) * 2. You can return any type you want, but if you don’t want to return anything at all, you do so by indicating that the method returns void. Here are some examples: boolean flag() { return true; } double naturalLogBase() { return 2.718; } void nothing() { return; } void nothing2() {} When the return type is void, then the return keyword is used only to exit the method, and is therefore unnecessary when you reach the end of the method. You can return from a method at any point, but if you’ve given a non-void return type, then the compiler will force you (with error messages) to return the appropriate type of value regardless of where you return. At this point, it can look like a program is just a bunch of objects with methods that take other objects as arguments and send messages to those other objects. That is indeed much of Everything Is an Object 49 what goes on, but in the following chapter you’ll learn how to do the detailed low-level work by making decisions within a method. For this chapter, sending messages will suffice. Building a Java program There are several other issues you must understand before seeing your first Java program. Name visibility A problem in any programming language is the control of names. If you use a name in one module of the program, and another programmer uses the same name in another module, how do you distinguish one name from another and prevent the two names from “clashing”? In C this is a particular problem because a program is often an unmanageable sea of names. C++ classes (on which Java classes are based) nest functions within classes so they cannot clash with function names nested within other classes. However, C++ still allows global data and global functions, so clashing is still possible. To solve this problem, C++ introduced namespaces using additional keywords. Java was able to avoid all of this by taking a fresh approach. To produce an unambiguous name for a library, the Java creators want you to use your Internet domain name in reverse since domain names are guaranteed to be unique. Since my domain name is MindView.net, my utility library of foibles would be named net.mindview.utility.foibles. After your reversed domain name, the dots are intended to represent subdirectories. In Java 1.0 and Java 1.1 the domain extensions com, edu, org, net, etc., were capitalized by convention, so the library would appear: NET.mindview.utility.foibles. Partway through the development of Java 2, however, it was discovered that this caused problems, so now the entire package name is lowercase. This mechanism means that all of your files automatically live in their own namespaces, and each class within a file must have a unique identifier—the language prevents name clashes for you. Using other components Whenever you want to use a predefined class in your program, the compiler must know how to locate it. Of course, the class might already exist in the same source-code file that it’s being called from. In that case, you simply use the class—even if the class doesn’t get defined until later in the file (Java eliminates the so-called “forward referencing” problem). What about a class that exists in some other file? You might think that the compiler should be smart enough to simply go and find it, but there is a problem. Imagine that you want to use a class with a particular name, but more than one definition for that class exists (presumably these are different definitions). Or worse, imagine that you’re writing a program, and as you’re building it you add a new class to your library that conflicts with the name of an existing class. To solve this problem, you must eliminate all potential ambiguities. This is accomplished by telling the Java compiler exactly what classes you want by using the import keyword. import tells the compiler to bring in a package, which is a library of classes. (In other languages, a library could consist of functions and data as well as classes, but remember that all code in Java must be written inside a class.) 50 Thinking in Java Bruce Eckel Most of the time you’ll be using components from the standard Java libraries that come with your compiler. With these, you don’t need to worry about long, reversed domain names; you just say, for example: import java.util.ArrayList; to tell the compiler that you want to use Java’s ArrayList class. However, util contains a number of classes, and you might want to use several of them without declaring them all explicitly. This is easily accomplished by using ‘*’ to indicate a wild card: import java.util.*; It is more common to import a collection of classes in this manner than to import classes individually. The static keyword Ordinarily, when you create a class you are describing how objects of that class look and how they will behave. You don’t actually get an object until you create one using new, and at that point storage is allocated and methods become available. There are two situations in which this approach is not sufficient. One is if you want to have only a single piece of storage for a particular field, regardless of how many objects of that class are created, or even if no objects are created. The other is if you need a method that isn’t associated with any particular object of this class. That is, you need a method that you can call even if no objects are created. You can achieve both of these effects with the static keyword. When you say something is static, it means that particular field or method is not tied to any particular object instance of that class. So even if you’ve never created an object of that class you can call a static method or access a static field. With ordinary, non-static fields and methods, you must create an object and use that object to access the field or method, since non-static fields and methods must know the particular object they are working with.4 Some object-oriented languages use the terms class data and class methods, meaning that the data and methods exist only for the class as a whole, and not for any particular objects of the class. Sometimes the Java literature uses these terms too. To make a field or method static, you simply place the keyword before the definition. For example, the following produces a static field and initializes it: class StaticTest { static int i = 47; } Now even if you make two StaticTest objects, there will still be only one piece of storage for StaticTest.i. Both objects will share the same i. Consider: StaticTest st1 = new StaticTest(); StaticTest st2 = new StaticTest(); 4 Of course, since static methods don’t need any objects to be created before they are used, they cannot directly access non-static members or methods by simply calling those other members without referring to a named object (since nonstatic members and methods must be tied to a particular object). Everything Is an Object 51 At this point, both st1.i and st2.i have the same value of 47 since they refer to the same piece of memory. There are two ways to refer to a static variable. As the preceding example indicates, you can name it via an object, by saying, for example, st2.i. You can also refer to it directly through its class name, something you cannot do with a non-static member. StaticTest.i++; The ++ operator adds one to the variable. At this point, both st1.i and st2.i will have the value 48. Using the class name is the preferred way to refer to a static variable. Not only does it emphasize that variable’s static nature, but in some cases it gives the compiler better opportunities for optimization. Similar logic applies to static methods. You can refer to a static method either through an object as you can with any method, or with the special additional syntax ClassName.method( ). You define a static method in a similar way: class Incrementable { static void increment() { StaticTest.i++; } } You can see that the Incrementable method increment( ) increments the static data i using the ++ operator. You can call increment( ) in the typical way, through an object: Incrementable sf = new Incrementable(); sf.increment(); Or, because increment( ) is a static method, you can call it directly through its class: Incrementable.increment(); Although static, when applied to a field, definitely changes the way the data is created (one for each class versus the non-static one for each object), when applied to a method it’s not so dramatic. An important use of static for methods is to allow you to call that method without creating an object. This is essential, as you will see, in defining the main( ) method that is the entry point for running an application. Your first Java program Finally, here’s the first complete program. It starts by printing a string, and then the date, using the Date class from the Java standard library. // HelloDate.java import java.util.*; public class HelloDate { public static void main(String[] args) { System.out.println("Hello, it’s: "); System.out.println(new Date()); } } 52 Thinking in Java Bruce Eckel At the beginning of each program file, you must place any necessary import statements to bring in extra classes you’ll need for the code in that file. Note that I say “extra”. That’s because there’s a certain library of classes that are automatically brought into every Java file: java.lang. Start up your Web browser and look at the documentation from Sun. (If you haven’t downloaded the JDK documentation from http://java.sun.com, do so now.5 Note that this documentation doesn’t come packed with the JDK; you must do a separate download to get it.) If you look at the list of the packages, you’ll see all the different class libraries that come with Java. Select java.lang. This will bring up a list of all the classes that are part of that library. Since java.lang is implicitly included in every Java code file, these classes are automatically available. There’s no Date class listed in java.lang, which means you must import another library to use that. If you don’t know the library where a particular class is, or if you want to see all of the classes, you can select “Tree” in the Java documentation. Now you can find every single class that comes with Java. Then you can use the browser’s “find” function to find Date. When you do you’ll see it listed as java.util.Date, which lets you know that it’s in the util library and that you must import java.util.* in order to use Date. If you go back to the beginning, select java.lang and then System, you’ll see that the System class has several fields, and if you select out, you’ll discover that it’s a static PrintStream object. Since it’s static, you don’t need to create anything with new. The out object is always there, and you can just use it. What you can do with this out object is determined by its type: PrintStream. Conveniently, PrintStream is shown in the description as a hyperlink, so if you click on that, you’ll see a list of all the methods you can call for PrintStream. There are quite a few, and these will be covered later in the book. For now all we’re interested in is println( ), which in effect means “Print what I’m giving you out to the console and end with a newline.” Thus, in any Java program you write you can write something like this: System.out.println("A String of things"); whenever you want to display information to the console. The name of the class is the same as the name of the file. When you’re creating a standalone program such as this one, one of the classes in the file must have the same name as the file. (The compiler complains if you don’t do this.) That class must contain a method called main( ) with this signature and return type: public static void main(String[] args) { The public keyword means that the method is available to the outside world (described in detail in the Access Control chapter). The argument to main( ) is an array of String objects. The args won’t be used in this program, but the Java compiler insists that they be there because they hold the arguments from the command line. The line that prints the date is quite interesting: System.out.println(new Date()); The argument is a Date object that is being created just to send its value (which is automatically converted to a String) to println( ). As soon as this statement is finished, that Date is unnecessary, and the garbage collector can come along and get it anytime. We don’t need to worry about cleaning it up. 5 The Java compiler and documentation from Sun tend to change regularly, and the best place to get them is directly from Sun. By downloading it yourself, you will get the most recent version. Everything Is an Object 53 When you look at the JDK documentation from http://java.sun.com, you will see that System has many other methods that allow you to produce interesting effects (one of Java’s most powerful assets is its large set of standard libraries). For example: //: object/ShowProperties.java public class ShowProperties { public static void main(String[] args) { System.getProperties().list(System.out); System.out.println(System.getProperty("user.name")); System.out.println( System.getProperty("java.library.path")); } } ///:~ The first line in main( ) displays all of the “properties” from the system where you are running the program, so it gives you environment information. The list( ) method sends the results to its argument, System.out. You will see later in the book that you can send the results elsewhere, to a file, for example. You can also ask for a specific property—in this case, the user name and java.library.path. (The unusual comments at the beginning and end will be explained a little later.) Compiling and running To compile and run this program, and all the other programs in this book, you must first have a Java programming environment. There are a number of third-party development environments, but in this book I will assume that you are using the Java Developer’s Kit (JDK) from Sun, which is free. If you are using another development system,6 you will need to look in the documentation for that system to determine how to compile and run programs. Get on the Internet and go to http://java.sun.com. There you will find information and links that will lead you through the process of downloading and installing the JDK for your particular platform. Once the JDK is installed, and you’ve set up your computer’s path information so that it will find javac and java, download and unpack the source code for this book (you can find it at www.MindView.net). This will create a subdirectory for each chapter in this book. Move to the subdirectory named objects and type: javac HelloDate.java This command should produce no response. If you get any kind of an error message, it means you haven’t installed the JDK properly and you need to investigate those problems. On the other hand, if you just get your command prompt back, you can type: java HelloDate and you’ll get the message and the date as output. This is the process you can use to compile and run each of the programs in this book. However, you will see that the source code for this book also has a file called build.xml in each chapter, and this contains “Ant” commands for automatically building the files for that 6 IBM’s “jikes” compiler is a common alternative, as it is significantly faster than Sun’s javac (although if you’re building groups of files using Ant, there’s not too much of a difference). There are also open-source projects to create Java compilers, runtime environments, and libraries. 54 Thinking in Java Bruce Eckel chapter. Buildfiles and Ant (including where to download it) are described more fully in the supplement you will find at http://MindView.net/Books/BetterJava, but once you have Ant installed (from http://jakarta.apache.org/ant) you can just type ‘ant’ at the command prompt to compile and run the programs in each chapter. If you haven’t installed Ant yet, you can just type the javac and java commands by hand. Comments and embedded documentation There are two types of comments in Java. The first is the traditional C-style comment that was inherited by C++. These comments begin with a /* and continue, possibly across many lines, until a */. Note that many programmers will begin each line of a continued comment with a *, so you’ll often see: /* This is a comment * that continues * across lines */ Remember, however, that everything inside the /* and */ is ignored, so there’s no difference in saying: /* This is a comment that continues across lines */ The second form of comment comes from C++. It is the single-line comment, which starts with a // and continues until the end of the line. This type of comment is convenient and commonly used because it’s easy. You don’t need to hunt on the keyboard to find / and then * (instead, you just press the same key twice), and you don’t need to close the comment. So you will often see: // This is a one-line comment Comment documentation Possibly the biggest problem with documenting code has been maintaining that documentation. If the documentation and the code are separate, it becomes tedious to change the documentation every time you change the code. The solution seems simple: Link the code to the documentation. The easiest way to do this is to put everything in the same file. To complete the picture, however, you need a special comment syntax to mark the documentation and a tool to extract those comments and put them in a useful form. This is what Java has done. The tool to extract the comments is called Javadoc, and it is part of the JDK installation. It uses some of the technology from the Java compiler to look for special comment tags that you put in your programs. It not only extracts the information marked by these tags, but it also pulls out the class name or method name that adjoins the comment. This way you can get away with the minimal amount of work to generate decent program documentation. The output of Javadoc is an HTML file that you can view with your Web browser. Thus, Javadoc allows you to create and maintain a single source file and automatically generate useful documentation. Because of Javadoc, you have a straightforward standard for creating documentation, so you can expect or even demand documentation with all Java libraries. Everything Is an Object 55 In addition, you can write your own Javadoc handlers, called doclets, if you want to perform special operations on the information processed by Javadoc (to produce output in a different format, for example). Doclets are introduced in the supplement at http://MindView.net/Books/BetterJava. What follows is only an introduction and overview of the basics of Javadoc. A thorough description can be found in the JDK documentation. When you unpack the documentation, look in the “tooldocs” subdirectory (or follow the “tooldocs” link). Syntax All of the Javadoc commands occur only within /** comments. The comments end with */ as usual. There are two primary ways to use Javadoc: Embed HTML or use “doc tags.” Standalone doc tags are commands that start with an ‘@’ and are placed at the beginning of a comment line. (A leading ‘*’, however, is ignored.) Inline doc tags can appear anywhere within a Javadoc comment and also start with an ‘@’ but are surrounded by curly braces. There are three “types” of comment documentation, which correspond to the element the comment precedes: class, field, or method. That is, a class comment appears right before the definition of a class, a field comment appears right in front of the definition of a field, and a method comment appears right in front of the definition of a method. As a simple example: //: object/Documentation1.java /** A class comment */ public class Documentation1 { /** A field comment */ public int i; /** A method comment */ public void f() {} } ///:~ Note that Javadoc will process comment documentation for only public and protected members. Comments for private and package-access members (see the Access Control chapter) are ignored, and you’ll see no output. (However, you can use the -private flag to include private members as well.) This makes sense, since only public and protected members are available outside the file, which is the client programmer’s perspective. The output for the preceding code is an HTML file that has the same standard format as all the rest of the Java documentation, so users will be comfortable with the format and can easily navigate your classes. It’s worth entering the preceding code, sending it through Javadoc, and viewing the resulting HTML file to see the results. Embedded HTML Javadoc passes HTML commands through to the generated HTML document. This allows you full use of HTML; however, the primary motive is to let you format code, such as: //: object/Documentation2.java /** *
     * System.out.println(new Date()); * 
    */ ///:~ You can also use HTML just as you would in any other Web document to format the regular text in your descriptions: 56 Thinking in Java Bruce Eckel //: object/Documentation3.java /** * You can even insert a list: *
      *
    1. Item one *
    2. Item two *
    3. Item three *
    */ ///:~ Note that within the documentation comment, asterisks at the beginning of a line are thrown away by Javadoc, along with leading spaces. Javadoc reformats everything so that it conforms to the standard documentation appearance. Don’t use headings such as

    or
    as embedded HTML, because Javadoc inserts its own headings and yours will interfere with them. All types of comment documentation—class, field, and method—can support embedded HTML. Some example tags Here are some of the Javadoc tags available for code documentation. Before trying to do anything serious using Javadoc, you should consult the Javadoc reference in the JDK documentation to learn all the different ways that you can use Javadoc. @see This tag allows you to refer to the documentation in other classes. Javadoc will generate HTML with the @see tags hyperlinked to the other documentation. The forms are: @see classname @see fully-qualified-classname @see fully-qualified-classname#method-name Each one adds a hyperlinked “See Also” entry to the generated documentation. Javadoc will not check the hyperlinks you give it to make sure they are valid. {@link package.class#member label} Very similar to @see, except that it can be used inline and uses the label as the hyperlink text rather than “See Also.” {@docRoot} Produces the relative path to the documentation root directory. Useful for explicit hyperlinking to pages in the documentation tree. {@inheritDoc} Inherits the documentation from the nearest base class of this class into the current doc comment. Everything Is an Object 57 @version This is of the form: @version version-information in which version-information is any significant information you see fit to include. When the - version flag is placed on the Javadoc command line, the version information will be called out specially in the generated HTML documentation. @author This is of the form: @author author-information in which author-information is, presumably, your name, but it could also include your email address or any other appropriate information. When the -author flag is placed on the Javadoc command line, the author information will be called out specially in the generated HTML documentation. You can have multiple author tags for a list of authors, but they must be placed consecutively. All the author information will be lumped together into a single paragraph in the generated HTML. @since This tag allows you to indicate the version of this code that began using a particular feature. You’ll see it appearing in the HTML Java documentation to indicate what version of the JDK is used. @param This is used for method documentation, and is of the form: @param parameter-name description in which parameter-name is the identifier in the method parameter list, and description is text that can continue on subsequent lines. The description is considered finished when a new documentation tag is encountered. You can have any number of these, presumably one for each parameter. @return This is used for method documentation, and looks like this: @return description in which description gives you the meaning of the return value. It can continue on subsequent lines. 58 Thinking in Java Bruce Eckel @throws Exceptions will be demonstrated in the Error Handling with Exceptions chapter. Briefly, they are objects that can be “thrown” out of a method if that method fails. Although only one exception object can emerge when you call a method, a particular method might produce any number of different types of exceptions, all of which need descriptions. So the form for the exception tag is: @throws fully-qualified-class-name description in which fully-qualified-class-name gives an unambiguous name of an exception class that’s defined somewhere, and description (which can continue on subsequent lines) tells you why this particular type of exception can emerge from the method call. @deprecated This is used to indicate features that were superseded by an improved feature. The deprecated tag is a suggestion that you no longer use this particular feature, since sometime in the future it is likely to be removed. A method that is marked @deprecated causes the compiler to issue a warning if it is used. In Java SE5, the @deprecated Javadoc tag has been superseded by the @Deprecated annotation (you’ll learn about these in the Annotations chapter). Documentation example Here is the first Java program again, this time with documentation comments added: //: object/HelloDate.java import java.util.*; /** The first Thinking in Java example program. * Displays a string and today’s date. * @author Bruce Eckel * @author www.MindView.net * @version 4.0 */ public class HelloDate { /** Entry point to class & application. * @param args array of string arguments * @throws exceptions No exceptions thrown */ public static void main(String[] args) { System.out.println("Hello, it’s: "); System.out.println(new Date()); } } /* Output: (55% match) Hello, it’s: Wed Oct 05 14:39:36 MDT 2005 *///:~ The first line of the file uses my own technique of putting a ‘//:’ as a special marker for the comment line containing the source file name. That line contains the path information to the file (object indicates this chapter) followed by the file name. The last line also finishes with a comment, and this one (‘///:~’) indicates the end of the source code listing, which allows it to be automatically updated into the text of this book after being checked with a compiler and executed. Everything Is an Object 59 The /* Output: tag indicates the beginning of the output that will be generated by this file. In this form, it can be automatically tested to verify its accuracy. In this case, the (55% match) indicates to the testing system that the output will be fairly different from one run to the next so it should only expect a 55 percent correlation with the output shown here. Most examples in this book that produce output will contain the output in this commented form, so you can see the output and know that it is correct. Coding style The style described in the Code Conventions for the Java Programming Language7 is to capitalize the first letter of a class name. If the class name consists of several words, they are run together (that is, you don’t use underscores to separate the names), and the first letter of each embedded word is capitalized, such as: class AllTheColorsOfTheRainbow { // ... This is sometimes called “camel-casing.” For almost everything else—methods, fields (member variables), and object reference names—the accepted style is just as it is for classes except that the first letter of the identifier is lowercase. For example: class AllTheColorsOfTheRainbow { int anIntegerRepresentingColors; void changeTheHueOfTheColor(int newHue) { // ... } // ... } The user must also type all these long names, so be merciful. The Java code you will see in the Sun libraries also follows the placement of open-and-close curly braces that you see used in this book. Summary The goal of this chapter is just enough Java to understand how to write a simple program. You’ve also gotten an overview of the language and some of its basic ideas. However, the examples so far have all been of the form “Do this, then do that, then do something else.” The next two chapters will introduce the basic operators used in Java programming, and then show you how to control the flow of your program. Exercises Normally, exercises will be distributed throughout the chapters, but in this chapter you were learning how to write basic programs so all the exercises were delayed until the end. The number in parentheses after each exercise number is an indicator of how difficult the exercise is, in a ranking from 1-10. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. 7 http://java.sun.com/docs/codeconv/index.html. To preserve space in this book and seminar presentations, not all of these guidelines could be followed, but you’ll see that the style I use here matches the Java standard as much as possible. 60 Thinking in Java Bruce Eckel Exercise 1: (2) Create a class containing an int and a char that are not initialized, and print their values to verify that Java performs default initialization. Exercise 2: (1) Following the HelloDate.java example in this chapter, create a “hello, world” program that simply displays that statement. You need only a single method in your class (the “main” one that gets executed when the program starts). Remember to make it static and to include the argument list, even though you don’t use the argument list. Compile the program with javac and run it using java. If you are using a different development environment than the JDK, learn how to compile and run programs in that environment. Exercise 3: (1) Find the code fragments involving ATypeName and turn them into a program that compiles and runs. Exercise 4: (1) Turn the DataOnly code fragments into a program that compiles and runs. Exercise 5: (1) Modify the previous exercise so that the values of the data in DataOnly are assigned to and printed in main( ). Exercise 6: (2) Write a program that includes and calls the storage( ) method defined as a code fragment in this chapter. Exercise 7: (1) Turn the Incrementable code fragments into a working program. Exercise 8: (3) Write a program that demonstrates that, no matter how many objects you create of a particular class, there is only one instance of a particular static field in that class. Exercise 9: (2) Write a program that demonstrates that autoboxing works for all the primitive types and their wrappers. Exercise 10: (2) Write a program that prints three arguments taken from the command line. To do this, you’ll need to index into the command-line array of Strings. Exercise 11: (1) Turn the AllTheColorsOfTheRainbow example into a program that compiles and runs. Exercise 12: (2) Find the code for the second version of HelloDate.java, which is the simple comment documentation example. Execute Javadoc on the file and view the results with your Web browser. Exercise 13: (1) Run Documentation1.java, Documentation2.java, and Documentation3.java through Javadoc. Verify the resulting documentation with your Web browser. Exercise 14: (1) Add an HTML list of items to the documentation in the previous exercise. Exercise 15: (1) Take the program in Exercise 2 and add comment documentation to it. Extract this comment documentation into an HTML file using Javadoc and view it with your Web browser. Everything Is an Object 61 Exercise 16: (1) In the Initialization & Cleanup chapter, locate the Overloading.java example and add Javadoc documentation. Extract this comment documentation into an HTML file using Javadoc and view it with your Web browser. 62 Thinking in Java Bruce Eckel Operators At the lowest level, data in Java is manipulated using operators. Because Java was inherited from C++, most of these operators will be familiar to C and C++ programmers. Java has also added some improvements and simplifications. If you’re familiar with C or C++ syntax, you can skim through this chapter and the next, looking for places where Java is different from those languages. However, if you find yourself floundering a bit in these two chapters, make sure you go through the multimedia seminar Thinking in C, freely downloadable from www.MindView.net. It contains audio lectures, slides, exercises, and solutions specifically designed to bring you up to speed with the fundamentals necessary to learn Java. Simpler print statements In the previous chapter, you were introduced to the Java print statement: System.out.println("Rather a lot to type"); You may observe that this is not only a lot to type (and thus many redundant tendon hits), but also rather noisy to read. Most languages before and after Java have taken a much simpler approach to such a commonly used statement. The Access Control chapter introduces the concept of the static import that was added to Java SE5, and creates a tiny library to simplify writing print statements. However, you don’t need to know those details in order to begin using that library. We can rewrite the program from the last chapter using this new library: //: operators/HelloDate.java import java.util.*; import static net.mindview.util.Print.*; public class HelloDate { public static void main(String[] args) { print("Hello, it’s: "); print(new Date()); } } /* Output: (55% match) Hello, it’s: Wed Oct 05 14:39:05 MDT 2005 *///:~ The results are much cleaner. Notice the insertion of the static keyword in the second import statement. In order to use this library, you must download this book’s code package from www.MindView.net or one of its mirrors. Unzip the code tree and add the root directory of that code tree to your computer’s CLASSPATH environment variable. (You’ll eventually get a full introduction to the classpath, but you might as well get used to struggling with it early. Alas, it is one of the more common battles you will have with Java.) Although the use of net.mindview.util.Print nicely simplifies most code, it is not justifiable everywhere. If there are only a small number of print statements in a program, I forego the import and write out the full System.out.println( ). Exercise 1: (1) Write a program that uses the “short” and normal form of print statement. Using Java operators An operator takes one or more arguments and produces a new value. The arguments are in a different form than ordinary method calls, but the effect is the same. Addition and unary plus (+), subtraction and unary minus (-), multiplication (*), division (/), and assignment (=) all work much the same in any programming language. All operators produce a value from their operands. In addition, some operators change the value of an operand. This is called a side effect. The most common use for operators that modify their operands is to generate the side effect, but you should keep in mind that the value produced is available for your use, just as in operators without side effects. Almost all operators work only with primitives. The exceptions are ‘=‘, ‘==‘ and ‘!=‘, which work with all objects (and are a point of confusion for objects). In addition, the String class supports ‘+’ and ‘+=‘. Precedence Operator precedence defines how an expression evaluates when several operators are present. Java has specific rules that determine the order of evaluation. The easiest one to remember is that multiplication and division happen before addition and subtraction. Programmers often forget the other precedence rules, so you should use parentheses to make the order of evaluation explicit. For example, look at statements (1) and (2): //: operators/Precedence.java public class Precedence { public static void main(String[] args) { int x = 1, y = 2, z = 3; int a = x + y - 2/2 + z; // (1) int b = x + (y - 2)/(2 + z); // (2) System.out.println("a = " + a + " b = " + b); } } /* Output: a=5b=1 *///:~ These statements look roughly the same, but from the output you can see that they have very different meanings which depend on the use of parentheses. Notice that the System.out.println( ) statement involves the ‘+’ operator. In this context, ‘+’ means “string concatenation” and, if necessary, “string conversion.” When the compiler sees a String followed by a ‘+’ followed by a non-String, it attempts to convert the nonString into a String. As you can see from the output, it successfully converts from int into String for a and b. 64 Thinking in Java Bruce Eckel Assignment Assignment is performed with the operator =. It means “Take the value of the right-hand side (often called the rvalue) and copy it into the left-hand side (often called the lvalue)”. An rvalue is any constant, variable, or expression that produces a value, but an lvalue must be a distinct, named variable. (That is, there must be a physical space to store the value.) For instance, you can assign a constant value to a variable: a = 4; but you cannot assign anything to a constant value—it cannot be an lvalue. (You can’t say 4 = a;.) Assignment of primitives is quite straightforward. Since the primitive holds the actual value and not a reference to an object, when you assign primitives, you copy the contents from one place to another. For example, if you say a = b for primitives, then the contents of b are copied into a. If you then go on to modify a, b is naturally unaffected by this modification. As a programmer, this is what you can expect for most situations. When you assign objects, however, things change. Whenever you manipulate an object, what you’re manipulating is the reference, so when you assign “from one object to another,” you’re actually copying a reference from one place to another. This means that if you say c = d for objects, you end up with both c and d pointing to the object that, originally, only d pointed to. Here’s an example that demonstrates this behavior: //: operators/Assignment.java // Assignment with objects is a bit tricky. import static net.mindview.util.Print.*; class Tank { int level; } public class Assignment { public static void main(String[] args) { Tank t1 = new Tank(); Tank t2 = new Tank(); t1.level = 9; t2.level = 47; print("1: t1.level: " + t1.level + ", t2.level: " + t2.level); t1 = t2; print("2: t1.level: " + t1.level + ", t2.level: " + t2.level); t1.level = 27; print("3: t1.level: " + t1.level + ", t2.level: " + t2.level); } } /* Output: 1: t1.level: 9, t2.level: 47 2: t1.level: 47, t2.level: 47 3: t1.level: 27, t2.level: 27 *///:~ The Tank class is simple, and two instances (t1 and t2) are created within main( ). The level field within each Tank is given a different value, and then t2 is assigned to t1, and t1 is changed. In many programming languages you expect t1 and t2 to be independent at all times, but because you’ve assigned a reference, changing the t1 object appears to change the t2 object as well! This is because both t1 and t2 contain the same reference, which is Operators 65 pointing to the same object. (The original reference that was in t1, that pointed to the object holding a value of 9, was overwritten during the assignment and effectively lost; its object will be cleaned up by the garbage collector.) This phenomenon is often called aliasing, and it’s a fundamental way that Java works with objects. But what if you don’t want aliasing to occur in this case? You could forego the assignment and say: t1.level = t2.level; This retains the two separate objects instead of discarding one and tying t1 and t2 to the same object. You’ll soon realize that manipulating the fields within objects is messy and goes against good object-oriented design principles. This is a nontrivial topic, so you should keep in mind that assignment for objects can add surprises. Exercise 2: (1) Create a class containing a float and use it to demonstrate aliasing. Aliasing during method calls Aliasing will also occur when you pass an object into a method: //: operators/PassObject.java // Passing objects to methods may not be // what you’re used to. import static net.mindview.util.Print.*; class Letter { char c; } public class PassObject { static void f(Letter y) { y.c = ‘z’; } public static void main(String[] args) { Letter x = new Letter(); x.c = ‘a’; print("1: x.c: " + x.c); f(x); print("2: x.c: " + x.c); } } /* Output: 1: x.c: a 2: x.c: z *///:~ In many programming languages, the method f( ) would appear to be making a copy of its argument Letter y inside the scope of the method. But once again a reference is being passed, so the line y.c = ‘z’; is actually changing the object outside of f( ). Aliasing and its solution is a complex issue which is covered in one of the online supplements for this book. However, you should be aware of it at this point so you can watch for pitfalls. 66 Thinking in Java Bruce Eckel Exercise 3: (1) Create a class containing a float and use it to demonstrate aliasing during method calls. Mathematical operators The basic mathematical operators are the same as the ones available in most programming languages: addition (+), subtraction (-), division (/), multiplication (*) and modulus (%, which produces the remainder from integer division). Integer division truncates, rather than rounds, the result. Java also uses the shorthand notation from C/C++ that performs an operation and an assignment at the same time. This is denoted by an operator followed by an equal sign, and is consistent with all the operators in the language (whenever it makes sense). For example, to add 4 to the variable x and assign the result to x, use: x += 4. This example shows the use of the mathematical operators: //: operators/MathOps.java // Demonstrates the mathematical operators. import java.util.*; import static net.mindview.util.Print.*; public class MathOps { public static void main(String[] args) { // Create a seeded random number generator: Random rand = new Random(47); int i, j, k; // Choose value from 1 to 100: j = rand.nextInt(100) + 1; print("j : " + j); k = rand.nextInt(100) + 1; print("k : " + k); i = j + k; print("j + k : " + i); i = j - k; print("j - k : " + i); i = k / j; print("k / j : " + i); i = k * j; print("k * j : " + i); i = k % j; print("k % j : " + i); j %= k; print("j %= k : " + j); // Floating-point number tests: float u, v, w; // Applies to doubles, too v = rand.nextFloat(); print("v : " + v); w = rand.nextFloat(); print("w : " + w); u = v + w; print("v + w : " + u); u = v - w; print("v - w : " + u); u = v * w; print("v * w : " + u); u = v / w; print("v / w : " + u); // The following also works for char, // byte, short, int, long, and double: Operators 67 u += v; print("u += v : " + u); u -= v; print("u -= v : " + u); u *= v; print("u *= v : " + u); u /= v; print("u /= v : " + u); } } /* Output: j : 59 k : 56 j + k : 115 j-k:3 k/j:0 k * j : 3304 k % j : 56 j %= k : 3 v : 0.5309454 w : 0.0534122 v + w : 0.5843576 v - w : 0.47753322 v * w : 0.028358962 v / w : 9.940527 u += v : 10.471473 u -= v : 9.940527 u *= v : 5.2778773 u /= v : 9.940527 *///:~ To generate numbers, the program first creates a Random object. If you create a Random object with no arguments, Java uses the current time as a seed for the random number generator, and will thus produce different output for each execution of the program. However, in the examples in this book, it is important that the output shown at the end of the examples be as consistent as possible, so that this output can be verified with external tools. By providing a seed (an initialization value for the random number generator that will always produce the same sequence for a particular seed value) when creating the Random object, the same random numbers will be generated each time the program is executed, so the output is verifiable.1 To generate more varying output, feel free to remove the seed in the examples in the book. The program generates a number of different types of random numbers with the Random object simply by calling the methods nextInt( ) and nextFloat( ) (you can also call nextLong( ) or nextDouble( )). The argument to nextInt( ) sets the upper bound on the generated number. The lower bound is zero, which we don’t want because of the possibility of a divide-by-zero, so the result is offset by one. Exercise 4: (2) Write a program that calculates velocity using a constant distance and a constant time. Unary minus and plus operators The unary minus (-) and unary plus (+) are the same operators as binary minus and plus. The compiler figures out which use is intended by the way you write the expression. For instance, the statement 1 The number 47 was considered a “magic number” at a college I attended, and it stuck. 68 Thinking in Java Bruce Eckel x = -a; has an obvious meaning. The compiler is able to figure out: x = a * -b; but the reader might get confused, so it is sometimes clearer to say: x = a * (-b); Unary minus inverts the sign on the data. Unary plus provides symmetry with unary minus, although it doesn’t have any effect. Auto increment and decrement Java, like C, has a number of shortcuts. Shortcuts can make code much easier to type, and either easier or harder to read. Two of the nicer shortcuts are the increment and decrement operators (often referred to as the auto-increment and auto-decrement operators). The decrement operator is -- and means “decrease by one unit.” The increment operator is ++ and means “increase by one unit.” If a is an int, for example, the expression ++a is equivalent to (a = a + 1). Increment and decrement operators not only modify the variable, but also produce the value of the variable as a result. There are two versions of each type of operator, often called the prefix and postfix versions. Preincrement means the ++ operator appears before the variable, and post-increment means the ++ operator appears after the variable. Similarly, pre-decrement means the -operator appears before the variable, and post-decrement means the -- operator appears after the variable. For pre-increment and pre-decrement (i.e., ++a or --a), the operation is performed and the value is produced. For post-increment and post-decrement (i.e., a++ or a--), the value is produced, then the operation is performed. As an example: //: operators/AutoInc.java // Demonstrates the ++ and -- operators. import static net.mindview.util.Print.*; public class AutoInc { public static void main(String[] args) { int i = 1; print("i : " + i); print("++i : " + ++i); // Pre-increment print("i++ : " + i++); // Post-increment print("i : " + i); print("--i : " + --i); // Pre-decrement print("i-- : " + i--); // Post-decrement print("i : " + i); } } /* Output: i:1 ++i : 2 i++ : 2 i:3 --i : 2 i-- : 2 i:1 *///:~ Operators 69 You can see that for the prefix form, you get the value after the operation has been performed, but with the postfix form, you get the value before the operation is performed. These are the only operators, other than those involving assignment, that have side effects— they change the operand rather than using just its value. The increment operator is one explanation for the name C++, implying “one step beyond C.” In an early Java speech, Bill Joy (one of the Java creators), said that “Java=C++--” (C plus plus minus minus), suggesting that Java is C++ with the unnecessary hard parts removed, and therefore a much simpler language. As you progress in this book, you’ll see that many parts are simpler, and yet in other ways Java isn’t much easier than C++. Relational operators Relational operators generate a boolean result. They evaluate the relationship between the values of the operands. A relational expression produces true if the relationship is true, and false if the relationship is untrue. The relational operators are less than (<), greater than (>), less than or equal to (<=), greater than or equal to (>=), equivalent (==) and not equivalent (!=). Equivalence and nonequivalence work with all primitives, but the other comparisons won’t work with type boolean. Because boolean values can only be true or false, “greater than” and “less than” doesn’t make sense. Testing object equivalence The relational operators == and != also work with all objects, but their meaning often confuses the first-time Java programmer. Here’s an example: //: operators/Equivalence.java public class Equivalence { public static void main(String[] args) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1 == n2); System.out.println(n1 != n2); } } /* Output: false true *///:~ The statement System.out.println(n1 == n2) will print the result of the boolean comparison within it. Surely the output should be “true” and then “false,” since both Integer objects are the same. But while the contents of the objects are the same, the references are not the same. The operators == and != compare object references, so the output is actually “false” and then “true.” Naturally, this surprises people at first. What if you want to compare the actual contents of an object for equivalence? You must use the special method equals( ) that exists for all objects (not primitives, which work fine with == and !=). Here’s how it’s used: //: operators/EqualsMethod.java public class EqualsMethod { public static void main(String[] args) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1.equals(n2)); 70 Thinking in Java Bruce Eckel } } /* Output: true *///:~ The result is now what you expect. Ah, but it’s not as simple as that. If you create your own class, like this: //: operators/EqualsMethod2.java // Default equals() does not compare contents. class Value { int i; } public class EqualsMethod2 { public static void main(String[] args) { Value v1 = new Value(); Value v2 = new Value(); v1.i = v2.i = 100; System.out.println(v1.equals(v2)); } } /* Output: false *///:~ things are confusing again: The result is false. This is because the default behavior of equals( ) is to compare references. So unless you override equals( ) in your new class you won’t get the desired behavior. Unfortunately, you won’t learn about overriding until the Reusing Classes chapter and about the proper way to define equals( ) until the Containers in Depth chapter, but being aware of the way equals( ) behaves might save you some grief in the meantime. Most of the Java library classes implement equals( ) so that it compares the contents of objects instead of their references. Exercise 5: (2) Create a class called Dog containing two Strings: name and says. In main( ), create two dog objects with names “spot” (who says, “Ruff!”) and “scruffy” (who says, “Wurf!”). Then display their names and what they say. Exercise 6: (3) Following Exercise 5, create a new Dog reference and assign it to spot’s object. Test for comparison using == and equals( ) for all references. Logical operators Each of the logical operators AND (&&), OR (||) and NOT (!) produces a boolean value of true or false based on the logical relationship of its arguments. This example uses the relational and logical operators: //: operators/Bool.java // Relational and logical operators. import java.util.*; import static net.mindview.util.Print.*; public class Bool { public static void main(String[] args) { Random rand = new Random(47); int i = rand.nextInt(100); Operators 71 int j = rand.nextInt(100); print("i = " + i); print("j = " + j); print("i > j is " + (i > j)); print("i < j is " + (i < j)); print("i >= j is " + (i >= j)); print("i <= j is " + (i <= j)); print("i == j is " + (i == j)); print("i != j is " + (i != j)); // Treating an int as a boolean is not legal Java: //! print("i && j is " + (i && j)); //! print("i || j is " + (i || j)); //! print("!i is " + !i); print("(i < 10) && (j < 10) is " + ((i < 10) && (j < 10)) ); print("(i < 10) || (j < 10) is " + ((i < 10) || (j < 10)) ); } } /* Output: i = 58 j = 55 i > j is true i < j is false i >= j is true i <= j is false i == j is false i != j is true (i < 10) && (j < 10) is false (i < 10) || (j < 10) is false *///:~ You can apply AND, OR, or NOT to boolean values only. You can’t use a non-boolean as if it were a boolean in a logical expression as you can in C and C++. You can see the failed attempts at doing this commented out with a ‘//!’ (this comment syntax enables automatic removal of comments to facilitate testing). The subsequent expressions, however, produce boolean values using relational comparisons, then use logical operations on the results. Note that a boolean value is automatically converted to an appropriate text form if it is used where a String is expected. You can replace the definition for int in the preceding program with any other primitive data type except boolean. Be aware, however, that the comparison of floating point numbers is very strict. A number that is the tiniest fraction different from another number is still “not equal.” A number that is the tiniest bit above zero is still nonzero. Exercise 7: (3) Write a program that simulates coin-flipping. Short-circuiting When dealing with logical operators, you run into a phenomenon called “short-circuiting.” This means that the expression will be evaluated only until the truth or falsehood of the entire expression can be unambiguously determined. As a result, the latter parts of a logical expression might not be evaluated. Here’s an example that demonstrates short-circuiting: //: operators/ShortCircuit.java // Demonstrates short-circuiting behavior // with logical operators. import static net.mindview.util.Print.*; 72 Thinking in Java Bruce Eckel public class ShortCircuit { static boolean test1(int val) { print("test1(" + val + ")"); print("result: " + (val < 1)); return val < 1; } static boolean test2(int val) { print("test2(" + val + ")"); print("result: " + (val < 2)); return val < 2; } static boolean test3(int val) { print("test3(" + val + ")"); print("result: " + (val < 3)); return val < 3; } public static void main(String[] args) { boolean b = test1(0) && test2(2) && test3(2); print("expression is " + b); } } /* Output: test1(0) result: true test2(2) result: false expression is false *///:~ Each test performs a comparison against the argument and returns true or false. It also prints information to show you that it’s being called. The tests are used in the expression: test1(0) && test2(2) && test3(2) You might naturally think that all three tests would be executed, but the output shows otherwise. The first test produced a true result, so the expression evaluation continues. However, the second test produced a false result. Since this means that the whole expression must be false, why continue evaluating the rest of the expression? It might be expensive. The reason for shortcircuiting, in fact, is that you can get a potential performance increase if all the parts of a logical expression do not need to be evaluated. Literals Ordinarily, when you insert a literal value into a program, the compiler knows exactly what type to make it. Sometimes, however, the type is ambiguous. When this happens, you must guide the compiler by adding some extra information in the form of characters associated with the literal value. The following code shows these characters: //: operators/Literals.java import static net.mindview.util.Print.*; public class Literals { public static void main(String[] args) { int i1 = 0x2f; // Hexadecimal (lowercase) print("i1: " + Integer.toBinaryString(i1)); int i2 = 0X2F; // Hexadecimal (uppercase) print("i2: " + Integer.toBinaryString(i2)); int i3 = 0177; // Octal (leading zero) print("i3: " + Integer.toBinaryString(i3)); char c = 0xffff; // max char hex value Operators 73 print("c: " + Integer.toBinaryString(c)); byte b = 0x7f; // max byte hex value print("b: " + Integer.toBinaryString(b)); short s = 0x7fff; // max short hex value print("s: " + Integer.toBinaryString(s)); long n1 = 200L; // long suffix long n2 = 200l; // long suffix (but can be confusing) long n3 = 200; float f1 = 1; float f2 = 1F; // float suffix float f3 = 1f; // float suffix double d1 = 1d; // double suffix double d2 = 1D; // double suffix // (Hex and Octal also work with long) } } /* Output: i1: 101111 i2: 101111 i3: 1111111 c: 1111111111111111 b: 1111111 s: 111111111111111 *///:~ A trailing character after a literal value establishes its type. Uppercase or lowercase L means long (however, using a lowercase l is confusing because it can look like the number one). Uppercase or lowercase F means float. Uppercase or lowercase D means double. Hexadecimal (base 16), which works with all the integral data types, is denoted by a leading 0x or 0X followed by 0-9 or a-f either in uppercase or lowercase. If you try to initialize a variable with a value bigger than it can hold (regardless of the numerical form of the value), the compiler will give you an error message. Notice in the preceding code the maximum possible hexadecimal values for char, byte, and short. If you exceed these, the compiler will automatically make the value an int and tell you that you need a narrowing cast for the assignment (casts are defined later in this chapter). You’ll know you’ve stepped over the line. Octal (base 8) is denoted by a leading zero in the number and digits from 0-7. There is no literal representation for binary numbers in C, C++, or Java. However, when working with hexadecimal and octal notation, it’s useful to display the binary form of the results. This is easily accomplished with the static toBinaryString( ) methods from the Integer and Long classes. Notice that when passing smaller types to Integer.toBinaryString( ), the type is automatically converted to an int. Exercise 8: (2) Show that hex and octal notations work with long values. Use Long.toBinaryString( ) to display the results. Exponential notation Exponents use a notation that I’ve always found rather dismaying: //: operators/Exponents.java // "e" means "10 to the power." public class Exponents { public static void main(String[] args) { // Uppercase and lowercase ‘e’ are the same: float expFloat = 1.39e-43f; expFloat = 1.39E-43f; 74 Thinking in Java Bruce Eckel System.out.println(expFloat); double expDouble = 47e47d; // ‘d’ is optional double expDouble2 = 47e47; // Automatically double System.out.println(expDouble); } } /* Output: 1.39E-43 4.7E48 *///:~ In science and engineering, ‘e’ refers to the base of natural logarithms, approximately 2.718. (A more precise double value is available in Java as Math.E.) This is used in exponentiation expressions such as 1.39 x e-43, which means 1.39 x 2.718-43. However, when the FORTRAN programming language was invented, they decided that e would mean “ten to the power”, which is an odd decision because FORTRAN was designed for science and engineering, and one would think its designers would be sensitive about introducing such an ambiguity.2 At any rate, this custom was followed in C, C++ and now Java. So if you’re used to thinking in terms of e as the base of natural logarithms, you must do a mental translation when you see an expression such as 1.39 e-43f in Java; it means 1.39 x 10-43. Note that you don’t need to use the trailing character when the compiler can figure out the appropriate type. With long n3 = 200; there’s no ambiguity, so an L after the 200 would be superfluous. However, with float f4 = 1e-43f; // 10 to the power the compiler normally takes exponential numbers as doubles, so without the trailing f, it will give you an error telling you that you must use a cast to convert double to float. Exercise 9: (1) Display the largest and smallest numbers for both float and double exponential notation. Bitwise operators The bitwise operators allow you to manipulate individual bits in an integral primitive data type. Bitwise operators perform Boolean algebra on the corresponding bits in the two arguments to produce the result. The bitwise operators come from C’s low-level orientation, where you often manipulate hardware directly and must set the bits in hardware registers. Java was originally designed to be embedded in TV set-top boxes, so this low-level orientation still made sense. However, you probably won’t use the bitwise operators much. 2 John Kirkham writes, “I started computing in 1962 using FORTRAN II on an IBM 1620. At that time, and throughout the 1960s and into the 1970s, FORTRAN was an all uppercase language. This probably started because many of the early input devices were old teletype units that used 5 bit Baudot code, which had no lowercase capability. The ‘E’ in the exponential notation was also always uppercase and was never confused with the natural logarithm base ‘e’, which is always lowercase. The ‘E’ simply stood for exponential, which was for the base of the number system used—usually 10. At the time octal was also widely used by programmers. Although I never saw it used, if I had seen an octal number in exponential notation I would have considered it to be base 8. The first time I remember seeing an exponential using a lowercase ‘e’ was in the late 1970s and I also found it confusing. The problem arose as lowercase crept into FORTRAN, not at its beginning. We actually had functions to use if you really wanted to use the natural logarithm base, but they were all uppercase.” Operators 75 The bitwise AND operator (&) produces a one in the output bit if both input bits are one; otherwise, it produces a zero. The bitwise OR operator (|) produces a one in the output bit if either input bit is a one and produces a zero only if both input bits are zero. The bitwise EXCLUSIVE OR, or XOR (^), produces a one in the output bit if one or the other input bit is a one, but not both. The bitwise NOT (~, also called the ones complement operator) is a unary operator; it takes only one argument. (All other bitwise operators are binary operators.) Bitwise NOT produces the opposite of the input bit—a one if the input bit is zero, a zero if the input bit is one. The bitwise operators and logical operators use the same characters, so it is helpful to have a mnemonic device to help you remember the meanings: Because bits are “small”, there is only one character in the bitwise operators. Bitwise operators can be combined with the = sign to unite the operation and assignment: &=, |= and ^= are all legitimate. (Since ~ is a unary operator, it cannot be combined with the = sign.) The boolean type is treated as a one-bit value, so it is somewhat different. You can perform a bitwise AND, OR, and XOR, but you can’t perform a bitwise NOT (presumably to prevent confusion with the logical NOT). For booleans, the bitwise operators have the same effect as the logical operators except that they do not short circuit. Also, bitwise operations on booleans include an XOR logical operator that is not included under the list of “logical” operators. You cannot use booleans in shift expressions, which are described next. Exercise 10: (3) Write a program with two constant values, one with alternating binary ones and zeroes, with a zero in the least-significant digit, and the second, also alternating, with a one in the least-significant digit (hint: It’s easiest to use hexadecimal constants for this). Take these two values and combine them in all possible ways using the bitwise operators, and display the results using Integer.toBinaryString( ). Shift operators The shift operators also manipulate bits. They can be used solely with primitive, integral types. The left-shift operator (<<) produces the operand to the left of the operator after it has been shifted to the left by the number of bits specified to the right of the operator (inserting zeroes at the lower-order bits). The signed right-shift operator (>>) produces the operand to the left of the operator after it has been shifted to the right by the number of bits specified to the right of the operator. The signed right shift >> uses sign extension: If the value is positive, zeroes are inserted at the higher-order bits; if the value is negative, ones are inserted at the higher-order bits. Java has also added the unsigned right shift >>>, which uses zero extension: Regardless of the sign, zeroes are inserted at the higher-order bits. This operator does not exist in C or C++. If you shift a char, byte, or short, it will be promoted to int before the shift takes place, and the result will be an int. Only the five low-order bits of the right-hand side will be used. This prevents you from shifting more than the number of bits in an int. If you’re operating on a long, you’ll get a long result. Only the six low-order bits of the right-hand side will be used, so you can’t shift more than the number of bits in a long. Shifts can be combined with the equal sign (<<= or >>= or >>>=). The lvalue is replaced by the lvalue shifted by the rvalue. There is a problem, however, with the unsigned right shift combined with assignment. If you use it with byte or short, you don’t get the correct results. Instead, these are promoted to int and right shifted, but then truncated as they are assigned back into their variables, so you get -1 in those cases. The following example demonstrates this: 76 Thinking in Java Bruce Eckel //: operators/URShift.java // Test of unsigned right shift. import static net.mindview.util.Print.*; public class URShift { public static void main(String[] args) { int i = -1; print(Integer.toBinaryString(i)); i >>>= 10; print(Integer.toBinaryString(i)); long l = -1; print(Long.toBinaryString(l)); l >>>= 10; print(Long.toBinaryString(l)); short s = -1; print(Integer.toBinaryString(s)); s >>>= 10; print(Integer.toBinaryString(s)); byte b = -1; print(Integer.toBinaryString(b)); b >>>= 10; print(Integer.toBinaryString(b)); b = -1; print(Integer.toBinaryString(b)); print(Integer.toBinaryString(b>>>10)); } } /* Output: 11111111111111111111111111111111 1111111111111111111111 1111111111111111111111111111111111111111111111111111111111111111 111111111111111111111111111111111111111111111111111111 11111111111111111111111111111111 11111111111111111111111111111111 11111111111111111111111111111111 11111111111111111111111111111111 11111111111111111111111111111111 1111111111111111111111 *///:~ In the last shift, the resulting value is not assigned back into b, but is printed directly, so the correct behavior occurs. Here’s an example that demonstrates the use of all the operators involving bits: //: operators/BitManipulation.java // Using the bitwise operators. import java.util.*; import static net.mindview.util.Print.*; public class BitManipulation { public static void main(String[] args) { Random rand = new Random(47); int i = rand.nextInt(); int j = rand.nextInt(); printBinaryInt("-1", -1); printBinaryInt("+1", +1); int maxpos = 2147483647; printBinaryInt("maxpos", maxpos); int maxneg = -2147483648; printBinaryInt("maxneg", maxneg); printBinaryInt("i", i); printBinaryInt("~i", ~i); Operators 77 printBinaryInt("-i", -i); printBinaryInt("j", j); printBinaryInt("i & j", i & j); printBinaryInt("i | j", i | j); printBinaryInt("i ^ j", i ^ j); printBinaryInt("i << 5", i << 5); printBinaryInt("i >> 5", i >> 5); printBinaryInt("(~i) >> 5", (~i) >> 5); printBinaryInt("i >>> 5", i >>> 5); printBinaryInt("(~i) >>> 5", (~i) >>> 5); long l = rand.nextLong(); long m = rand.nextLong(); printBinaryLong("-1L", -1L); printBinaryLong("+1L", +1L); long ll = 9223372036854775807L; printBinaryLong("maxpos", ll); long lln = -9223372036854775808L; printBinaryLong("maxneg", lln); printBinaryLong("l", l); printBinaryLong("~l", ~l); printBinaryLong("-l", -l); printBinaryLong("m", m); printBinaryLong("l & m", l & m); printBinaryLong("l | m", l | m); printBinaryLong("l ^ m", l ^ m); printBinaryLong("l << 5", l << 5); printBinaryLong("l >> 5", l >> 5); printBinaryLong("(~l) >> 5", (~l) >> 5); printBinaryLong("l >>> 5", l >>> 5); printBinaryLong("(~l) >>> 5", (~l) >>> 5); } static void printBinaryInt(String s, int i) { print(s + ", int: " + i + ", binary:\n " + Integer.toBinaryString(i)); } static void printBinaryLong(String s, long l) { print(s + ", long: " + l + ", binary:\n " + Long.toBinaryString(l)); } } /* Output: -1, int: -1, binary: 11111111111111111111111111111111 +1, int: 1, binary: 1 maxpos, int: 2147483647, binary: 1111111111111111111111111111111 maxneg, int: -2147483648, binary: 10000000000000000000000000000000 i, int: -1172028779, binary: 10111010001001000100001010010101 ~i, int: 1172028778, binary: 1000101110110111011110101101010 -i, int: 1172028779, binary: 1000101110110111011110101101011 j, int: 1717241110, binary: 1100110010110110000010100010110 i & j, int: 570425364, binary: 100010000000000000000000010100 i | j, int: -25213033, binary: 11111110011111110100011110010111 i ^ j, int: -595638397, binary: 11011100011111110100011110000011 78 Thinking in Java Bruce Eckel i << 5, int: 1149784736, binary: 1000100100010000101001010100000 i >> 5, int: -36625900, binary: 11111101110100010010001000010100 (~i) >> 5, int: 36625899, binary: 10001011101101110111101011 i >>> 5, int: 97591828, binary: 101110100010010001000010100 (~i) >>> 5, int: 36625899, binary: 10001011101101110111101011 ... *///:~ The two methods at the end, printBinaryInt( ) and printBinaryLong( ), take an int or a long, respectively, and print it out in binary format along with a descriptive string. As well as demonstrating the effect of all the bitwise operators for int and long, this example also shows the minimum, maximum, +1, and -1 values for int and long so you can see what they look like. Note that the high bit represents the sign: 0 means positive and 1 means negative. The output for the int portion is displayed above. The binary representation of the numbers is referred to as signed twos complement. Exercise 11: (3) Start with a number that has a binary one in the most significant position (hint: Use a hexadecimal constant). Using the signed right-shift operator, right shift it all the way through all of its binary positions, each time displaying the result using Integer.toBinaryString( ). Exercise 12: (3) Start with a number that is all binary ones. Left shift it, then use the unsigned right-shift operator to right shift through all of its binary positions, each time displaying the result using Integer.toBinaryString( ). Exercise 13: (1) Write a method that displays char values in binary form. Demonstrate it using several different characters. Ternary if-else operator The ternary operator, also called the conditional operator, is unusual because it has three operands. It is truly an operator because it produces a value, unlike the ordinary if-else statement that you’ll see in the next section of this chapter. The expression is of the form: boolean-exp ? value0 : value1 If boolean-exp evaluates to true, value0 is evaluated, and its result becomes the value produced by the operator. If boolean-exp is false, value1 is evaluated and its result becomes the value produced by the operator. Of course, you could use an ordinary if-else statement (described later), but the ternary operator is much terser. Although C (where this operator originated) prides itself on being a terse language, and the ternary operator might have been introduced partly for efficiency, you should be somewhat wary of using it on an everyday basis—it’s easy to produce unreadable code. The conditional operator is different from if-else because it produces a value. Here’s an example comparing the two: //: operators/TernaryIfElse.java Operators 79 import static net.mindview.util.Print.*; public class TernaryIfElse { static int ternary(int i) { return i < 10 ? i * 100 : i * 10; } static int standardIfElse(int i) { if(i < 10) return i * 100; else return i * 10; } public static void main(String[] args) { print(ternary(9)); print(ternary(10)); print(standardIfElse(9)); print(standardIfElse(10)); } } /* Output: 900 100 900 100 *///:~ You can see that this code in ternary( ) is more compact than what you’d need to write without the ternary operator, in standardIfElse( ). However, standardIfElse( ) is easier to understand, and doesn’t require a lot more typing. So be sure to ponder your reasons when choosing the ternary operator—it’s generally warranted when you’re setting a variable to one of two values. String operator + and += There’s one special usage of an operator in Java: The + and += operators can be used to concatenate strings, as you’ve already seen. It seems a natural use of these operators even though it doesn’t fit with the traditional way that they are used. This capability seemed like a good idea in C++, so operator overloading was added to C++ to allow the C++ programmer to add meanings to almost any operator. Unfortunately, operator overloading combined with some of the other restrictions in C++ turns out to be a fairly complicated feature for programmers to design into their classes. Although operator overloading would have been much simpler to implement in Java than it was in C++ (as has been demonstrated in the C# language, which does have straightforward operator overloading), this feature was still considered too complex, so Java programmers cannot implement their own overloaded operators like C++ and C# programmers can. The use of the String operators has some interesting behavior. If an expression begins with a String, then all operands that follow must be Strings (remember that the compiler automatically turns a double-quoted sequence of characters into a String): //: operators/StringOperators.java import static net.mindview.util.Print.*; public class StringOperators { public static void main(String[] args) { int x = 0, y = 1, z = 2; String s = "x, y, z "; print(s + x + y + z); print(x + " " + s); // Converts x to a String 80 Thinking in Java Bruce Eckel s += "(summed) = "; // Concatenation operator print(s + (x + y + z)); print("" + x); // Shorthand for Integer.toString() } } /* Output: x, y, z 012 0 x, y, z x, y, z (summed) = 3 0 *///:~ Note that the output from the first print statement is ‘o12’ instead of just ‘3’, which is what you’d get if it was summing the integers. This is because the Java compiler converts x, y, and z into their String representations and concatenates those strings, instead of adding them together first. The second print statement converts the leading variable into a String, so the string conversion does not depend on what comes first. Finally, you see the use of the += operator to append a string to s, and the use of parentheses to control the order of evaluation of the expression so that the ints are actually summed before they are displayed. Notice the last example in main( ): you will sometimes see an empty String followed by a + and a primitive as a way to perform the conversion without calling the more cumbersome explicit method (Integer.toString( ), in this case). Common pitfalls when using operators One of the pitfalls when using operators is attempting to leave out the parentheses when you are even the least bit uncertain about how an expression will evaluate. This is still true in Java. An extremely common error in C and C++ looks like this: while(x = y) { // .... } The programmer was clearly trying to test for equivalence (==) rather than do an assignment. In C and C++ the result of this assignment will always be true if y is nonzero, and you’ll probably get an infinite loop. In Java, the result of this expression is not a boolean, but the compiler expects a boolean and won’t convert from an int, so it will conveniently give you a compile-time error and catch the problem before you ever try to run the program. So the pitfall never happens in Java. (The only time you won’t get a compiletime error is when x and y are boolean, in which case x = y is a legal expression, and in the preceding example, probably an error.) A similar problem in C and C++ is using bitwise AND and OR instead of the logical versions. Bitwise AND and OR use one of the characters (& or |) while logical AND and OR use two (&& and ||). Just as with = and ==, it’s easy to type just one character instead of two. In Java, the compiler again prevents this, because it won’t let you cavalierly use one type where it doesn’t belong. Casting operators The word cast is used in the sense of “casting into a mold.” Java will automatically change one type of data into another when appropriate. For instance, if you assign an integral value to a floating point variable, the compiler will automatically convert the int to a float. Casting Operators 81 allows you to make this type conversion explicit, or to force it when it wouldn’t normally happen. To perform a cast, put the desired data type inside parentheses to the left of any value. You can see this in the following example: //: operators/Casting.java public class Casting { public static void main(String[] args) { int i = 200; long lng = (long)i; lng = i; // "Widening," so cast not really required long lng2 = (long)200; lng2 = 200; // A "narrowing conversion": i = (int)lng2; // Cast required } } ///:~ As you can see, it’s possible to perform a cast on a numeric value as well as on a variable. Notice that you can introduce superfluous casts; for example, the compiler will automatically promote an int value to a long when necessary. However, you are allowed to use superfluous casts to make a point or to clarify your code. In other situations, a cast may be essential just to get the code to compile. In C and C++, casting can cause some headaches. In Java, casting is safe, with the exception that when you perform a so-called narrowing conversion (that is, when you go from a data type that can hold more information to one that doesn’t hold as much), you run the risk of losing information. Here the compiler forces you to use a cast, in effect saying, “This can be a dangerous thing to do—if you want me to do it anyway you must make the cast explicit.” With a widening conversion an explicit cast is not needed, because the new type will more than hold the information from the old type so that no information is ever lost. Java allows you to cast any primitive type to any other primitive type, except for boolean, which doesn’t allow any casting at all. Class types do not allow casting. To convert one to the other, there must be special methods. (You’ll find out later in this book that objects can be cast within a family of types; an Oak can be cast to a Tree and vice versa, but not to a foreign type such as a Rock.) Truncation and rounding When you are performing narrowing conversions, you must pay attention to issues of truncation and rounding. For example, if you cast from a floating point value to an integral value, what does Java do? For example, if you have the value 29.7 and you cast it to an int, is the resulting value 30 or 29? The answer to this can be seen in this example: //: operators/CastingNumbers.java // What happens when you cast a float // or double to an integral value? import static net.mindview.util.Print.*; public class CastingNumbers { public static void main(String[] args) { double above = 0.7, below = 0.4; float fabove = 0.7f, fbelow = 0.4f; print("(int)above: " + (int)above); print("(int)below: " + (int)below); print("(int)fabove: " + (int)fabove); 82 Thinking in Java Bruce Eckel print("(int)fbelow: " + (int)fbelow); } } /* Output: (int)above: 0 (int)below: 0 (int)fabove: 0 (int)fbelow: 0 *///:~ So the answer is that casting from a float or double to an integral value always truncates the number. If instead you want the result to be rounded, use the round( ) methods in java.lang.Math: //: operators/RoundingNumbers.java // Rounding floats and doubles. import static net.mindview.util.Print.*; public class RoundingNumbers { public static void main(String[] args) { double above = 0.7, below = 0.4; float fabove = 0.7f, fbelow = 0.4f; print("Math.round(above): " + Math.round(above)); print("Math.round(below): " + Math.round(below)); print("Math.round(fabove): " + Math.round(fabove)); print("Math.round(fbelow): " + Math.round(fbelow)); } } /* Output: Math.round(above): 1 Math.round(below): 0 Math.round(fabove): 1 Math.round(fbelow): 0 *///:~ Since the round( ) is part of java.lang, you don’t need an extra import to use it. Promotion You’ll discover that if you perform any mathematical or bitwise operations on primitive data types that are smaller than an int (that is, char, byte, or short), those values will be promoted to int before performing the operations, and the resulting value will be of type int. So if you want to assign back into the smaller type, you must use a cast. (And, since you’re assigning back into a smaller type, you might be losing information.) In general, the largest data type in an expression is the one that determines the size of the result of that expression; if you multiply a float and a double, the result will be double; if you add an int and a long, the result will be long. Java has no “sizeof” In C and C++, the sizeof( ) operator tells you the number of bytes allocated for data items. The most compelling reason for sizeof( ) in C and C++ is for portability. Different data types might be different sizes on different machines, so the programmer must discover how big those types are when performing operations that are sensitive to size. For example, one computer might store integers in 32 bits, whereas another might store integers as 16 bits. Programs could store larger values in integers on the first machine. As you might imagine, portability is a huge headache for C and C++ programmers. Operators 83 Java does not need a sizeof( ) operator for this purpose, because all the data types are the same size on all machines. You do not need to think about portability on this level—it is designed into the language. A compendium of operators The following example shows which primitive data types can be used with particular operators. Basically, it is the same example repeated over and over, but using different primitive data types. The file will compile without error because the lines that fail are commented out with a //!. //: operators/AllOps.java // Tests all the operators on all the primitive data types // to show which ones are accepted by the Java compiler. public class AllOps { // To accept the results of a boolean test: void f(boolean b) {} void boolTest(boolean x, boolean y) { // Arithmetic operators: //! x = x * y; //! x = x / y; //! x = x % y; //! x = x + y; //! x = x - y; //! x++; //! x--; //! x = +y; //! x = -y; // Relational and logical: //! f(x > y); //! f(x >= y); //! f(x < y); //! f(x <= y); f(x == y); f(x != y); f(!y); x = x && y; x = x || y; // Bitwise operators: //! x = ~y; x = x & y; x = x | y; x = x ^ y; //! x = x << 1; //! x = x >> 1; //! x = x >>> 1; // Compound assignment: //! x += y; //! x -= y; //! x *= y; //! x /= y; //! x %= y; //! x <<= 1; //! x >>= 1; //! x >>>= 1; x &= y; x ^= y; x |= y; // Casting: 84 Thinking in Java Bruce Eckel //! char c = (char)x; //! byte b = (byte)x; //! short s = (short)x; //! int i = (int)x; //! long l = (long)x; //! float f = (float)x; //! double d = (double)x; } void charTest(char x, char y) { // Arithmetic operators: x = (char)(x * y); x = (char)(x / y); x = (char)(x % y); x = (char)(x + y); x = (char)(x - y); x++; x--; x = (char)+y; x = (char)-y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x= (char)~y; x = (char)(x & y); x = (char)(x | y); x = (char)(x ^ y); x = (char)(x << 1); x = (char)(x >> 1); x = (char)(x >>> 1); // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean bl = (boolean)x; byte b = (byte)x; short s = (short)x; int i = (int)x; long l = (long)x; float f = (float)x; double d = (double)x; } void byteTest(byte x, byte y) { // Arithmetic operators: x = (byte)(x* y); x = (byte)(x / y); x = (byte)(x % y); Operators 85 x = (byte)(x + y); x = (byte)(x - y); x++; x--; x = (byte)+ y; x = (byte)- y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = (byte)~y; x = (byte)(x & y); x = (byte)(x | y); x = (byte)(x ^ y); x = (byte)(x << 1); x = (byte)(x >> 1); x = (byte)(x >>> 1); // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean bl = (boolean)x; char c = (char)x; short s = (short)x; int i = (int)x; long l = (long)x; float f = (float)x; double d = (double)x; } void shortTest(short x, short y) { // Arithmetic operators: x = (short)(x * y); x = (short)(x / y); x = (short)(x % y); x = (short)(x + y); x = (short)(x - y); x++; x--; x = (short)+y; x = (short)-y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); 86 Thinking in Java Bruce Eckel //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = (short)~y; x = (short)(x & y); x = (short)(x | y); x = (short)(x ^ y); x = (short)(x << 1); x = (short)(x >> 1); x = (short)(x >>> 1); // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean bl = (boolean)x; char c = (char)x; byte b = (byte)x; int i = (int)x; long l = (long)x; float f = (float)x; double d = (double)x; } void intTest(int x, int y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = ~y; x = x & y; x = x | y; x = x ^ y; x = x << 1; x = x >> 1; x = x >>> 1; // Compound assignment: x += y; Operators 87 x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean bl = (boolean)x; char c = (char)x; byte b = (byte)x; short s = (short)x; long l = (long)x; float f = (float)x; double d = (double)x; } void longTest(long x, long y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = ~y; x = x & y; x = x | y; x = x ^ y; x = x << 1; x = x >> 1; x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean bl = (boolean)x; char c = (char)x; 88 Thinking in Java Bruce Eckel byte b = (byte)x; short s = (short)x; int i = (int)x; float f = (float)x; double d = (double)x; } void floatTest(float x, float y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: //! x = ~y; //! x = x & y; //! x = x | y; //! x = x ^ y; //! x = x << 1; //! x = x >> 1; //! x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; //! x <<= 1; //! x >>= 1; //! x >>>= 1; //! x &= y; //! x ^= y; //! x |= y; // Casting: //! boolean bl = (boolean)x; char c = (char)x; byte b = (byte)x; short s = (short)x; int i = (int)x; long l = (long)x; double d = (double)x; } void doubleTest(double x, double y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; Operators 89 x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: //! x = ~y; //! x = x & y; //! x = x | y; //! x = x ^ y; //! x = x << 1; //! x = x >> 1; //! x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; //! x <<= 1; //! x >>= 1; //! x >>>= 1; //! x &= y; //! x ^= y; //! x |= y; // Casting: //! boolean bl = (boolean)x; char c = (char)x; byte b = (byte)x; short s = (short)x; int i = (int)x; long l = (long)x; float f = (float)x; } } ///:~ Note that boolean is quite limited. You can assign to it the values true and false, and you can test it for truth or falsehood, but you cannot add booleans or perform any other type of operation on them. In char, byte, and short, you can see the effect of promotion with the arithmetic operators. Each arithmetic operation on any of those types produces an int result, which must be explicitly cast back to the original type (a narrowing conversion that might lose information) to assign back to that type. With int values, however, you do not need to cast, because everything is already an int. Don’t be lulled into thinking everything is safe, though. If you multiply two ints that are big enough, you’ll overflow the result. The following example demonstrates this: //: operators/Overflow.java // Surprise! Java lets you overflow. public class Overflow { public static void main(String[] args) { 90 Thinking in Java Bruce Eckel int big = Integer.MAX_VALUE; System.out.println("big = " + big); int bigger = big * 4; System.out.println("bigger = " + bigger); } } /* Output: big = 2147483647 bigger = -4 *///:~ You get no errors or warnings from the compiler, and no exceptions at run time. Java is good, but it’s not that good. Compound assignments do not require casts for char, byte, or short, even though they are performing promotions that have the same results as the direct arithmetic operations. On the other hand, the lack of the cast certainly simplifies the code. You can see that, with the exception of boolean, any primitive type can be cast to any other primitive type. Again, you must be aware of the effect of a narrowing conversion when casting to a smaller type; otherwise, you might unknowingly lose information during the cast. Exercise 14: (3) Write a method that takes two String arguments and uses all the boolean comparisons to compare the two Strings and print the results. For the == and !=, also perform the equals( ) test. In main( ), call your method with some different String objects. Summary If you’ve had experience with any languages that use C-like syntax, you can see that the operators in Java are so similar that there is virtually no learning curve. If you found this chapter challenging, make sure you view the multimedia presentation Thinking in C, available at www.MindView.net. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Operators 91 Controlling Execution Like a sentient creature, a program must manipulate its world and make choices during execution. In Java you make choices with execution control statements. Java uses all of C’s execution control statements, so if you’ve programmed with C or C++, then most of what you see will be familiar. Most procedural programming languages have some kind of control statements, and there is often overlap among languages. In Java, the keywords include if-else, while, do-while, for, return, break, and a selection statement called switch. Java does not, however, support the much-maligned goto (which can still be the most expedient way to solve certain types of problems). You can still do a goto-like jump, but it is much more constrained than a typical goto. true and false All conditional statements use the truth or falsehood of a conditional expression to determine the execution path. An example of a conditional expression is a == b. This uses the conditional operator == to see if the value of a is equivalent to the value of b. The expression returns true or false. Any of the relational operators you’ve seen in the previous chapter can be used to produce a conditional statement. Note that Java doesn’t allow you to use a number as a boolean, even though it’s allowed in C and C++ (where truth is nonzero and falsehood is zero). If you want to use a non-boolean in a boolean test, such as if(a), you must first convert it to a boolean value by using a conditional expression, such as if(a != 0). if-else The if-else statement is the most basic way to control program flow. The else is optional, so you can use if in two forms: if(Boolean-expression) statement or if(Boolean-expression) statement else statement The Boolean-expression must produce a boolean result. The statement is either a simple statement terminated by a semicolon, or a compound statement, which is a group of simple statements enclosed in braces. Whenever the word “statement” is used, it always implies that the statement can be simple or compound. As an example of if-else, here is a test( ) method that will tell you whether a guess is above, below, or equivalent to a target number: //: control/IfElse.java import static net.mindview.util.Print.*; public class IfElse { static int result = 0; static void test(int testval, int target) { if(testval > target) result = +1; else if(testval < target) result = -1; else result = 0; // Match } public static void main(String[] args) { test(10, 5); print(result); test(5, 10); print(result); test(5, 5); print(result); } } /* Output: 1 -1 0 *///:~ In the middle of test( ), you’ll also see an “else if,” which is not a new keyword but just an else followed by a new if statement. Although Java, like C and C++ before it, is a “free-form” language, it is conventional to indent the body of a control flow statement so the reader can easily determine where it begins and ends. Iteration Looping is controlled by while, do-while and for, which are sometimes classified as iteration statements. A statement repeats until the controlling Boolean-expression evaluates to false. The form for a while loop is: while(Boolean-expression) statement The Boolean-expression is evaluated once at the beginning of the loop and again before each further iteration of the statement. Here’s a simple example that generates random numbers until a particular condition is met: //: control/WhileTest.java // Demonstrates the while loop. public class WhileTest { static boolean condition() { boolean result = Math.random() < 0.99; System.out.print(result + ", "); return result; } public static void main(String[] args) { while(condition()) System.out.println("Inside ‘while’"); System.out.println("Exited ‘while’"); } } /* (Execute to see output) *///:~ 94 Thinking in Java Bruce Eckel The condition( ) method uses the static method random( ) in the Math library, which generates a double value between 0 and 1. (It includes 0, but not 1.) The result value comes from the comparison operator <, which produces a boolean result. If you print a boolean value, you automatically get the appropriate string “true” or “false.” The conditional expression for the while says: “repeat the statements in the body as long as condition( ) returns true.” do-while The form for do-while is do statement while(Boolean-expression); The sole difference between while and do-while is that the statement of the do-while always executes at least once, even if the expression evaluates to false the first time. In a while, if the conditional is false the first time the statement never executes. In practice, dowhile is less common than while. for A for loop is perhaps the most commonly used form of iteration. This loop performs initialization before the first iteration. Then it performs conditional testing and, at the end of each iteration, some form of “stepping.” The form of the for loop is: for(initialization; Boolean-expression; step) statement Any of the expressions initialization, Boolean-expression or step can be empty. The expression is tested before each iteration, and as soon as it evaluates to false, execution will continue at the line following the for statement. At the end of each loop, the step executes. for loops are usually used for “counting” tasks: //: control/ListCharacters.java // Demonstrates "for" loop by listing // all the lowercase ASCII letters. public class ListCharacters { public static void main(String[] args) { for(char c = 0; c < 128; c++) if(Character.isLowerCase(c)) System.out.println("value: " + (int)c + " character: " + c); } } /* Output: value: 97 character: a value: 98 character: b value: 99 character: c value: 100 character: d value: 101 character: e value: 102 character: f value: 103 character: g value: 104 character: h value: 105 character: i value: 106 character: j ... Controlling Execution 95 *///:~ Note that the variable c is defined at the point where it is used, inside the control expression of the for loop, rather than at the beginning of main( ). The scope of c is the statement controlled by the for. This program also uses the java.lang.Character “wrapper” class, which not only wraps the primitive char type in an object, but also provides other utilities. Here, the static isLowerCase( ) method is used to detect whether the character in question is a lowercase letter. Traditional procedural languages like C require that all variables be defined at the beginning of a block so that when the compiler creates a block, it can allocate space for those variables. In Java and C++, you can spread your variable declarations throughout the block, defining them at the point that you need them. This allows a more natural coding style and makes code easier to understand. Exercise 1: (1) Write a program that prints values from 1 to 100. Exercise 2: (2) Write a program that generates 25 random int values. For each value, use an if-else statement to classify it as greater than, less than, or equal to a second randomly generated value. Exercise 3: (1) Modify Exercise 2 so that your code is surrounded by an “infinite” while loop. It will then run until you interrupt it from the keyboard (typically by pressing ControlC). Exercise 4: (3) Write a program that uses two nested for loops and the modulus operator (%) to detect and print prime numbers (integral numbers that are not evenly divisible by any other numbers except for themselves and 1). Exercise 5: (4) Repeat Exercise 10 from the previous chapter, using the ternary operator and a bitwise test to display the ones and zeroes, instead of Integer.toBinaryString( ). The comma operator Earlier in this chapter I stated that the comma operator (not the comma separator, which is used to separate definitions and method arguments) has only one use in Java: in the control expression of a for loop. In both the initialization and step portions of the control expression, you can have a number of statements separated by commas, and those statements will be evaluated sequentially. Using the comma operator, you can define multiple variables within a for statement, but they must be of the same type: //: control/CommaOperator.java public class CommaOperator { public static void main(String[] args) { for(int i = 1, j = i + 10; i < 5; i++, j = i * 2) { System.out.println("i = " + i + " j = " + j); } } } /* Output: i = 1 j = 11 i=2j=4 96 Thinking in Java Bruce Eckel i=3j=6 i=4j=8 *///:~ The int definition in the for statement covers both i and j. The initialization portion can have any number of definitions of one type. The ability to define variables in a control expression is limited to the for loop. You cannot use this approach with any of the other selection or iteration statements. You can see that in both the initialization and step portions, the statements are evaluated in sequential order. Foreach syntax Java SE5 introduces a new and more succinct for syntax, for use with arrays and containers (you’ll learn more about these in the Arrays and Containers in Depth chapter). This is often called the foreach syntax, and it means that you don’t have to create an int to count through a sequence of items—the foreach produces each item for you, automatically. For example, suppose you have an array of float and you’d like to select each element in that array: //: control/ForEachFloat.java import java.util.*; public class ForEachFloat { public static void main(String[] args) { Random rand = new Random(47); float f[] = new float[10]; for(int i = 0; i < 10; i++) f[i] = rand.nextFloat(); for(float x : f) System.out.println(x); } } /* Output: 0.72711575 0.39982635 0.5309454 0.0534122 0.16020656 0.57799757 0.18847865 0.4170137 0.51660204 0.73734957 *///:~ The array is populated using the old for loop, because it must be accessed with an index. You can see the foreach syntax in the line: for(float x : f) { This defines a variable x of type float and sequentially assigns each element of f to x. Any method that returns an array is a candidate for use with foreach. For example, the String class has a method toCharArray( ) that returns an array of char, so you can easily iterate through the characters in a string: Controlling Execution 97 //: control/ForEachString.java public class ForEachString { public static void main(String[] args) { for(char c : "An African Swallow".toCharArray() ) System.out.print(c + " "); } } /* Output: An African Swallow *///:~ As you’ll see in the Holding Your Objects chapter, foreach will also work with any object that is Iterable. Many for statements involve stepping through a sequence of integral values, like this: for(int i = 0; i < 100; i++) For these, the foreach syntax won’t work unless you want to create an array of int first. To simplify this task, I’ve created a method called range( ) in net.mindview.util.Range that automatically generates the appropriate array. My intent is for range( ) to be used as a static import: //: control/ForEachInt.java import static net.mindview.util.Range.*; import static net.mindview.util.Print.*; public class ForEachInt { public static void main(String[] args) { for(int i : range(10)) // 0..9 printnb(i + " "); print(); for(int i : range(5, 10)) // 5..9 printnb(i + " "); print(); for(int i : range(5, 20, 3)) // 5..20 step 3 printnb(i + " "); print(); } } /* Output: 0123456789 56789 5 8 11 14 17 *///:~ The range( ) method has been overloaded, which means the same method name can be used with different argument lists (you’ll learn about overloading soon). The first overloaded form of range( ) just starts at zero and produces values up to but not including the top end of the range. The second form starts at the first value and goes until one less than the second, and the third form has a step value so it increases by that value. range( ) is a very simple version of what’s called a generator, which you’ll see later in the book. Note that although range( ) allows the use of the foreach syntax in more places, and thus arguably increases readability, it is a little less efficient, so if you are tuning for performance you may want to use a profiler, which is a tool that measures the performance of your code. You’ll note the use of printnb( ) in addition to print( ). The printnb( ) method does not emit a newline, so it allows you to output a line in pieces. 98 Thinking in Java Bruce Eckel The foreach syntax not only saves time when typing in code. More importantly, it is far easier to read and says what you are trying to do (get each element of the array) rather than giving the details of how you are doing it (“I’m creating this index so I can use it to select each of the array elements.”). The foreach syntax will be used whenever possible in this book. return Several keywords represent unconditional branching, which simply means that the branch happens without any test. These include return, break, continue, and a way to jump to a labeled statement which is similar to the goto in other languages. The return keyword has two purposes: It specifies what value a method will return (if it doesn’t have a void return value) and it causes the current method to exit, returning that value. The preceding test( ) method can be rewritten to take advantage of this: //: control/IfElse2.java import static net.mindview.util.Print.*; public class IfElse2 { static int test(int testval, int target) { if(testval > target) return +1; else if(testval < target) return -1; else return 0; // Match } public static void main(String[] args) { print(test(10, 5)); print(test(5, 10)); print(test(5, 5)); } } /* Output: 1 -1 0 *///:~ There’s no need for else, because the method will not continue after executing a return. If you do not have a return statement in a method that returns void, there’s an implicit return at the end of that method, so it’s not always necessary to include a return statement. However, if your method states it will return anything other than void, you must ensure every code path will return a value. Exercise 6: (2) Modify the two test( ) methods in the previous two programs so that they take two extra arguments, begin and end, and so that testval is tested to see if it is within the range between (and including) begin and end. break and continue You can also control the flow of the loop inside the body of any of the iteration statements by using break and continue. break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to the beginning of the loop to begin the next iteration. Controlling Execution 99 This program shows examples of break and continue within for and while loops: //: control/BreakAndContinue.java // Demonstrates break and continue keywords. import static net.mindview.util.Range.*; public class BreakAndContinue { public static void main(String[] args) { for(int i = 0; i < 100; i++) { if(i == 74) break; // Out of for loop if(i % 9 != 0) continue; // Next iteration System.out.print(i + " "); } System.out.println(); // Using foreach: for(int i : range(100)) { if(i == 74) break; // Out of for loop if(i % 9 != 0) continue; // Next iteration System.out.print(i + " "); } System.out.println(); int i = 0; // An "infinite loop": while(true) { i++; int j = i * 27; if(j == 1269) break; // Out of loop if(i % 10 != 0) continue; // Top of loop System.out.print(i + " "); } } } /* Output: 0 9 18 27 36 45 54 63 72 0 9 18 27 36 45 54 63 72 10 20 30 40 *///:~ In the for loop, the value of i never gets to 100 because the break statement breaks out of the loop when i is 74. Normally, you’d use a break like this only if you didn’t know when the terminating condition was going to occur. The continue statement causes execution to go back to the top of the iteration loop (thus incrementing i) whenever i is not evenly divisible by 9. When it is, the value is printed. The second for loop shows the use of foreach, and that it produces the same results. Finally, you see an “infinite” while loop that would, in theory, continue forever. However, inside the loop there is a break statement that will break out of the loop. In addition, you’ll see that the continue statement moves control back to the top of the loop without completing anything after that continue statement. (Thus printing happens in the second loop only when the value of i is divisible by 10.) In the output, the value 0 is printed, because 0 % 9 produces 0. A second form of the infinite loop is for(;;). The compiler treats both while(true) and for(;;) in the same way, so whichever one you use is a matter of programming taste. Exercise 7: (1) Modify Exercise 1 so that the program exits by using the break keyword at value 99. Try using return instead. 100 Thinking in Java Bruce Eckel The infamous “goto” The goto keyword has been present in programming languages from the beginning. Indeed, goto was the genesis of program control in assembly language: “If condition A, then jump here; otherwise, jump there.” If you read the assembly code that is ultimately generated by virtually any compiler, you’ll see that program control contains many jumps (the Java compiler produces its own “assembly code,” but this code is run by the Java Virtual Machine rather than directly on a hardware CPU). A goto is a jump at the source-code level, and that’s what brought it into disrepute. If a program will always jump from one point to another, isn’t there some way to reorganize the code so the flow of control is not so jumpy? goto fell into true disfavor with the publication of the famous “Goto considered harmful” paper by Edsger Dijkstra, and since then gotobashing has been a popular sport, with advocates of the cast-out keyword scurrying for cover. As is typical in situations like this, the middle ground is the most fruitful. The problem is not the use of goto, but the overuse of goto; in rare situations goto is actually the best way to structure control flow. Although goto is a reserved word in Java, it is not used in the language; Java has no goto. However, it does have something that looks a bit like a jump tied in with the break and continue keywords. It’s not a jump but rather a way to break out of an iteration statement. The reason it’s often thrown in with discussions of goto is because it uses the same mechanism: a label. A label is an identifier followed by a colon, like this: label1: The only place a label is useful in Java is right before an iteration statement. And that means right before—it does no good to put any other statement between the label and the iteration. And the sole reason to put a label before an iteration is if you’re going to nest another iteration or a switch (which you’ll learn about shortly) inside it. That’s because the break and continue keywords will normally interrupt only the current loop, but when used with a label, they’ll interrupt the loops up to where the label exists: label1: outer-iteration { inner-iteration { //... break; // (1) //... continue; // (2) //... continue label1; // (3) //... break label1; // (4) } } In (1), the break breaks out of the inner iteration and you end up in the outer iteration. In (2), the continue moves back to the beginning of the inner iteration. But in (3), the continue label1 breaks out of the inner iteration and the outer iteration, all the way back to label1. Then it does in fact continue the iteration, but starting at the outer iteration. In (4), the break label1 also breaks all the way out to label1, but it does not reenter the iteration. It actually does break out of both iterations. Controlling Execution 101 Here is an example using for loops: //: control/LabeledFor.java // For loops with "labeled break" and "labeled continue." import static net.mindview.util.Print.*; public class LabeledFor { public static void main(String[] args) { int i = 0; outer: // Can’t have statements here for(; true ;) { // infinite loop inner: // Can’t have statements here for(; i < 10; i++) { print("i = " + i); if(i == 2) { print("continue"); continue; } if(i == 3) { print("break"); i++; // Otherwise i never // gets incremented. break; } if(i == 7) { print("continue outer"); i++; // Otherwise i never // gets incremented. continue outer; } if(i == 8) { print("break outer"); break outer; } for(int k = 0; k < 5; k++) { if(k == 3) { print("continue inner"); continue inner; } } } } // Can’t break or continue to labels here } } /* Output: i=0 continue inner i=1 continue inner i=2 continue i=3 break i=4 continue inner i=5 continue inner i=6 continue inner i=7 continue outer i=8 break outer 102 Thinking in Java Bruce Eckel *///:~ Note that break breaks out of the for loop, and that the increment expression doesn’t occur until the end of the pass through the for loop. Since break skips the increment expression, the increment is performed directly in the case of i == 3. The continue outer statement in the case of i == 7 also goes to the top of the loop and also skips the increment, so it too is incremented directly. If not for the break outer statement, there would be no way to get out of the outer loop from within an inner loop, since break by itself can break out of only the innermost loop. (The same is true for continue.) Of course, in the cases where breaking out of a loop will also exit the method, you can simply use a return. Here is a demonstration of labeled break and continue statements with while loops: //: control/LabeledWhile.java // While loops with "labeled break" and "labeled continue." import static net.mindview.util.Print.*; public class LabeledWhile { public static void main(String[] args) { int i = 0; outer: while(true) { print("Outer while loop"); while(true) { i++; print("i = " + i); if(i == 1) { print("continue"); continue; } if(i == 3) { print("continue outer"); continue outer; } if(i == 5) { print("break"); break; } if(i == 7) { print("break outer"); break outer; } } } } } /* Output: Outer while loop i=1 continue i=2 i=3 continue outer Outer while loop i=4 i=5 break Outer while loop Controlling Execution 103 i=6 i=7 break outer *///:~ The same rules hold true for while: 1. A plain continue goes to the top of the innermost loop and continues. 2. A labeled continue goes to the label and reenters the loop right after that label. 3. A break “drops out of the bottom” of the loop. 4. A labeled break drops out of the bottom of the end of the loop denoted by the label. It’s important to remember that the only reason to use labels in Java is when you have nested loops and you want to break or continue through more than one nested level. In Dijkstra’s “Goto considered harmful” paper, what he specifically objected to was the labels, not the goto. He observed that the number of bugs seems to increase with the number of labels in a program, and that labels and gotos make programs difficult to analyze. Note that Java labels don’t suffer from this problem, since they are constrained in their placement and can’t be used to transfer control in an ad hoc manner. It’s also interesting to note that this is a case where a language feature is made more useful by restricting the power of the statement. switch The switch is sometimes called a selection statement. The switch statement selects from among pieces of code based on the value of an integral expression. Its general form is: switch(integral-selector) { case integral-value1 : statement; break; case integral-value2 : statement; break; case integral-value3 : statement; break; case integral-value4 : statement; break; case integral-value5 : statement; break; // ... default: statement; } Integral-selector is an expression that produces an integral value. The switch compares the result of integral-selector to each integral-value. If it finds a match, the corresponding statement (a single statement or multiple statements; braces are not required) executes. If no match occurs, the default statement executes. You will notice in the preceding definition that each case ends with a break, which causes execution to jump to the end of the switch body. This is the conventional way to build a switch statement, but the break is optional. If it is missing, the code for the following case statements executes until a break is encountered. Although you don’t usually want this kind of behavior, it can be useful to an experienced programmer. Note that the last statement, following the default, doesn’t have a break because the execution just falls through to where the break would have taken it anyway. You could put a break at the end of the default statement with no harm if you considered it important for style’s sake. The switch statement is a clean way to implement multiway selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value, such as int or char. If you want to use, for example, a string or a floating 104 Thinking in Java Bruce Eckel point number as a selector, it won’t work in a switch statement. For non-integral types, you must use a series of if statements. At the end of the next chapter, you’ll see that Java SE5’s new enum feature helps ease this restriction, as enums are designed to work nicely with switch. Here’s an example that creates letters randomly and determines whether they’re vowels or consonants: //: control/VowelsAndConsonants.java // Demonstrates the switch statement. import java.util.*; import static net.mindview.util.Print.*; public class VowelsAndConsonants { public static void main(String[] args) { Random rand = new Random(47); for(int i = 0; i < 100; i++) { int c = rand.nextInt(26) + ‘a’; printnb((char)c + ", " + c + ": "); switch(c) { case ‘a’: case ‘e’: case ‘i’: case ‘o’: case ‘u’: print("vowel"); break; case ‘y’: case ‘w’: print("Sometimes a vowel"); break; default: print("consonant"); } } } } /* Output: y, 121: Sometimes a vowel n, 110: consonant z, 122: consonant b, 98: consonant r, 114: consonant n, 110: consonant y, 121: Sometimes a vowel g, 103: consonant c, 99: consonant f, 102: consonant o, 111: vowel w, 119: Sometimes a vowel z, 122: consonant ... *///:~ Since Random.nextInt(26) generates a value between 0 and 26, you need only add an offset of ‘a’ to produce the lowercase letters. The single-quoted characters in the case statements also produce integral values that are used for comparison. Notice how the cases can be “stacked” on top of each other to provide multiple matches for a particular piece of code. You should also be aware that it’s essential to put the break statement at the end of a particular case; otherwise, control will simply drop through and continue processing on the next case. Controlling Execution 105 In the statement: int c = rand.nextInt(26) + ‘a’; Random.nextInt( ) produces a random int value from 0 to 25, which is added to the value of ‘a’. This means that ‘a’ is automatically converted to an int to perform the addition. In order to print c as a character, it must be cast to char; otherwise, you’ll produce integral output. Exercise 8: (2) Create a switch statement that prints a message for each case, and put the switch inside a for loop that tries each case. Put a break after each case and test it, then remove the breaks and see what happens. Exercise 9: (4) A Fibonacci sequence is the sequence of numbers 1, 1, 2, 3, 5, 8, 13, 21, 34, and so on, where each number (from the third on) is the sum of the previous two. Create a method that takes an integer as an argument and displays that many Fibonacci numbers starting from the beginning, e.g., If you run java Fibonacci 5 (where Fibonacci is the name of the class) the output will be: 1, 1, 2, 3, 5. Exercise 10: (5) A vampire number has an even number of digits and is formed by multiplying a pair of numbers containing half the number of digits of the result. The digits are taken from the original number in any order. Pairs of trailing zeroes are not allowed. Examples include: 1260 = 21 * 60 1827 = 21 * 87 2187 = 27 * 81 Write a program that finds all the 4-digit vampire numbers. (Suggested by Dan Forhan.) Summary This chapter concludes the study of fundamental features that appear in most programming languages: calculation, operator precedence, type casting, and selection and iteration. Now you’re ready to begin taking steps that move you closer to the world of object-oriented programming. The next chapter will cover the important issues of initialization and cleanup of objects, followed in the subsequent chapter by the essential concept of implementation hiding. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. 106 Thinking in Java Bruce Eckel Initialization & Cleanup As the computer revolution progresses, “unsafe” programming has become one of the major culprits that makes programming expensive. Two of these safety issues are initialization and cleanup. Many C bugs occur when the programmer forgets to initialize a variable. This is especially true with libraries when users don’t know how to initialize a library component, or even that they must. Cleanup is a special problem because it’s easy to forget about an element when you’re done with it, since it no longer concerns you. Thus, the resources used by that element are retained and you can easily end up running out of resources (most notably, memory). C++ introduced the concept of a constructor, a special method automatically called when an object is created. Java also adopted the constructor, and in addition has a garbage collector that automatically releases memory resources when they’re no longer being used. This chapter examines the issues of initialization and cleanup, and their support in Java. Guaranteed initialization with the constructor You can imagine creating a method called initialize( ) for every class you write. The name is a hint that it should be called before using the object. Unfortunately, this means the user must remember to call that method. In Java, the class designer can guarantee initialization of every object by providing a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed. The next challenge is what to name this method. There are two issues. The first is that any name you use could clash with a name you might like to use as a member in the class. The second is that because the compiler is responsible for calling the constructor, it must always know which method to call. The C++ solution seems the easiest and most logical, so it’s also used in Java: The name of the constructor is the same as the name of the class. It makes sense that such a method will be called automatically during initialization. Here’s a simple class with a constructor: //: initialization/SimpleConstructor.java // Demonstration of a simple constructor. class Rock { Rock() { // This is the constructor System.out.print("Rock "); } } public class SimpleConstructor { public static void main(String[] args) { for(int i = 0; i < 10; i++) new Rock(); } } /* Output: Rock Rock Rock Rock Rock Rock Rock Rock Rock Rock *///:~ Now, when an object is created: new Rock(); storage is allocated and the constructor is called. It is guaranteed that the object will be properly initialized before you can get your hands on it. Note that the coding style of making the first letter of all methods lowercase does not apply to constructors, since the name of the constructor must match the name of the class exactly. A constructor that takes no arguments is called the default constructor. The Java documents typically use the term no-arg constructor, but “default constructor” has been in use for many years before Java appeared, so I will tend to use that. But like any method, the constructor can also have arguments to allow you to specify how an object is created. The preceding example can easily be changed so the constructor takes an argument: //: initialization/SimpleConstructor2.java // Constructors can have arguments. class Rock2 { Rock2(int i) { System.out.print("Rock " + i + " "); } } public class SimpleConstructor2 { public static void main(String[] args) { for(int i = 0; i < 8; i++) new Rock2(i); } } /* Output: Rock 0 Rock 1 Rock 2 Rock 3 Rock 4 Rock 5 Rock 6 Rock 7 *///:~ Constructor arguments provide you with a way to provide parameters for the initialization of an object. For example, if the class Tree has a constructor that takes a single integer argument denoting the height of the tree, you create a Tree object like this: Tree t = new Tree(12); // 12-foot tree If Tree(int) is your only constructor, then the compiler won’t let you create a Tree object any other way. Constructors eliminate a large class of problems and make the code easier to read. In the preceding code fragment, for example, you don’t see an explicit call to some initialize( ) method that is conceptually separate from creation. In Java, creation and initialization are unified concepts—you can’t have one without the other. The constructor is an unusual type of method because it has no return value. This is distinctly different from a void return value, in which the method returns nothing but you still have the option to make it return something else. Constructors return nothing and you don’t have an option (the new expression does return a reference to the newly created object, but the constructor itself has no return value). If there were a return value, and if you could select your own, the compiler would somehow need to know what to do with that return value. 108 Thinking in Java Bruce Eckel Exercise 1: (1) Create a class containing an uninitialized String reference. Demonstrate that this reference is initialized by Java to null. Exercise 2: (2) Create a class with a String field that is initialized at the point of definition, and another one that is initialized by the constructor. What is the difference between the two approaches? Method overloading One of the important features in any programming language is the use of names. When you create an object, you give a name to a region of storage. A method is a name for an action. You refer to all objects and methods by using names. Well-chosen names create a system that is easier for people to understand and change. It’s a lot like writing prose—the goal is to communicate with your readers. A problem arises when mapping the concept of nuance in human language onto a programming language. Often, the same word expresses a number of different meanings—it’s overloaded. This is useful, especially when it comes to trivial differences. You say, “Wash the shirt,” “Wash the car,” and “Wash the dog.” It would be silly to be forced to say, “shirtWash the shirt,” “carWash the car,” and “dogWash the dog” just so the listener doesn’t need to make any distinction about the action performed. Most human languages are redundant, so even if you miss a few words, you can still determine the meaning. You don’t need unique identifiers—you can deduce meaning from context. Most programming languages (C in particular) require you to have a unique identifier for each method (often called functions in those languages). So you could not have one function called print( ) for printing integers and another called print( ) for printing floats—each function requires a unique name. In Java (and C++), another factor forces the overloading of method names: the constructor. Because the constructor’s name is predetermined by the name of the class, there can be only one constructor name. But what if you want to create an object in more than one way? For example, suppose you build a class that can initialize itself in a standard way or by reading information from a file. You need two constructors, the default constructor and one that takes a String as an argument, which is the name of the file from which to initialize the object. Both are constructors, so they must have the same name—the name of the class. Thus, method overloading is essential to allow the same method name to be used with different argument types. And although method overloading is a must for constructors, it’s a general convenience and can be used with any method. Here’s an example that shows both overloaded constructors and overloaded methods: //: initialization/Overloading.java // Demonstration of both constructor // and ordinary method overloading. import static net.mindview.util.Print.*; class Tree { int height; Tree() { print("Planting a seedling"); height = 0; } Tree(int initialHeight) { height = initialHeight; print("Creating new Tree that is " + height + " feet tall"); Initialization & Cleanup 109 } void info() { print("Tree is " + height + " feet tall"); } void info(String s) { print(s + ": Tree is " + height + " feet tall"); } } public class Overloading { public static void main(String[] args) { for(int i = 0; i < 5; i++) { Tree t = new Tree(i); t.info(); t.info("overloaded method"); } // Overloaded constructor: new Tree(); } } /* Output: Creating new Tree that is 0 feet tall Tree is 0 feet tall overloaded method: Tree is 0 feet tall Creating new Tree that is 1 feet tall Tree is 1 feet tall overloaded method: Tree is 1 feet tall Creating new Tree that is 2 feet tall Tree is 2 feet tall overloaded method: Tree is 2 feet tall Creating new Tree that is 3 feet tall Tree is 3 feet tall overloaded method: Tree is 3 feet tall Creating new Tree that is 4 feet tall Tree is 4 feet tall overloaded method: Tree is 4 feet tall Planting a seedling *///:~ A Tree object can be created either as a seedling, with no argument, or as a plant grown in a nursery, with an existing height. To support this, there is a default constructor, and one that takes the existing height. You might also want to call the info( ) method in more than one way. For example, if you have an extra message you want printed, you can use info(String), and info( ) if you have nothing more to say. It would seem strange to give two separate names to what is obviously the same concept. Fortunately, method overloading allows you to use the same name for both. Distinguishing overloaded methods If the methods have the same name, how can Java know which method you mean? There’s a simple rule: Each overloaded method must take a unique list of argument types. If you think about this for a second, it makes sense. How else could a programmer tell the difference between two methods that have the same name, other than by the types of their arguments? Even differences in the ordering of arguments are sufficient to distinguish two methods, although you don’t normally want to take this approach because it produces difficult-tomaintain code: 110 Thinking in Java Bruce Eckel //: initialization/OverloadingOrder.java // Overloading based on the order of the arguments. import static net.mindview.util.Print.*; public class OverloadingOrder { static void f(String s, int i) { print("String: " + s + ", int: " + i); } static void f(int i, String s) { print("int: " + i + ", String: " + s); } public static void main(String[] args) { f("String first", 11); f(99, "Int first"); } } /* Output: String: String first, int: 11 int: 99, String: Int first *///:~ The two f( ) methods have identical arguments, but the order is different, and that’s what makes them distinct. Overloading with primitives A primitive can be automatically promoted from a smaller type to a larger one, and this can be slightly confusing in combination with overloading. The following example demonstrates what happens when a primitive is handed to an overloaded method: //: initialization/PrimitiveOverloading.java // Promotion of primitives and overloading. import static net.mindview.util.Print.*; public class PrimitiveOverloading { void f1(char x) { printnb("f1(char) "); } void f1(byte x) { printnb("f1(byte) "); } void f1(short x) { printnb("f1(short) "); } void f1(int x) { printnb("f1(int) "); } void f1(long x) { printnb("f1(long) "); } void f1(float x) { printnb("f1(float) "); } void f1(double x) { printnb("f1(double) "); } void f2(byte x) { printnb("f2(byte) "); } void f2(short x) { printnb("f2(short) "); } void f2(int x) { printnb("f2(int) "); } void f2(long x) { printnb("f2(long) "); } void f2(float x) { printnb("f2(float) "); } void f2(double x) { printnb("f2(double) "); } void f3(short x) { printnb("f3(short) "); } void f3(int x) { printnb("f3(int) "); } void f3(long x) { printnb("f3(long) "); } void f3(float x) { printnb("f3(float) "); } void f3(double x) { printnb("f3(double) "); } void f4(int x) { printnb("f4(int) "); } void f4(long x) { printnb("f4(long) "); } void f4(float x) { printnb("f4(float) "); } void f4(double x) { printnb("f4(double) "); } void f5(long x) { printnb("f5(long) "); } Initialization & Cleanup 111 void f5(float x) { printnb("f5(float) "); } void f5(double x) { printnb("f5(double) "); } void f6(float x) { printnb("f6(float) "); } void f6(double x) { printnb("f6(double) "); } void f7(double x) { printnb("f7(double) "); } void testConstVal() { printnb("5: "); f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5); print(); } void testChar() { char x = ‘x’; printnb("char: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } void testByte() { byte x = 0; printnb("byte: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } void testShort() { short x = 0; printnb("short: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } void testInt() { int x = 0; printnb("int: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } void testLong() { long x = 0; printnb("long: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } void testFloat() { float x = 0; printnb("float: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } void testDouble() { double x = 0; printnb("double: "); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); print(); } public static void main(String[] args) { PrimitiveOverloading p = new PrimitiveOverloading(); p.testConstVal(); p.testChar(); p.testByte(); p.testShort(); p.testInt(); p.testLong(); p.testFloat(); p.testDouble(); } } /* Output: 5: f1(int) f2(int) f3(int) f4(int) f5(long) f6(float) f7(double) char: f1(char) f2(int) f3(int) f4(int) f5(long) f6(float) f7(double) byte: f1(byte) f2(byte) f3(short) f4(int) f5(long) f6(float) f7(double) 112 Thinking in Java Bruce Eckel short: f1(short) f2(short) f3(short) f4(int) f5(long) f6(float) f7(double) int: f1(int) f2(int) f3(int) f4(int) f5(long) f6(float) f7(double) long: f1(long) f2(long) f3(long) f4(long) f5(long) f6(float) f7(double) float: f1(float) f2(float) f3(float) f4(float) f5(float) f6(float) f7(double) double: f1(double) f2(double) f3(double) f4(double) f5(double) f6(double) f7(double) *///:~ You can see that the constant value 5 is treated as an int, so if an overloaded method is available that takes an int, it is used. In all other cases, if you have a data type that is smaller than the argument in the method, that data type is promoted. char produces a slightly different effect, since if it doesn’t find an exact char match, it is promoted to int. What happens if your argument is bigger than the argument expected by the overloaded method? A modification of the preceding program gives the answer: //: initialization/Demotion.java // Demotion of primitives and overloading. import static net.mindview.util.Print.*; public class Demotion { void f1(char x) { print("f1(char)"); } void f1(byte x) { print("f1(byte)"); } void f1(short x) { print("f1(short)"); } void f1(int x) { print("f1(int)"); } void f1(long x) { print("f1(long)"); } void f1(float x) { print("f1(float)"); } void f1(double x) { print("f1(double)"); } void f2(char x) { print("f2(char)"); } void f2(byte x) { print("f2(byte)"); } void f2(short x) { print("f2(short)"); } void f2(int x) { print("f2(int)"); } void f2(long x) { print("f2(long)"); } void f2(float x) { print("f2(float)"); } void f3(char x) { print("f3(char)"); } void f3(byte x) { print("f3(byte)"); } void f3(short x) { print("f3(short)"); } void f3(int x) { print("f3(int)"); } void f3(long x) { print("f3(long)"); } void f4(char x) { print("f4(char)"); } void f4(byte x) { print("f4(byte)"); } void f4(short x) { print("f4(short)"); } void f4(int x) { print("f4(int)"); } void f5(char x) { print("f5(char)"); } void f5(byte x) { print("f5(byte)"); } void f5(short x) { print("f5(short)"); } void f6(char x) { print("f6(char)"); } void f6(byte x) { print("f6(byte)"); } void f7(char x) { print("f7(char)"); } void testDouble() { double x = 0; print("double argument:"); f1(x);f2((float)x);f3((long)x);f4((int)x); Initialization & Cleanup 113 f5((short)x);f6((byte)x);f7((char)x); } public static void main(String[] args) { Demotion p = new Demotion(); p.testDouble(); } } /* Output: double argument: f1(double) f2(float) f3(long) f4(int) f5(short) f6(byte) f7(char) *///:~ Here, the methods take narrower primitive values. If your argument is wider, then you must perform a narrowing conversion with a cast. If you don’t do this, the compiler will issue an error message. Overloading on return values It is common to wonder, “Why only class names and method argument lists? Why not distinguish between methods based on their return values?” For example, these two methods, which have the same name and arguments, are easily distinguished from each other: void f() {} int f() { return 1; } This might work fine as long as the compiler could unequivocally determine the meaning from the context, as in int x = f( ). However, you can also call a method and ignore the return value. This is often referred to as calling a method for its side effect, since you don’t care about the return value, but instead want the other effects of the method call. So if you call the method this way: f(); how can Java determine which f( ) should be called? And how could someone reading the code see it? Because of this sort of problem, you cannot use return value types to distinguish overloaded methods. Default constructors As mentioned previously, a default constructor (a.k.a. a “no-arg” constructor) is one without arguments that is used to create a “default object.” If you create a class that has no constructors, the compiler will automatically create a default constructor for you. For example: //: initialization/DefaultConstructor.java class Bird {} public class DefaultConstructor { public static void main(String[] args) { Bird b = new Bird(); // Default! 114 Thinking in Java Bruce Eckel } } ///:~ The expression new Bird() creates a new object and calls the default constructor, even though one was not explicitly defined. Without it, you would have no method to call to build the object. However, if you define any constructors (with or without arguments), the compiler will not synthesize one for you: //: initialization/NoSynthesis.java class Bird2 { Bird2(int i) {} Bird2(double d) {} } public class NoSynthesis { public static void main(String[] args) { //! Bird2 b = new Bird2(); // No default Bird2 b2 = new Bird2(1); Bird2 b3 = new Bird2(1.0); } } ///:~ If you say: new Bird2() the compiler will complain that it cannot find a constructor that matches. When you don’t put in any constructors, it’s as if the compiler says, “You are bound to need some constructor, so let me make one for you.” But if you write a constructor, the compiler says, “You’ve written a constructor so you know what you’re doing; if you didn’t put in a default it’s because you meant to leave it out.” Exercise 3: (1) Create a class with a default constructor (one that takes no arguments) that prints a message. Create an object of this class. Exercise 4: (1) Add an overloaded constructor to the previous exercise that takes a String argument and prints it along with your message. Exercise 5: (2) Create a class called Dog with an overloaded bark( ) method. This method should be overloaded based on various primitive data types, and print different types of barking, howling, etc., depending on which overloaded version is called. Write a main( ) that calls all the different versions. Exercise 6: (1) Modify the previous exercise so that two of the overloaded methods have two arguments (of two different types), but in reversed order relative to each other. Verify that this works. Exercise 7: (1) Create a class without a constructor, and then create an object of that class in main( ) to verify that the default constructor is automatically synthesized. Initialization & Cleanup 115 The this keyword If you have two objects of the same type called a and b, you might wonder how it is that you can call a method peel( ) for both those objects: //: initialization/BananaPeel.java class Banana { void peel(int i) { /* ... */ } } public class BananaPeel { public static void main(String[] args) { Banana a = new Banana(), b = new Banana(); a.peel(1); b.peel(2); } } ///:~ If there’s only one method called peel( ), how can that method know whether it’s being called for the object a or b? To allow you to write the code in a convenient object-oriented syntax in which you “send a message to an object,” the compiler does some undercover work for you. There’s a secret first argument passed to the method peel( ), and that argument is the reference to the object that’s being manipulated. So the two method calls become something like: Banana.peel(a, 1); Banana.peel(b, 2); This is internal and you can’t write these expressions and get the compiler to accept them, but it gives you an idea of what’s happening. Suppose you’re inside a method and you’d like to get the reference to the current object. Since that reference is passed secretly by the compiler, there’s no identifier for it. However, for this purpose there’s a keyword: this. The this keyword—which can be used only inside a non-static method—produces the reference to the object that the method has been called for. You can treat the reference just like any other object reference. Keep in mind that if you’re calling a method of your class from within another method of your class, you don’t need to use this. You simply call the method. The current this reference is automatically used for the other method. Thus you can say: //: initialization/Apricot.java public class Apricot { void pick() { /* ... */ } void pit() { pick(); /* ... */ } } ///:~ Inside pit( ), you could say this.pick( ) but there’s no need to.1 The compiler does it for you automatically. The this keyword is used only for those special cases in which you need to explicitly use the reference to the current object. For example, it’s often used in return statements when you want to return the reference to the current object: 1 Some people will obsessively put this in front of every method call and field reference, arguing that it makes it “clearer and more explicit.” Don’t do it. There’s a reason that we use high-level languages: They do things for us. If you put this in when it’s not necessary, you will confuse and annoy everyone who reads your code, since all the rest of the code they’ve read won’t use this everywhere. People expect this to be used only when it is necessary. Following a consistent and straightforward coding style saves time and money. 116 Thinking in Java Bruce Eckel //: initialization/Leaf.java // Simple use of the "this" keyword. public class Leaf { int i = 0; Leaf increment() { i++; return this; } void print() { System.out.println("i = " + i); } public static void main(String[] args) { Leaf x = new Leaf(); x.increment().increment().increment().print(); } } /* Output: i=3 *///:~ Because increment( ) returns the reference to the current object via the this keyword, multiple operations can easily be performed on the same object. The this keyword is also useful for passing the current object to another method: //: initialization/PassingThis.java class Person { public void eat(Apple apple) { Apple peeled = apple.getPeeled(); System.out.println("Yummy"); } } class Peeler { static Apple peel(Apple apple) { // ... remove peel return apple; // Peeled } } class Apple { Apple getPeeled() { return Peeler.peel(this); } } public class PassingThis { public static void main(String[] args) { new Person().eat(new Apple()); } } /* Output: Yummy *///:~ Apple needs to call Peeler.peel( ), which is a foreign utility method that performs an operation that, for some reason, needs to be external to Apple (perhaps the external method can be applied across many different classes, and you don’t want to repeat the code). To pass itself to the foreign method, it must use this. Exercise 8: (1) Create a class with two methods. Within the first method, call the second method twice: the first time without using this, and the second time using this—just to see it working; you should not use this form in practice. Initialization & Cleanup 117 Calling constructors from constructors When you write several constructors for a class, there are times when you’d like to call one constructor from another to avoid duplicating code. You can make such a call by using the this keyword. Normally, when you say this, it is in the sense of “this object” or “the current object,” and by itself it produces the reference to the current object. In a constructor, the this keyword takes on a different meaning when you give it an argument list. It makes an explicit call to the constructor that matches that argument list. Thus you have a straightforward way to call other constructors: //: initialization/Flower.java // Calling constructors with "this" import static net.mindview.util.Print.*; public class Flower { int petalCount = 0; String s = "initial value"; Flower(int petals) { petalCount = petals; print("Constructor w/ int arg only, petalCount= " + petalCount); } Flower(String ss) { print("Constructor w/ String arg only, s = " + ss); s = ss; } Flower(String s, int petals) { this(petals); //! this(s); // Can’t call two! this.s = s; // Another use of "this" print("String & int args"); } Flower() { this("hi", 47); print("default constructor (no args)"); } void printPetalCount() { //! this(11); // Not inside non-constructor! print("petalCount = " + petalCount + " s = "+ s); } public static void main(String[] args) { Flower x = new Flower(); x.printPetalCount(); } } /* Output: Constructor w/ int arg only, petalCount= 47 String & int args default constructor (no args) petalCount = 47 s = hi *///:~ The constructor Flower(String s, int petals) shows that, while you can call one constructor using this, you cannot call two. In addition, the constructor call must be the first thing you do, or you’ll get a compiler error message. This example also shows another way you’ll see this used. Since the name of the argument s and the name of the member data s are the same, there’s an ambiguity. You can resolve it 118 Thinking in Java Bruce Eckel using this.s, to say that you’re referring to the member data. You’ll often see this form used in Java code, and it’s used in numerous places in this book. In printPetalCount( ) you can see that the compiler won’t let you call a constructor from inside any method other than a constructor. Exercise 9: (1) Create a class with two (overloaded) constructors. Using this, call the second constructor inside the first one. The meaning of static With the this keyword in mind, you can more fully understand what it means to make a method static. It means that there is no this for that particular method. You cannot call non-static methods from inside static methods2 (although the reverse is possible), and you can call a static method for the class itself, without any object. In fact, that’s primarily what a static method is for. It’s as if you’re creating the equivalent of a global method. However, global methods are not permitted in Java, and putting the static method inside a class allows it access to other static methods and to static fields. Some people argue that static methods are not object-oriented, since they do have the semantics of a global method; with a static method, you don’t send a message to an object, since there’s no this. This is probably a fair argument, and if you find yourself using a lot of static methods, you should probably rethink your strategy. However, statics are pragmatic, and there are times when you genuinely need them, so whether or not they are “proper OOP” should be left to the theoreticians. Cleanup: finalization and garbage collection Programmers know about the importance of initialization, but often forget the importance of cleanup. After all, who needs to clean up an int? But with libraries, simply “letting go” of an object once you’re done with it is not always safe. Of course, Java has the garbage collector to reclaim the memory of objects that are no longer used. Now consider an unusual case: Suppose your object allocates “special” memory without using new. The garbage collector only knows how to release memory allocated with new, so it won’t know how to release the object’s “special” memory. To handle this case, Java provides a method called finalize( ) that you can define for your class. Here’s how it’s supposed to work. When the garbage collector is ready to release the storage used for your object, it will first call finalize( ), and only on the next garbage-collection pass will it reclaim the object’s memory. So if you choose to use finalize( ), it gives you the ability to perform some important cleanup at the time of garbage collection. This is a potential programming pitfall because some programmers, especially C++ programmers, might initially mistake finalize( ) for the destructor in C++, which is a function that is always called when an object is destroyed. It is important to distinguish between C++ and Java here, because in C++, objects always get destroyed (in a bug-free program), whereas in Java, objects do not always get garbage collected. Or, put another way: 1. Your objects might not get garbage collected. 2 The one case in which this is possible occurs if you pass a reference to an object into the static method (the static method could also create its own object). Then, via the reference (which is now effectively this), you can call non-static methods and access non-static fields. But typically, if you want to do something like this, you’ll just make an ordinary, non-static method. Initialization & Cleanup 119 2. Garbage collection is not destruction. If you remember this, you will stay out of trouble. What it means is that if there is some activity that must be performed before you no longer need an object, you must perform that activity yourself. Java has no destructor or similar concept, so you must create an ordinary method to perform this cleanup. For example, suppose that in the process of creating your object, it draws itself on the screen. If you don’t explicitly erase its image from the screen, it might never get cleaned up. If you put some kind of erasing functionality inside finalize( ), then if an object is garbage collected and finalize( ) is called (and there’s no guarantee this will happen), then the image will first be removed from the screen, but if it isn’t, the image will remain. You might find that the storage for an object never gets released because your program never nears the point of running out of storage. If your program completes and the garbage collector never gets around to releasing the storage for any of your objects, that storage will be returned to the operating system en masse as the program exits. This is a good thing, because garbage collection has some overhead, and if you never do it, you never incur that expense. What is finalize() for? So, if you should not use finalize( ) as a general-purpose cleanup method, what good is it? A third point to remember is: 3. Garbage collection is only about memory. That is, the sole reason for the existence of the garbage collector is to recover memory that your program is no longer using. So any activity that is associated with garbage collection, most notably your finalize( ) method, must also be only about memory and its deallocation. Does this mean that if your object contains other objects, finalize( ) should explicitly release those objects? Well, no—the garbage collector takes care of the release of all object memory regardless of how the object is created. It turns out that the need for finalize( ) is limited to special cases in which your object can allocate storage in some way other than creating an object. But, you might observe, everything in Java is an object, so how can this be? It would seem that finalize( ) is in place because of the possibility that you’ll do something Clike by allocating memory using a mechanism other than the normal one in Java. This can happen primarily through native methods, which are a way to call non-Java code from Java. (Native methods are covered in Appendix B in the electronic 2nd edition of this book, available at www.MindView.net.) C and C++ are the only languages currently supported by native methods, but since they can call subprograms in other languages, you can effectively call anything. Inside the non-Java code, C’s malloc( ) family of functions might be called to allocate storage, and unless you call free( ), that storage will not be released, causing a memory leak. Of course, free( ) is a C and C++ function, so you’d need to call it in a native method inside your finalize( ). After reading this, you probably get the idea that you won’t use finalize( ) much.3 You’re correct; it is not the appropriate place for normal cleanup to occur. So where should normal cleanup be performed? 3 Joshua Bloch goes further in his section titled “avoid finalizers”: “Finalizers are unpredictable, often dangerous, and generally unnecessary.” Effective JavaTM Programming Language Guide, p. 20 (Addison-Wesley, 2001). 120 Thinking in Java Bruce Eckel You must perform cleanup To clean up an object, the user of that object must call a cleanup method at the point the cleanup is desired. This sounds pretty straightforward, but it collides a bit with the C++ concept of the destructor. In C++, all objects are destroyed. Or rather, all objects should be destroyed. If the C++ object is created as a local (i.e., on the stack—not possible in Java), then the destruction happens at the closing curly brace of the scope in which the object was created. If the object was created using new (like in Java), the destructor is called when the programmer calls the C++ operator delete (which doesn’t exist in Java). If the C++ programmer forgets to call delete, the destructor is never called, and you have a memory leak, plus the other parts of the object never get cleaned up. This kind of bug can be very difficult to track down, and is one of the compelling reasons to move from C++ to Java. In contrast, Java doesn’t allow you to create local objects—you must always use new. But in Java, there’s no “delete” for releasing the object, because the garbage collector releases the storage for you. So from a simplistic standpoint, you could say that because of garbage collection, Java has no destructor. You’ll see as this book progresses, however, that the presence of a garbage collector does not remove the need for or the utility of destructors. (And you should never call finalize( ) directly, so that’s not a solution.) If you want some kind of cleanup performed other than storage release, you must still explicitly call an appropriate method in Java, which is the equivalent of a C++ destructor without the convenience. Remember that neither garbage collection nor finalization is guaranteed. If the JVM isn’t close to running out of memory, then it might not waste time recovering memory through garbage collection. The termination condition In general, you can’t rely on finalize( ) being called, and you must create separate “cleanup” methods and call them explicitly. So it appears that finalize( ) is only useful for obscure memory cleanup that most programmers will never use. However, there is an interesting use of finalize( ) that does not rely on it being called every time. This is the verification of the termination condition4 of an object. At the point that you’re no longer interested in an object—when it’s ready to be cleaned up— that object should be in a state whereby its memory can be safely released. For example, if the object represents an open file, that file should be closed by the programmer before the object is garbage collected. If any portions of the object are not properly cleaned up, then you have a bug in your program that can be very difficult to find. finalize( ) can be used to eventually discover this condition, even if it isn’t always called. If one of the finalizations happens to reveal the bug, then you discover the problem, which is all you really care about. Here’s a simple example of how you might use it: //: initialization/TerminationCondition.java // Using finalize() to detect an object that // hasn’t been properly cleaned up. class Book { boolean checkedOut = false; Book(boolean checkOut) { checkedOut = checkOut; } 4 A term coined by Bill Venners (www.Artima.com) during a seminar that he and I were giving together. Initialization & Cleanup 121 void checkIn() { checkedOut = false; } protected void finalize() { if(checkedOut) System.out.println("Error: checked out"); // Normally, you’ll also do this: // super.finalize(); // Call the base-class version } } public class TerminationCondition { public static void main(String[] args) { Book novel = new Book(true); // Proper cleanup: novel.checkIn(); // Drop the reference, forget to clean up: new Book(true); // Force garbage collection & finalization: System.gc(); } } /* Output: Error: checked out *///:~ The termination condition is that all Book objects are supposed to be checked in before they are garbage collected, but in main( ), a programmer error doesn’t check in one of the books. Without finalize( ) to verify the termination condition, this can be a difficult bug to find. Note that System.gc( ) is used to force finalization. But even if it isn’t, it’s highly probable that the errant Book will eventually be discovered through repeated executions of the program (assuming the program allocates enough storage to cause the garbage collector to execute). You should generally assume that the base-class version of finalize( ) will also be doing something important, and call it using super, as you can see in Book.finalize( ). In this case, it is commented out because it requires exception handling, which we haven’t covered yet. Exercise 10: (2) Create a class with a finalize( ) method that prints a message. In main( ), create an object of your class. Explain the behavior of your program. Exercise 11: (4) Modify the previous exercise so that your finalize( ) will always be called. Exercise 12: (4) Create a class called Tank that can be filled and emptied, and has a termination condition that it must be empty when the object is cleaned up. Write a finalize( ) that verifies this termination condition. In main( ), test the possible scenarios that can occur when your Tank is used. How a garbage collector works If you come from a programming language where allocating objects on the heap is expensive, you may naturally assume that Java’s scheme of allocating everything (except primitives) on the heap is also expensive. However, it turns out that the garbage collector can have a significant impact on increasing the speed of object creation. This might sound a bit odd at first—that storage release affects storage allocation—but it’s the way some JVMs work, and it 122 Thinking in Java Bruce Eckel means that allocating storage for heap objects in Java can be nearly as fast as creating storage on the stack in other languages. For example, you can think of the C++ heap as a yard where each object stakes out its own piece of turf. This real estate can become abandoned sometime later and must be reused. In some JVMs, the Java heap is quite different; it’s more like a conveyor belt that moves forward every time you allocate a new object. This means that object storage allocation is remarkably rapid. The “heap pointer” is simply moved forward into virgin territory, so it’s effectively the same as C++’s stack allocation. (Of course, there’s a little extra overhead for bookkeeping, but it’s nothing like searching for storage.) You might observe that the heap isn’t in fact a conveyor belt, and if you treat it that way, you’ll start paging memory—moving it on and off disk, so that you can appear to have more memory than you actually do. Paging significantly impacts performance. Eventually, after you create enough objects, you’ll run out of memory. The trick is that the garbage collector steps in, and while it collects the garbage it compacts all the objects in the heap so that you’ve effectively moved the “heap pointer” closer to the beginning of the conveyor belt and farther away from a page fault. The garbage collector rearranges things and makes it possible for the high-speed, infinite-free-heap model to be used while allocating storage. To understand garbage collection in Java, it’s helpful learn how garbage-collection schemes work in other systems. A simple but slow garbage-collection technique is called reference counting. This means that each object contains a reference counter, and every time a reference is attached to that object, the reference count is increased. Every time a reference goes out of scope or is set to null, the reference count is decreased. Thus, managing reference counts is a small but constant overhead that happens throughout the lifetime of your program. The garbage collector moves through the entire list of objects, and when it finds one with a reference count of zero it releases that storage (however, reference counting schemes often release an object as soon as the count goes to zero). The one drawback is that if objects circularly refer to each other they can have nonzero reference counts while still being garbage. Locating such self-referential groups requires significant extra work for the garbage collector. Reference counting is commonly used to explain one kind of garbage collection, but it doesn’t seem to be used in any JVM implementations. In faster schemes, garbage collection is not based on reference counting. Instead, it is based on the idea that any non-dead object must ultimately be traceable back to a reference that lives either on the stack or in static storage. The chain might go through several layers of objects. Thus, if you start in the stack and in the static storage area and walk through all the references, you’ll find all the live objects. For each reference that you find, you must trace into the object that it points to and then follow all the references in that object, tracing into the objects they point to, etc., until you’ve moved through the entire Web that originated with the reference on the stack or in static storage. Each object that you move through must still be alive. Note that there is no problem with detached self-referential groups—these are simply not found, and are therefore automatically garbage. In the approach described here, the JVM uses an adaptive garbage-collection scheme, and what it does with the live objects that it locates depends on the variant currently being used. One of these variants is stop-and-copy. This means that—for reasons that will become apparent—the program is first stopped (this is not a background collection scheme). Then, each live object is copied from one heap to another, leaving behind all the garbage. In addition, as the objects are copied into the new heap, they are packed end-to-end, thus compacting the new heap (and allowing new storage to simply be reeled off the end as previously described). Of course, when an object is moved from one place to another, all references that point at the object must be changed. The reference that goes from the heap or the static storage area to the object can be changed right away, but there can be other references pointing to this object Initialization & Cleanup 123 that will be encountered later during the “walk.” These are fixed up as they are found (you could imagine a table that maps old addresses to new ones). There are two issues that make these so-called “copy collectors” inefficient. The first is the idea that you have two heaps and you slosh all the memory back and forth between these two separate heaps, maintaining twice as much memory as you actually need. Some JVMs deal with this by allocating the heap in chunks as needed and simply copying from one chunk to another. The second issue is the copying process itself. Once your program becomes stable, it might be generating little or no garbage. Despite that, a copy collector will still copy all the memory from one place to another, which is wasteful. To prevent this, some JVMs detect that no new garbage is being generated and switch to a different scheme (this is the “adaptive” part). This other scheme is called mark-and-sweep, and it’s what earlier versions of Sun’s JVM used all the time. For general use, mark-and-sweep is fairly slow, but when you know you’re generating little or no garbage, it’s fast. Mark-and-sweep follows the same logic of starting from the stack and static storage, and tracing through all the references to find live objects. However, each time it finds a live object, that object is marked by setting a flag in it, but the object isn’t collected yet. Only when the marking process is finished does the sweep occur. During the sweep, the dead objects are released. However, no copying happens, so if the collector chooses to compact a fragmented heap, it does so by shuffling objects around. “Stop-and-copy” refers to the idea that this type of garbage collection is not done in the background; instead, the program is stopped while the garbage collection occurs. In the Sun literature you’ll find many references to garbage collection as a low-priority background process, but it turns out that the garbage collection was not implemented that way in earlier versions of the Sun JVM. Instead, the Sun garbage collector stopped the program when memory got low. Mark-and-sweep also requires that the program be stopped. As previously mentioned, in the JVM described here memory is allocated in big blocks. If you allocate a large object, it gets its own block. Strict stop-and-copy requires copying every live object from the source heap to a new heap before you can free the old one, which translates to lots of memory. With blocks, the garbage collection can typically copy objects to dead blocks as it collects. Each block has a generation count to keep track of whether it’s alive. In the normal case, only the blocks created since the last garbage collection are compacted; all other blocks get their generation count bumped if they have been referenced from somewhere. This handles the normal case of lots of short-lived temporary objects. Periodically, a full sweep is made—large objects are still not copied (they just get their generation count bumped), and blocks containing small objects are copied and compacted. The JVM monitors the efficiency of garbage collection and if it becomes a waste of time because all objects are long-lived, then it switches to mark-andsweep. Similarly, the JVM keeps track of how successful mark-and-sweep is, and if the heap starts to become fragmented, it switches back to stop-and-copy. This is where the “adaptive” part comes in, so you end up with a mouthful: “Adaptive generational stop-and-copy mark-andsweep.” There are a number of additional speedups possible in a JVM. An especially important one involves the operation of the loader and what is called a just-in-time (JIT) compiler. A JIT compiler partially or fully converts a program into native machine code so that it doesn’t need to be interpreted by the JVM and thus runs much faster. When a class must be loaded (typically, the first time you want to create an object of that class), the .class file is located, and the bytecodes for that class are brought into memory. At this point, one approach is to simply JIT compile all the code, but this has two drawbacks: It takes a little more time, which, compounded throughout the life of the program, can add up; and it increases the size of the executable (bytecodes are significantly more compact than expanded JIT code), and this might cause paging, which definitely slows down a program. An alternative approach is lazy evaluation, which means that the code is not JIT compiled until necessary. Thus, code 124 Thinking in Java Bruce Eckel that never gets executed might never be JIT compiled. The Java HotSpot technologies in recent JDKs take a similar approach by increasingly optimizing a piece of code each time it is executed, so the more the code is executed, the faster it gets. Member initialization Java goes out of its way to guarantee that variables are properly initialized before they are used. In the case of a method’s local variables, this guarantee comes in the form of a compiletime error. So if you say: void f() { int i; i++; // Error -- i not initialized } you’ll get an error message that says that i might not have been initialized. Of course, the compiler could have given i a default value, but an uninitialized local variable is probably a programmer error, and a default value would have covered that up. Forcing the programmer to provide an initialization value is more likely to catch a bug. If a primitive is a field in a class, however, things are a bit different. As you saw in the Everything Is an Object chapter, each primitive field of a class is guaranteed to get an initial value. Here’s a program that verifies this, and shows the values: //: initialization/InitialValues.java // Shows default initial values. import static net.mindview.util.Print.*; public class InitialValues { boolean t; char c; byte b; short s; int i; long l; float f; double d; InitialValues reference; void printInitialValues() { print("Data type Initial value"); print("boolean " + t); print("char [" + c + "]"); print("byte " + b); print("short " + s); print("int " + i); print("long " + l); print("float " + f); print("double " + d); print("reference " + reference); } public static void main(String[] args) { InitialValues iv = new InitialValues(); iv.printInitialValues(); /* You could also say: new InitialValues().printInitialValues(); */ } } /* Output: Data type Initial value Initialization & Cleanup 125 boolean char byte short int long float double reference *///:~ false [] 0 0 0 0 0.0 0.0 null You can see that even though the values are not specified, they automatically get initialized (the char value is a zero, which prints as a space). So at least there’s no threat of working with uninitialized variables. When you define an object reference inside a class without initializing it to a new object, that reference is given a special value of null. Specifying initialization What happens if you want to give a variable an initial value? One direct way to do this is simply to assign the value at the point you define the variable in the class. (Notice you cannot do this in C++, although C++ novices always try.) Here the field definitions in class InitialValues are changed to provide initial values: //: initialization/InitialValues2.java // Providing explicit initial values. public class InitialValues2 { boolean bool = true; char ch = ‘x’; byte b = 47; short s = 0xff; int i = 999; long lng = 1; float f = 3.14f; double d = 3.14159; } ///:~ You can also initialize non-primitive objects in this same way. If Depth is a class, you can create a variable and initialize it like so: //: initialization/Measurement.java class Depth {} public class Measurement { Depth d = new Depth(); // ... } ///:~ If you haven’t given d an initial value and you try to use it anyway, you’ll get a runtime error called an exception (covered in the Error Handling with Exceptions chapter). You can even call a method to provide an initialization value: //: initialization/MethodInit.java public class MethodInit { int i = f(); int f() { return 11; } 126 Thinking in Java Bruce Eckel } ///:~ This method can have arguments, of course, but those arguments cannot be other class members that haven’t been initialized yet. Thus, you can do this: //: initialization/MethodInit2.java public class MethodInit2 { int i = f(); int j = g(i); int f() { return 11; } int g(int n) { return n * 10; } } ///:~ But you cannot do this: //: initialization/MethodInit3.java public class MethodInit3 { //! int j = g(i); // Illegal forward reference int i = f(); int f() { return 11; } int g(int n) { return n * 10; } } ///:~ This is one place in which the compiler, appropriately, does complain about forward referencing, since this has to do with the order of initialization and not the way the program is compiled. This approach to initialization is simple and straightforward. It has the limitation that every object of type InitialValues will get these same initialization values. Sometimes this is exactly what you need, but at other times you need more flexibility. Constructor initialization The constructor can be used to perform initialization, and this gives you greater flexibility in your programming because you can call methods and perform actions at run time to determine the initial values. There’s one thing to keep in mind, however: You aren’t precluding the automatic initialization, which happens before the constructor is entered. So, for example, if you say: //: initialization/Counter.java public class Counter { int i; Counter() { i = 7; } // ... } ///:~ then i will first be initialized to 0, then to 7. This is true with all the primitive types and with object references, including those that are given explicit initialization at the point of definition. For this reason, the compiler doesn’t try to force you to initialize elements in the constructor at any particular place, or before they are used—initialization is already guaranteed. Order of initialization Within a class, the order of initialization is determined by the order that the variables are defined within the class. The variable definitions may be scattered throughout and in Initialization & Cleanup 127 between method definitions, but the variables are initialized before any methods can be called—even the constructor. For example: //: initialization/OrderOfInitialization.java // Demonstrates initialization order. import static net.mindview.util.Print.*; // When the constructor is called to create a // Window object, you’ll see a message: class Window { Window(int marker) { print("Window(" + marker + ")"); } } class House { Window w1 = new Window(1); // Before constructor House() { // Show that we’re in the constructor: print("House()"); w3 = new Window(33); // Reinitialize w3 } Window w2 = new Window(2); // After constructor void f() { print("f()"); } Window w3 = new Window(3); // At end } public class OrderOfInitialization { public static void main(String[] args) { House h = new House(); h.f(); // Shows that construction is done } } /* Output: Window(1) Window(2) Window(3) House() Window(33) f() *///:~ In House, the definitions of the Window objects are intentionally scattered about to prove that they’ll all get initialized before the constructor is entered or anything else can happen. In addition, w3 is reinitialized inside the constructor. From the output, you can see that the w3 reference gets initialized twice: once before and once during the constructor call. (The first object is dropped, so it can be garbage collected later.) This might not seem efficient at first, but it guarantees proper initialization—what would happen if an overloaded constructor were defined that did not initialize w3 and there wasn’t a “default” initialization for w3 in its definition? static data initialization There’s only a single piece of storage for a static, regardless of how many objects are created. You can’t apply the static keyword to local variables, so it only applies to fields. If a field is a static primitive and you don’t initialize it, it gets the standard initial value for its type. If it’s a reference to an object, the default initialization value is null. If you want to place initialization at the point of definition, it looks the same as for nonstatics. 128 Thinking in Java Bruce Eckel To see when the static storage gets initialized, here’s an example: //: initialization/StaticInitialization.java // Specifying initial values in a class definition. import static net.mindview.util.Print.*; class Bowl { Bowl(int marker) { print("Bowl(" + marker + ")"); } void f1(int marker) { print("f1(" + marker + ")"); } } class Table { static Bowl bowl1 = new Bowl(1); Table() { print("Table()"); bowl2.f1(1); } void f2(int marker) { print("f2(" + marker + ")"); } static Bowl bowl2 = new Bowl(2); } class Cupboard { Bowl bowl3 = new Bowl(3); static Bowl bowl4 = new Bowl(4); Cupboard() { print("Cupboard()"); bowl4.f1(2); } void f3(int marker) { print("f3(" + marker + ")"); } static Bowl bowl5 = new Bowl(5); } public class StaticInitialization { public static void main(String[] args) { print("Creating new Cupboard() in main"); new Cupboard(); print("Creating new Cupboard() in main"); new Cupboard(); table.f2(1); cupboard.f3(1); } static Table table = new Table(); static Cupboard cupboard = new Cupboard(); } /* Output: Bowl(1) Bowl(2) Table() f1(1) Bowl(4) Bowl(5) Bowl(3) Cupboard() f1(2) Creating new Cupboard() in main Bowl(3) Initialization & Cleanup 129 Cupboard() f1(2) Creating new Cupboard() in main Bowl(3) Cupboard() f1(2) f2(1) f3(1) *///:~ Bowl allows you to view the creation of a class, and Table and Cupboard have static members of Bowl scattered through their class definitions. Note that Cupboard creates a non-static Bowl bowl3 prior to the static definitions. From the output, you can see that the static initialization occurs only if it’s necessary. If you don’t create a Table object and you never refer to Table.bowl1 or Table.bowl2, the static Bowl bowl1 and bowl2 will never be created. They are initialized only when the first Table object is created (or the first static access occurs). After that, the static objects are not reinitialized. The order of initialization is statics first, if they haven’t already been initialized by a previous object creation, and then the non-static objects. You can see the evidence of this in the output. To execute main( ) (a static method), the StaticInitialization class must be loaded, and its static fields table and cupboard are then initialized, which causes those classes to be loaded, and since they both contain static Bowl objects, Bowl is then loaded. Thus, all the classes in this particular program get loaded before main( ) starts. This is usually not the case, because in typical programs you won’t have everything linked together by statics as you do in this example. To summarize the process of creating an object, consider a class called Dog: 1. Even though it doesn’t explicitly use the static keyword, the constructor is actually a static method. So the first time an object of type Dog is created, or the first time a static method or static field of class Dog is accessed, the Java interpreter must locate Dog.class, which it does by searching through the classpath. 2. As Dog.class is loaded (creating a Class object, which you’ll learn about later), all of its static initializers are run. Thus, static initialization takes place only once, as the Class object is loaded for the first time. 3. When you create a new Dog( ), the construction process for a Dog object first allocates enough storage for a Dog object on the heap. 4. This storage is wiped to zero, automatically setting all the primitives in that Dog object to their default values (zero for numbers and the equivalent for boolean and char) and the references to null. 5. Any initializations that occur at the point of field definition are executed. 6. Constructors are executed. As you shall see in the Reusing Classes chapter, this might actually involve a fair amount of activity, especially when inheritance is involved. Explicit static initialization Java allows you to group other static initializations inside a special “static clause” (sometimes called a static block) in a class. It looks like this: 130 Thinking in Java Bruce Eckel //: initialization/Spoon.java public class Spoon { static int i; static { i = 47; } } ///:~ It appears to be a method, but it’s just the static keyword followed by a block of code. This code, like other static initializations, is executed only once: the first time you make an object of that class or the first time you access a static member of that class (even if you never make an object of that class). For example: //: initialization/ExplicitStatic.java // Explicit static initialization with the "static" clause. import static net.mindview.util.Print.*; class Cup { Cup(int marker) { print("Cup(" + marker + ")"); } void f(int marker) { print("f(" + marker + ")"); } } class Cups { static Cup cup1; static Cup cup2; static { cup1 = new Cup(1); cup2 = new Cup(2); } Cups() { print("Cups()"); } } public class ExplicitStatic { public static void main(String[] args) { print("Inside main()"); Cups.cup1.f(99); // (1) } // static Cups cups1 = new Cups(); // (2) // static Cups cups2 = new Cups(); // (2) } /* Output: Inside main() Cup(1) Cup(2) f(99) *///:~ The static initializers for Cups run when either the access of the static object cup1 occurs on the line marked (1), or if line (1) is commented out and the lines marked (2) are uncommented. If both (1) and (2) are commented out, the static initialization for Cups never occurs, as you can see from the output. Also, it doesn’t matter if one or both of the lines marked (2) are uncommented; the static initialization only occurs once. Exercise 13: (1) Verify the statements in the previous paragraph. Initialization & Cleanup 131 Exercise 14: (1) Create a class with a static String field that is initialized at the point of definition, and another one that is initialized by the static block. Add a static method that prints both fields and demonstrates that they are both initialized before they are used. Non-static instance initialization Java provides a similar syntax, called instance initialization, for initializing non-static variables for each object. Here’s an example: //: initialization/Mugs.java // Java "Instance Initialization." import static net.mindview.util.Print.*; class Mug { Mug(int marker) { print("Mug(" + marker + ")"); } void f(int marker) { print("f(" + marker + ")"); } } public class Mugs { Mug mug1; Mug mug2; { mug1 = new Mug(1); mug2 = new Mug(2); print("mug1 & mug2 initialized"); } Mugs() { print("Mugs()"); } Mugs(int i) { print("Mugs(int)"); } public static void main(String[] args) { print("Inside main()"); new Mugs(); print("new Mugs() completed"); new Mugs(1); print("new Mugs(1) completed"); } } /* Output: Inside main() Mug(1) Mug(2) mug1 & mug2 initialized Mugs() new Mugs() completed Mug(1) Mug(2) mug1 & mug2 initialized Mugs(int) new Mugs(1) completed *///:~ You can see that the instance initialization clause: { 132 Thinking in Java Bruce Eckel mug1 = new Mug(1); mug2 = new Mug(2); print("mug1 & mug2 initialized"); } looks exactly like the static initialization clause except for the missing static keyword. This syntax is necessary to support the initialization of anonymous inner classes (see the Inner Classes chapter), but it also allows you to guarantee that certain operations occur regardless of which explicit constructor is called. From the output, you can see that the instance initialization clause is executed before either one of the constructors. Exercise 15: (1) Create a class with a String that is initialized using instance initialization. Array initialization An array is simply a sequence of either objects or primitives that are all the same type and are packaged together under one identifier name. Arrays are defined and used with the squarebrackets indexing operator [ ]. To define an array reference, you simply follow your type name with empty square brackets: int[] a1; You can also put the square brackets after the identifier to produce exactly the same meaning: int a1[]; This conforms to expectations from C and C++ programmers. The former style, however, is probably a more sensible syntax, since it says that the type is “an int array.” That style will be used in this book. The compiler doesn’t allow you to tell it how big the array is. This brings us back to that issue of “references.” All that you have at this point is a reference to an array (you’ve allocated enough storage for that reference), and there’s been no space allocated for the array object itself. To create storage for the array, you must write an initialization expression. For arrays, initialization can appear anywhere in your code, but you can also use a special kind of initialization expression that must occur at the point where the array is created. This special initialization is a set of values surrounded by curly braces. The storage allocation (the equivalent of using new) is taken care of by the compiler in this case. For example: int[] a1 = { 1, 2, 3, 4, 5 }; So why would you ever define an array reference without an array? int[] a2; Well, it’s possible to assign one array to another in Java, so you can say: a2 = a1; What you’re really doing is copying a reference, as demonstrated here: //: initialization/ArraysOfPrimitives.java import static net.mindview.util.Print.*; Initialization & Cleanup 133 public class ArraysOfPrimitives { public static void main(String[] args) { int[] a1 = { 1, 2, 3, 4, 5 }; int[] a2; a2 = a1; for(int i = 0; i < a2.length; i++) a2[i] = a2[i] + 1; for(int i = 0; i < a1.length; i++) print("a1[" + i + "] = " + a1[i]); } } /* Output: a1[0] = 2 a1[1] = 3 a1[2] = 4 a1[3] = 5 a1[4] = 6 *///:~ You can see that a1 is given an initialization value but a2 is not; a2 is assigned later—in this case, to another array. Since a2 and a1 are then aliased to the same array, the changes made via a2 are seen in a1. All arrays have an intrinsic member (whether they’re arrays of objects or arrays of primitives) that you can query—but not change—to tell you how many elements there are in the array. This member is length. Since arrays in Java, like C and C++, start counting from element zero, the largest element you can index is length - 1. If you go out of bounds, C and C++ quietly accept this and allow you to stomp all over your memory, which is the source of many infamous bugs. However, Java protects you against such problems by causing a runtime error (an exception) if you step out of bounds.5 What if you don’t know how many elements you’re going to need in your array while you’re writing the program? You simply use new to create the elements in the array. Here, new works even though it’s creating an array of primitives (new won’t create a non-array primitive): //: initialization/ArrayNew.java // Creating arrays with new. import java.util.*; import static net.mindview.util.Print.*; public class ArrayNew { public static void main(String[] args) { int[] a; Random rand = new Random(47); a = new int[rand.nextInt(20)]; print("length of a = " + a.length); print(Arrays.toString(a)); } } /* Output: length of a = 18 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] *///:~ The size of the array is chosen at random by using the Random.nextInt( ) method, which produces a value between zero and that of its argument. Because of the randomness, it’s clear 5 Of course, checking every array access costs time and code and there’s no way to turn it off, which means that array accesses might be a source of inefficiency in your program if they occur at a critical juncture. For Internet security and programmer productivity, the Java designers saw that this was a worthwhile trade-off. Although you may be tempted to write code that you think might make array accesses more efficient, this is a waste of time because automatic compile-time and runtime optimizations will speed array accesses. 134 Thinking in Java Bruce Eckel that array creation is actually happening at run time. In addition, the output of this program shows that array elements of primitive types are automatically initialized to “empty” values. (For numerics and char, this is zero, and for boolean, it’s false.) The Arrays.toString( ) method, which is part of the standard java.util library, produces a printable version of a one-dimensional array. Of course, in this case the array could also have been defined and initialized in the same statement: int[] a = new int[rand.nextInt(20)]; This is the preferred way to do it, if you can. If you create a non-primitive array, you create an array of references. Consider the wrapper type Integer, which is a class and not a primitive: //: initialization/ArrayClassObj.java // Creating an array of nonprimitive objects. import java.util.*; import static net.mindview.util.Print.*; public class ArrayClassObj { public static void main(String[] args) { Random rand = new Random(47); Integer[] a = new Integer[rand.nextInt(20)]; print("length of a = " + a.length); for(int i = 0; i < a.length; i++) a[i] = rand.nextInt(500); // Autoboxing print(Arrays.toString(a)); } } /* Output: (Sample) length of a = 18 [55, 193, 361, 461, 429, 368, 200, 22, 207, 288, 128, 51, 89, 309, 278, 498, 361, 20] *///:~ Here, even after new is called to create the array: Integer[] a = new Integer[rand.nextInt(20)]; it’s only an array of references, and the initialization is not complete until the reference itself is initialized by creating a new Integer object (via autoboxing, in this case): a[i] = rand.nextInt(500); If you forget to create the object, however, you’ll get an exception at run time when you try to use the empty array location. It’s also possible to initialize arrays of objects by using the curly brace-enclosed list. There are two forms: //: initialization/ArrayInit.java // Array initialization. import java.util.*; public class ArrayInit { public static void main(String[] args) { Integer[] a = { Initialization & Cleanup 135 new Integer(1), new Integer(2), 3, // Autoboxing }; Integer[] b = new Integer[]{ new Integer(1), new Integer(2), 3, // Autoboxing }; System.out.println(Arrays.toString(a)); System.out.println(Arrays.toString(b)); } } /* Output: [1, 2, 3] [1, 2, 3] *///:~ In both cases, the final comma in the list of initializers is optional. (This feature makes for easier maintenance of long lists.) Although the first form is useful, it’s more limited because it can only be used at the point where the array is defined. You can use the second and third forms anywhere, even inside a method call. For example, you could create an array of String objects to pass to the main( ) of another method, to provide alternate command-line arguments to that main( ): //: initialization/DynamicArray.java // Array initialization. public class DynamicArray { public static void main(String[] args) { Other.main(new String[]{ "fiddle", "de", "dum" }); } } class Other { public static void main(String[] args) { for(String s : args) System.out.print(s + " "); } } /* Output: fiddle de dum *///:~ The array created for the argument of Other.main( ) is created at the point of the method call, so you can even provide alternate arguments at the time of the call. Exercise 16: (1) Create an array of String objects and assign a String to each element. Print the array by using a for loop. Exercise 17: (2) Create a class with a constructor that takes a String argument. During construction, print the argument. Create an array of object references to this class, but don’t actually create objects to assign into the array. When you run the program, notice whether the initialization messages from the constructor calls are printed. Exercise 18: (1) Complete the previous exercise by creating objects to attach to the array of references. 136 Thinking in Java Bruce Eckel Variable argument lists The second form provides a convenient syntax to create and call methods that can produce an effect similar to C’s variable argument lists (known as “varargs” in C). These can include unknown quantities of arguments as well as unknown types. Since all classes are ultimately inherited from the common root class Object (a subject you will learn more about as this book progresses), you can create a method that takes an array of Object and call it like this: //: initialization/VarArgs.java // Using array syntax to create variable argument lists. class A {} public class VarArgs { static void printArray(Object[] args) { for(Object obj : args) System.out.print(obj + " "); System.out.println(); } public static void main(String[] args) { printArray(new Object[]{ new Integer(47), new Float(3.14), new Double(11.11) }); printArray(new Object[]{"one", "two", "three" }); printArray(new Object[]{new A(), new A(), new A()}); } } /* Output: (Sample) 47 3.14 11.11 one two three A@1a46e30 A@3e25a5 A@19821f *///:~ You can see that print( ) takes an array of Object, then steps through the array using the foreach syntax and prints each one. The standard Java library classes produce sensible output, but the objects of the classes created here print the class name, followed by an ‘@’ sign and hexadecimal digits. Thus, the default behavior (if you don’t define a toString( ) method for your class, which will be described later in the book) is to print the class name and the address of the object. You may see pre-Java SE5 code written like the above in order to produce variable argument lists. In Java SE5, however, this long-requested feature was finally added, so you can now use ellipses to define a variable argument list, as you can see in printArray( ): //: initialization/NewVarArgs.java // Using array syntax to create variable argument lists. public class NewVarArgs { static void printArray(Object... args) { for(Object obj : args) System.out.print(obj + " "); System.out.println(); } public static void main(String[] args) { // Can take individual elements: printArray(new Integer(47), new Float(3.14), new Double(11.11)); printArray(47, 3.14F, 11.11); printArray("one", "two", "three"); printArray(new A(), new A(), new A()); // Or an array: Initialization & Cleanup 137 printArray((Object[])new Integer[]{ 1, 2, 3, 4 }); printArray(); // Empty list is OK } } /* Output: (75% match) 47 3.14 11.11 47 3.14 11.11 one two three A@1bab50a A@c3c749 A@150bd4d 1234 *///:~ With varargs, you no longer have to explicitly write out the array syntax—the compiler will actually fill it in for you when you specify varargs. You’re still getting an array, which is why print( ) is able to use foreach to iterate through the array. However, it’s more than just an automatic conversion from a list of elements to an array. Notice the second-t0-last line in the program, where an array of Integer (created using autoboxing) is cast to an Object array (to remove a compiler warning) and passed to printArray( ). Clearly, the compiler sees that this is already an array and performs no conversion on it. So if you have a group of items you can pass them in as a list, and if you already have an array it will accept that as the variable argument list. The last line of the program shows that it’s possible to pass zero arguments to a vararg list. This is helpful when you have optional trailing arguments: //: initialization/OptionalTrailingArguments.java public class OptionalTrailingArguments { static void f(int required, String... trailing) { System.out.print("required: " + required + " "); for(String s : trailing) System.out.print(s + " "); System.out.println(); } public static void main(String[] args) { f(1, "one"); f(2, "two", "three"); f(0); } } /* Output: required: 1 one required: 2 two three required: 0 *///:~ This also shows how you can use varargs with a specified type other than Object. Here, all the varargs must be String objects. It’s possible to use any type of argument in varargs, including a primitive type. The following example also shows that the vararg list becomes an array, and if there’s nothing in the list it’s an array of size zero: //: initialization/VarargType.java public class VarargType { static void f(Character... args) { System.out.print(args.getClass()); System.out.println(" length " + args.length); } static void g(int... args) { System.out.print(args.getClass()); System.out.println(" length " + args.length); } public static void main(String[] args) { 138 Thinking in Java Bruce Eckel f(‘a’); f(); g(1); g(); System.out.println("int[]: " + new int[0].getClass()); } } /* Output: class [Ljava.lang.Character; length 1 class [Ljava.lang.Character; length 0 class [I length 1 class [I length 0 int[]: class [I *///:~ The getClass( ) method is part of Object, and will be explored fully in the Type Information chapter. It produces the class of an object, and when you print this class, you see an encoded string representing the class type. The leading ‘[‘ indicates that this is an array of the type that follows. The ‘I’ is for a primitive int; to double-check, I created an array of int in the last line and printed its type. This verifies that using varargs does not depend on autoboxing, but that it actually uses the primitive types. Varargs do work in harmony with autoboxing, however. For example: //: initialization/AutoboxingVarargs.java public class AutoboxingVarargs { public static void f(Integer... args) { for(Integer i : args) System.out.print(i + " "); System.out.println(); } public static void main(String[] args) { f(new Integer(1), new Integer(2)); f(4, 5, 6, 7, 8, 9); f(10, new Integer(11), 12); } } /* Output: 12 456789 10 11 12 *///:~ Notice that you can mix the types together in a single argument list, and autoboxing selectively promotes the int arguments to Integer. Varargs complicate the process of overloading, although it seems safe enough at first: //: initialization/OverloadingVarargs.java public class OverloadingVarargs { static void f(Character... args) { System.out.print("first"); for(Character c : args) System.out.print(" " + c); System.out.println(); } static void f(Integer... args) { System.out.print("second"); for(Integer i : args) System.out.print(" " + i); System.out.println(); Initialization & Cleanup 139 } static void f(Long... args) { System.out.println("third"); } public static void main(String[] args) { f(‘a’, ‘b’, ‘c’); f(1); f(2, 1); f(0); f(0L); //! f(); // Won’t compile -- ambiguous } } /* Output: first a b c second 1 second 2 1 second 0 third *///:~ In each case, the compiler is using autoboxing to match the overloaded method, and it calls the most specifically matching method. But when you call f( ) without arguments, it has no way of knowing which one to call. Although this error is understandable, it will probably surprise the client programmer. You might try solving the problem by adding a non-vararg argument to one of the methods: //: initialization/OverloadingVarargs2.java // {CompileTimeError} (Won’t compile) public class OverloadingVarargs2 { static void f(float i, Character... args) { System.out.println("first"); } static void f(Character... args) { System.out.print("second"); } public static void main(String[] args) { f(1, ‘a’); f(‘a’, ‘b’); } } ///:~ The {CompileTimeError} comment tag excludes the file from this book’s Ant build. If you compile it by hand you’ll see the error message: reference to f is ambiguous, both method f(float,java.lang.Character...) in OverloadingVarargs2 and method f(java.lang.Character...) in OverloadingVarargs2 match If you give both methods a non-vararg argument, it works: //: initialization/OverloadingVarargs3.java public class OverloadingVarargs3 { static void f(float i, Character... args) { System.out.println("first"); } static void f(char c, Character... args) { 140 Thinking in Java Bruce Eckel System.out.println("second"); } public static void main(String[] args) { f(1, ‘a’); f(‘a’, ‘b’); } } /* Output: first second *///:~ You should generally only use a variable argument list on one version of an overloaded method. Or consider not doing it at all. Exercise 19: (2) Write a method that takes a vararg String array. Verify that you can pass either a comma-separated list of Strings or a String[] into this method. Exercise 20: (1) Create a main( ) that uses varargs instead of the ordinary main( ) syntax. Print all the elements in the resulting args array. Test it with various numbers of command-line arguments. Enumerated types An apparently small addition in Java SE5 is the enum keyword, which makes your life much easier when you need to group together and use a set of enumerated types. In the past you would have created a set of constant integral values, but these do not naturally restrict themselves to your set and thus are riskier and more difficult to use. Enumerated types are a common enough need that C, C++, and a number of other languages have always had them. Before Java SE5, Java programmers were forced to know a lot and be quite careful when they wanted to properly produce the enum effect. Now Java has enum, too, and it’s much more full-featured than what you find in C/C++. Here’s a simple example: //: initialization/Spiciness.java public enum Spiciness { NOT, MILD, MEDIUM, HOT, FLAMING } ///:~ This creates an enumerated type called Spiciness with five named values. Because the instances of enumerated types are constants, they are in all capital letters by convention (if there are multiple words in a name, they are separated by underscores). To use an enum, you create a reference of that type and assign it to an instance: //: initialization/SimpleEnumUse.java public class SimpleEnumUse { public static void main(String[] args) { Spiciness howHot = Spiciness.MEDIUM; System.out.println(howHot); } } /* Output: MEDIUM *///:~ The compiler automatically adds useful features when you create an enum. For example, it creates a toString( ) so that you can easily display the name of an enum instance, which is how the print statement above produced its output. The compiler also creates an ordinal( ) Initialization & Cleanup 141 method to indicate the declaration order of a particular enum constant, and a static values( ) method that produces an array of values of the enum constants in the order that they were declared: //: initialization/EnumOrder.java public class EnumOrder { public static void main(String[] args) { for(Spiciness s : Spiciness.values()) System.out.println(s + ", ordinal " + s.ordinal()); } } /* Output: NOT, ordinal 0 MILD, ordinal 1 MEDIUM, ordinal 2 HOT, ordinal 3 FLAMING, ordinal 4 *///:~ Although enums appear to be a new data type, the keyword only produces some compiler behavior while generating a class for the enum, so in many ways you can treat an enum as if it were any other class. In fact, enums are classes and have their own methods. An especially nice feature is the way that enums can be used inside switch statements: //: initialization/Burrito.java public class Burrito { Spiciness degree; public Burrito(Spiciness degree) { this.degree = degree;} public void describe() { System.out.print("This burrito is "); switch(degree) { case NOT: System.out.println("not spicy at all."); break; case MILD: case MEDIUM: System.out.println("a little hot."); break; case HOT: case FLAMING: default: System.out.println("maybe too hot."); } } public static void main(String[] args) { Burrito plain = new Burrito(Spiciness.NOT), greenChile = new Burrito(Spiciness.MEDIUM), jalapeno = new Burrito(Spiciness.HOT); plain.describe(); greenChile.describe(); jalapeno.describe(); } } /* Output: This burrito is not spicy at all. This burrito is a little hot. This burrito is maybe too hot. *///:~ Since a switch is intended to select from a limited set of possibilities, it’s an ideal match for an enum. Notice how the enum names can produce a much clearer indication of what the program means to do. 142 Thinking in Java Bruce Eckel In general you can use an enum as if it were another way to create a data type, and then just put the results to work. That’s the point, so you don’t have to think too hard about them. Before the introduction of enum in Java SE5, you had to go to a lot of effort to make an equivalent enumerated type that was safe to use. This is enough for you to understand and use basic enums, but we’ll look more deeply at them later in the book—they have their own chapter: Enumerated Types. Exercise 21: (1) Create an enum of the least-valuable six types of paper currency. Loop through the values( ) and print each value and its ordinal( ). Exercise 22: (2) Write a switch statement for the enum in the previous example. For each case, output a description of that particular currency. Summary This seemingly elaborate mechanism for initialization, the constructor, should give you a strong hint about the critical importance placed on initialization in the language. As Bjarne Stroustrup, the inventor of C++, was designing that language, one of the first observations he made about productivity in C was that improper initialization of variables causes a significant portion of programming problems. These kinds of bugs are hard to find, and similar issues apply to improper cleanup. Because constructors allow you to guarantee proper initialization and cleanup (the compiler will not allow an object to be created without the proper constructor calls), you get complete control and safety. In C++, destruction is quite important because objects created with new must be explicitly destroyed. In Java, the garbage collector automatically releases the memory for all objects, so the equivalent cleanup method in Java isn’t necessary much of the time (but when it is, you must do it yourself). In cases where you don’t need destructor-like behavior, Java’s garbage collector greatly simplifies programming and adds much-needed safety in managing memory. Some garbage collectors can even clean up other resources like graphics and file handles. However, the garbage collector does add a runtime cost, the expense of which is difficult to put into perspective because of the historical slowness of Java interpreters. Although Java has had significant performance increases over time, the speed problem has taken its toll on the adoption of the language for certain types of programming problems. Because of the guarantee that all objects will be constructed, there’s actually more to the constructor than what is shown here. In particular, when you create new classes using either composition or inheritance, the guarantee of construction also holds, and some additional syntax is necessary to support this. You’ll learn about composition, inheritance, and how they affect constructors in future chapters. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Initialization & Cleanup 143 Access Control Access control (or implementation hiding) is about “not getting it right the first time.” All good writers—including those who write software—know that a piece of work isn’t good until it’s been rewritten, often many times. If you leave a piece of code in a drawer for a while and come back to it, you may see a much better way to do it. This is one of the prime motivations for refactoring, which rewrites working code in order to make it more readable, understandable, and thus maintainable.1 There is a tension, however, in this desire to change and improve your code. There are often consumers (client programmers) who rely on some aspect of your code staying the same. So you want to change it; they want it to stay the same. Thus a primary consideration in objectoriented design is to “separate the things that change from the things that stay the same.” This is particularly important for libraries. Consumers of that library must rely on the part they use, and know that they won’t need to rewrite code if a new version of the library comes out. On the flip side, the library creator must have the freedom to make modifications and improvements with the certainty that the client code won’t be affected by those changes. This can be achieved through convention. For example, the library programmer must agree not to remove existing methods when modifying a class in the library, since that would break the client programmer’s code. The reverse situation is thornier, however. In the case of a field, how can the library creator know which fields have been accessed by client programmers? This is also true with methods that are only part of the implementation of a class, and not meant to be used directly by the client programmer. What if the library creator wants to rip out an old implementation and put in a new one? Changing any of those members might break a client programmer’s code. Thus the library creator is in a strait jacket and can’t change anything. To solve this problem, Java provides access specifiers to allow the library creator to say what is available to the client programmer and what is not. The levels of access control from “most access” to “least access” are public, protected, package access (which has no keyword), and private. From the previous paragraph you might think that, as a library designer, you’ll want to keep everything as “private” as possible, and expose only the methods that you want the client programmer to use. This is exactly right, even though it’s often counterintuitive for people who program in other languages (especially C) and who are used to accessing everything without restriction. By the end of this chapter you should be convinced of the value of access control in Java. The concept of a library of components and the control over who can access the components of that library is not complete, however. There’s still the question of how the components are bundled together into a cohesive library unit. This is controlled with the package keyword in Java, and the access specifiers are affected by whether a class is in the same package or in a separate package. So to begin this chapter, you’ll learn how library components are placed into packages. Then you’ll be able to understand the complete meaning of the access specifiers. 1 See Refactoring: Improving the Design of Existing Code, by Martin Fowler, et al. (Addison-Wesley, 1999). Occasionally someone will argue against refactoring, suggesting that code which works is perfectly good and it’s a waste of time to refactor it. The problem with this way of thinking is that the lion’s share of a project’s time and money is not in the initial writing of the code, but in maintaining it. Making code easier to understand translates into very significant dollars. package: the library unit A package contains a group of classes, organized together under a single namespace. For example, there’s a utility library that’s part of the standard Java distribution, organized under the namespace java.util. One of the classes in java.util is called ArrayList. One way to use an ArrayList is to specify the full name java.util.ArrayList. //: access/FullQualification.java public class FullQualification { public static void main(String[] args) { java.util.ArrayList list = new java.util.ArrayList(); } } ///:~ This rapidly becomes tedious, so you’ll probably want to use the import keyword instead. If you want to import a single class, you can name that class in the import statement: //: access/SingleImport.java import java.util.ArrayList; public class SingleImport { public static void main(String[] args) { ArrayList list = new java.util.ArrayList(); } } ///:~ Now you can use ArrayList with no qualification. However, none of the other classes in java.util are available. To import everything, you simply use the ‘*’ as you’ve been seeing in the rest of the examples in this book: import java.util.*; The reason for all this importing is to provide a mechanism to manage namespaces. The names of all your class members are insulated from each other. A method f( ) inside a class A will not clash with an f( ) that has the same signature in class B. But what about the class names? Suppose you create a Stack class that is installed on a machine that already has a Stack class that’s written by someone else? This potential clashing of names is why it’s important to have complete control over the namespaces in Java, and to create a unique identifier combination for each class. Most of the examples thus far in this book have existed in a single file and have been designed for local use, so they haven’t bothered with package names. These examples have actually been in packages: the “unnamed” or default package. This is certainly an option, and for simplicity’s sake this approach will be used whenever possible throughout the rest of this book. However, if you’re planning to create libraries or programs that are friendly to other Java programs on the same machine, you must think about preventing class name clashes. When you create a source-code file for Java, it’s commonly called a compilation unit (sometimes a translation unit). Each compilation unit must have a name ending in .java, and inside the compilation unit there can be a public class that must have the same name as the file (including capitalization, but excluding the .java file name extension). There can be only one public class in each compilation unit; otherwise, the compiler will complain. If there are additional classes in that compilation unit, they are hidden from the world outside that package because they’re not public, and they comprise “support” classes for the main public class. 146 Thinking in Java Bruce Eckel Code organization When you compile a .java file, you get an output file for each class in the .java file. Each output file has the name of a class in the .java file, but with an extension of .class. Thus you can end up with quite a few .class files from a small number of .java files. If you’ve programmed with a compiled language, you might be used to the compiler spitting out an intermediate form (usually an “obj” file) that is then packaged together with others of its kind using a linker (to create an executable file) or a librarian (to create a library). That’s not how Java works. A working program is a bunch of .class files, which can be packaged and compressed into a Java ARchive (JAR) file (using Java’s jar archiver). The Java interpreter is responsible for finding, loading, and interpreting2 these files. A library is a group of these class files. Each source file usually has a public class and any number of non-public classes, so there’s one public component for each source file. If you want to say that all these components (each in its own separate .java and .class files) belong together, that’s where the package keyword comes in. If you use a package statement, it must appear as the first non-comment in the file. When you say: package access; you’re stating that this compilation unit is part of a library named access. Put another way, you’re saying that the public class name within this compilation unit is under the umbrella of the name access, and anyone who wants to use that name must either fully specify the name or use the import keyword in combination with access, using the choices given previously. (Note that the convention for Java package names is to use all lowercase letters, even for intermediate words.) For example, suppose the name of the file is MyClass.java. This means there can be one and only one public class in that file, and the name of that class must be MyClass (including the capitalization): //: access/mypackage/MyClass.java package access.mypackage; public class MyClass { // ... } ///:~ Now, if someone wants to use MyClass or, for that matter, any of the other public classes in access, they must use the import keyword to make the name or names in access available. The alternative is to give the fully qualified name: //: access/QualifiedMyClass.java public class QualifiedMyClass { public static void main(String[] args) { access.mypackage.MyClass m = new access.mypackage.MyClass(); } } ///:~ 2 There’s nothing in Java that forces the use of an interpreter. There exist native-code Java compilers that generate a single executable file. Access Control 147 The import keyword can make this much cleaner: //: access/ImportedMyClass.java import access.mypackage.*; public class ImportedMyClass { public static void main(String[] args) { MyClass m = new MyClass(); } } ///:~ It’s worth keeping in mind that what the package and import keywords allow you to do, as a library designer, is to divide up the single global namespace so you won’t have clashing names, no matter how many people get on the Internet and start writing classes in Java. Creating unique package names You might observe that, since a package never really gets “packaged” into a single file, a package can be made up of many .class files, and things could get a bit cluttered. To prevent this, a logical thing to do is to place all the .class files for a particular package into a single directory; that is, use the hierarchical file structure of the operating system to your advantage. This is one way that Java references the problem of clutter; you’ll see the other way later when the jar utility is introduced. Collecting the package files into a single subdirectory solves two other problems: creating unique package names, and finding those classes that might be buried in a directory structure someplace. This is accomplished by encoding the path of the location of the .class file into the name of the package. By convention, the first part of the package name is the reversed Internet domain name of the creator of the class. Since Internet domain names are guaranteed to be unique, if you follow this convention, your package name will be unique and you’ll never have a name clash. (That is, until you lose the domain name to someone else who starts writing Java code with the same path names as you did.) Of course, if you don’t have your own domain name, then you must fabricate an unlikely combination (such as your first and last name) to create unique package names. If you’ve decided to start publishing Java code, it’s worth the relatively small effort to get a domain name. The second part of this trick is resolving the package name into a directory on your machine, so that when the Java program runs and it needs to load the .class file, it can locate the directory where the .class file resides. The Java interpreter proceeds as follows. First, it finds the environment variable CLASSPATH3 (set via the operating system, and sometimes by the installation program that installs Java or a Java-based tool on your machine). CLASSPATH contains one or more directories that are used as roots in a search for .class files. Starting at that root, the interpreter will take the package name and replace each dot with a slash to generate a path name off of the CLASSPATH root (so package foo.bar.baz becomes foo\bar\baz or foo/bar/baz or possibly something else, depending on your operating system). This is then concatenated to the various entries in the CLASSPATH. That’s where it looks for the .class file with the name corresponding to the class you’re trying to create. (It also searches some standard directories relative to where the Java interpreter resides.) To understand this, consider my domain name, which is MindView.net. By reversing this and making it all lowercase, net.mindview establishes my unique global name for my classes. (The com, edu, org, etc., extensions were formerly capitalized in Java packages, but this was changed in Java 2 so the entire package name is lowercase.) I can further subdivide 3 When referring to the environment variable, capital letters will be used (CLASSPATH). 148 Thinking in Java Bruce Eckel this by deciding that I want to create a library named simple, so I’ll end up with a package name: package net.mindview.simple; Now this package name can be used as an umbrella namespace for the following two files: //: net/mindview/simple/Vector.java // Creating a package. package net.mindview.simple; public class Vector { public Vector() { System.out.println("net.mindview.simple.Vector"); } } ///:~ As mentioned before, the package statement must be the first non-comment code in the file. The second file looks much the same: //: net/mindview/simple/List.java // Creating a package. package net.mindview.simple; public class List { public List() { System.out.println("net.mindview.simple.List"); } } ///:~ Both of these files are placed in the subdirectory on my system: C:\DOC\JavaT\net\mindview\simple (Notice that the first comment line in every file in this book establishes the directory location of that file in the source-code tree—this is used by the automatic code-extraction tool for this book.) If you walk back through this path, you can see the package name net.mindview.simple, but what about the first portion of the path? That’s taken care of by the CLASSPATH environment variable, which is, on my machine: CLASSPATH=.;D:\JAVA\LIB;C:\DOC\JavaT You can see that the CLASSPATH can contain a number of alternative search paths. There’s a variation when using JAR files, however. You must put the actual name of the JAR file in the classpath, not just the path where it’s located. So for a JAR named grape.jar your classpath would include: CLASSPATH=.;D:\JAVA\LIB;C:\flavors\grape.jar Once the classpath is set up properly, the following file can be placed in any directory: //: access/LibTest.java // Uses the library. import net.mindview.simple.*; Access Control 149 public class LibTest { public static void main(String[] args) { Vector v = new Vector(); List l = new List(); } } /* Output: net.mindview.simple.Vector net.mindview.simple.List *///:~ When the compiler encounters the import statement for the simple library, it begins searching at the directories specified by CLASSPATH, looking for subdirectory net/mindview/simple, then seeking the compiled files of the appropriate names (Vector.class for Vector, and List.class for List). Note that both the classes and the desired methods in Vector and List must be public. Setting the CLASSPATH has been such a trial for beginning Java users (it was for me, when I started) that Sun made the JDK in later versions of Java a bit smarter. You’ll find that when you install it, even if you don’t set the CLASSPATH, you’ll be able to compile and run basic Java programs. To compile and run the source-code package for this book (available at www.MindView.net), however, you will need to add the base directory of the book’s code tree to your CLASSPATH. Exercise 1: (1) Create a class in a package. Create an instance of your class outside of that package. Collisions What happens if two libraries are imported via ‘*’ and they include the same names? For example, suppose a program does this: import net.mindview.simple.*; import java.util.*; Since java.util.* also contains a Vector class, this causes a potential collision. However, as long as you don’t write the code that actually causes the collision, everything is OK—this is good, because otherwise you might end up doing a lot of typing to prevent collisions that would never happen. The collision does occur if you now try to make a Vector: Vector v = new Vector(); Which Vector class does this refer to? The compiler can’t know, and the reader can’t know either. So the compiler complains and forces you to be explicit. If I want the standard Java Vector, for example, I must say: java.util.Vector v = new java.util.Vector(); Since this (along with the CLASSPATH) completely specifies the location of that Vector, there’s no need for the import java.util.* statement unless I’m using something else from java.util. Alternatively, you can use the single-class import form to prevent clashes—as long as you don’t use both colliding names in the same program (in which case you must fall back to fully specifying the names). 150 Thinking in Java Bruce Eckel Exercise 2: (1) Take the code fragments in this section and turn them into a program, and verify that collisions do in fact occur. A custom tool library With this knowledge, you can now create your own libraries of tools to reduce or eliminate duplicate code. Consider, for example, the alias we’ve been using for System.out.println( ), to reduce typing. This can be part of a class called Print so that you end up with a readable static import: //: net/mindview/util/Print.java // Print methods that can be used without // qualifiers, using Java SE5 static imports: package net.mindview.util; import java.io.*; public class Print { // Print with a newline: public static void print(Object obj) { System.out.println(obj); } // Print a newline by itself: public static void print() { System.out.println(); } // Print with no line break: public static void printnb(Object obj) { System.out.print(obj); } // The new Java SE5 printf() (from C): public static PrintStream printf(String format, Object... args) { return System.out.printf(format, args); } } ///:~ You can use the printing shorthand to print anything, either with a newline (print( )) or without a newline (printnb( )). You can guess that the location of this file must be in a directory that starts at one of the CLASSPATH locations, then continues into net/mindview. After compiling, the static print( ) and printnb( ) methods can be used anywhere on your system with an import static statement: //: access/PrintTest.java // Uses the static printing methods in Print.java. import static net.mindview.util.Print.*; public class PrintTest { public static void main(String[] args) { print("Available from now on!"); print(100); print(100L); print(3.14159); } } /* Output: Available from now on! 100 100 3.14159 Access Control 151 *///:~ A second component of this library can be the range( ) methods, introduced in the Controlling Execution chapter, that allow the use of the foreach syntax for simple integer sequences: //: net/mindview/util/Range.java // Array creation methods that can be used without // qualifiers, using Java SE5 static imports: package net.mindview.util; public class Range { // Produce a sequence [0..n) public static int[] range(int n) { int[] result = new int[n]; for(int i = 0; i < n; i++) result[i] = i; return result; } // Produce a sequence [start..end) public static int[] range(int start, int end) { int sz = end - start; int[] result = new int[sz]; for(int i = 0; i < sz; i++) result[i] = start + i; return result; } // Produce a sequence [start..end) incrementing by step public static int[] range(int start, int end, int step) { int sz = (end - start)/step; int[] result = new int[sz]; for(int i = 0; i < sz; i++) result[i] = start + (i * step); return result; } } ///:~ From now on, whenever you come up with a useful new utility, you can add it to your own library. You’ll see more components added to the net.mindview.util library throughout the book. Using imports to change behavior A feature that is missing from Java is C’s conditional compilation, which allows you to change a switch and get different behavior without changing any other code. The reason such a feature was left out of Java is probably because it is most often used in C to solve crossplatform issues: Different portions of the code are compiled depending on the target platform. Since Java is intended to be automatically cross-platform, such a feature should not be necessary. However, there are other valuable uses for conditional compilation. A very common use is for debugging code. The debugging features are enabled during development and disabled in the shipping product. You can accomplish this by changing the package that’s imported in order to change the code used in your program from the debug version to the production version. This technique can be used for any kind of conditional code. Exercise 3: (2) Create two packages: debug and debugoff, containing an identical class with a debug( ) method. The first version displays its String argument to the console, the 152 Thinking in Java Bruce Eckel second does nothing. Use a static import line to import the class into a test program, and demonstrate the conditional compilation effect. Package caveat It’s worth remembering that anytime you create a package, you implicitly specify a directory structure when you give the package a name. The package must live in the directory indicated by its name, which must be a directory that is searchable starting from the CLASSPATH. Experimenting with the package keyword can be a bit frustrating at first, because unless you adhere to the package-name to directory-path rule, you’ll get a lot of mysterious runtime messages about not being able to find a particular class, even if that class is sitting there in the same directory. If you get a message like this, try commenting out the package statement, and if it runs, you’ll know where the problem lies. Note that compiled code is often placed in a different directory than source code, but the path to the compiled code must still be found by the JVM using the CLASSPATH. Java access specifiers The Java access specifiers public, protected, and private are placed in front of each definition for each member in your class, whether it’s a field or a method. Each access specifier only controls the access for that particular definition. If you don’t provide an access specifier, it means “package access.” So one way or another, everything has some kind of access control. In the following sections, you’ll learn about the various types of access. Package access All the examples before this chapter used no access specifiers. The default access has no keyword, but it is commonly referred to as package access (and sometimes “friendly”). It means that all the other classes in the current package have access to that member, but to all the classes outside of this package, the member appears to be private. Since a compilation unit—a file—can belong only to a single package, all the classes within a single compilation unit are automatically available to each other via package access. Package access allows you to group related classes together in a package so that they can easily interact with each other. When you put classes together in a package, thus granting mutual access to their package-access members, you “own” the code in that package. It makes sense that only code that you own should have package access to other code that you own. You could say that package access gives a meaning or a reason for grouping classes together in a package. In many languages the way you organize your definitions in files can be arbitrary, but in Java you’re compelled to organize them in a sensible fashion. In addition, you’ll probably want to exclude classes that shouldn’t have access to the classes being defined in the current package. The class controls the code that has access to its members. Code from another package can’t just come around and say, “Hi, I’m a friend of Bob’s!” and expect to be shown the protected, package-access, and private members of Bob. The only way to grant access to a member is to: 1. Make the member public. Then everybody, everywhere, can access it. Access Control 153 2. Give the member package access by leaving off any access specifier, and put the other classes in the same package. Then the other classes in that package can access the member. 3. As you’ll see in the Reusing Classes chapter, when inheritance is introduced, an inherited class can access a protected member as well as a public member (but not private members). It can access package-access members only if the two classes are in the same package. But don’t worry about inheritance and protected right now. 4. Provide “accessor/mutator” methods (also known as “get/set” methods) that read and change the value. This is the most civilized approach in terms of OOP, and it is fundamental to JavaBeans, as you’ll see in the Graphical User Interfaces chapter. public: interface access When you use the public keyword, it means that the member declaration that immediately follows public is available to everyone, in particular to the client programmer who uses the library. Suppose you define a package dessert containing the following compilation unit: //: access/dessert/Cookie.java // Creates a library. package access.dessert; public class Cookie { public Cookie() { System.out.println("Cookie constructor"); } void bite() { System.out.println("bite"); } } ///:~ Remember, the class file produced by Cookie.java must reside in a subdirectory called dessert, in a directory under access (indicating the Access Control chapter of this book) that must be under one of the CLASSPATH directories. Don’t make the mistake of thinking that Java will always look at the current directory as one of the starting points for searching. If you don’t have a ‘.’ as one of the paths in your CLASSPATH, Java won’t look there. Now if you create a program that uses Cookie: //: access/Dinner.java // Uses the library. import access.dessert.*; public class Dinner { public static void main(String[] args) { Cookie x = new Cookie(); //! x.bite(); // Can’t access } } /* Output: Cookie constructor *///:~ you can create a Cookie object, since its constructor is public and the class is public. (We’ll look more at the concept of a public class later.) However, the bite( ) member is inaccessible inside Dinner.java since bite( ) provides access only within package dessert, so the compiler prevents you from using it. 154 Thinking in Java Bruce Eckel The default package You might be surprised to discover that the following code compiles, even though it would appear that it breaks the rules: //: access/Cake.java // Accesses a class in a separate compilation unit. class Cake { public static void main(String[] args) { Pie x = new Pie(); x.f(); } } /* Output: Pie.f() *///:~ In a second file in the same directory: //: access/Pie.java // The other class. class Pie { void f() { System.out.println("Pie.f()"); } } ///:~ You might initially view these as completely foreign files, and yet Cake is able to create a Pie object and call its f( ) method. (Note that you must have ‘.’ in your CLASSPATH in order for the files to compile.) You’d typically think that Pie and f( ) have package access and are therefore not available to Cake. They do have package access—that part is correct. The reason that they are available in Cake.java is because they are in the same directory and have no explicit package name. Java treats files like this as implicitly part of the “default package” for that directory, and thus they provide package access to all the other files in that directory. private: you can’t touch that! The private keyword means that no one can access that member except the class that contains that member, inside methods of that class. Other classes in the same package cannot access private members, so it’s as if you’re even insulating the class against yourself. On the other hand, it’s not unlikely that a package might be created by several people collaborating together, so private allows you to freely change that member without concern that it will affect another class in the same package. The default package access often provides an adequate amount of hiding; remember, a packageaccess member is inaccessible to the client programmer using the class. This is nice, since the default access is the one that you normally use (and the one that you’ll get if you forget to add any access control). Thus, you’ll typically think about access for the members that you explicitly want to make public for the client programmer, and as a result, you might initially think that you won’t use the private keyword very often, since it’s tolerable to get away without it. However, it turns out that the consistent use of private is very important, especially where multithreading is concerned. (As you’ll see in the Concurrency chapter.) Here’s an example of the use of private: //: access/IceCream.java // Demonstrates "private" keyword. Access Control 155 class Sundae { private Sundae() {} static Sundae makeASundae() { return new Sundae(); } } public class IceCream { public static void main(String[] args) { //! Sundae x = new Sundae(); Sundae x = Sundae.makeASundae(); } } ///:~ This shows an example in which private comes in handy: You might want to control how an object is created and prevent someone from directly accessing a particular constructor (or all of them). In the preceding example, you cannot create a Sundae object via its constructor; instead, you must call the makeASundae( ) method to do it for you.4 Any method that you’re certain is only a “helper” method for that class can be made private, to ensure that you don’t accidentally use it elsewhere in the package and thus prohibit yourself from changing or removing the method. Making a method private guarantees that you retain this option. The same is true for a private field inside a class. Unless you must expose the underlying implementation (which is less likely than you might think), you should make all fields private. However, just because a reference to an object is private inside a class doesn’t mean that some other object can’t have a public reference to the same object. (See the online supplements for this book to learn about aliasing issues.) protected: inheritance access Understanding the protected access specifier requires a jump ahead. First, you should be aware that you don’t need to understand this section to continue through this book up through inheritance (the Reusing Classes chapter). But for completeness, here is a brief description and example using protected. The protected keyword deals with a concept called inheritance, which takes an existing class— which we refer to as the base class—and adds new members to that class without touching the existing class. You can also change the behavior of existing members of the class. To inherit from a class, you say that your new class extends an existing class, like this: class Foo extends Bar { The rest of the class definition looks the same. If you create a new package and inherit from a class in another package, the only members you have access to are the public members of the original package. (Of course, if you perform the inheritance in the same package, you can manipulate all the members that have package access.) Sometimes the creator of the base class would like to take a particular member and grant access to derived classes but not the world in general. That’s what protected does. protected also gives package access—that is, other classes in the same package may access protected elements. 4 There’s another effect in this case: Since the default constructor is the only one defined, and it’s private, it will prevent inheritance of this class. (A subject that will be introduced later.) 156 Thinking in Java Bruce Eckel If you refer back to the file Cookie.java, the following class cannot call the package-access member bite( ): //: access/ChocolateChip.java // Can’t use package-access member from another package. import access.dessert.*; public class ChocolateChip extends Cookie { public ChocolateChip() { System.out.println("ChocolateChip constructor"); } public void chomp() { //! bite(); // Can’t access bite } public static void main(String[] args) { ChocolateChip x = new ChocolateChip(); x.chomp(); } } /* Output: Cookie constructor ChocolateChip constructor *///:~ One of the interesting things about inheritance is that if a method bite( ) exists in class Cookie, then it also exists in any class inherited from Cookie. But since bite( ) has package access and is in a foreign package, it’s unavailable to us in this one. Of course, you could make it public, but then everyone would have access, and maybe that’s not what you want. If you change the class Cookie as follows: //: access/cookie2/Cookie.java package access.cookie2; public class Cookie { public Cookie() { System.out.println("Cookie constructor"); } protected void bite() { System.out.println("bite"); } } ///:~ now bite( ) becomes accessible to anyone inheriting from Cookie: //: access/ChocolateChip2.java import access.cookie2.*; public class ChocolateChip2 extends Cookie { public ChocolateChip2() { System.out.println("ChocolateChip2 constructor"); } public void chomp() { bite(); } // Protected method public static void main(String[] args) { ChocolateChip2 x = new ChocolateChip2(); x.chomp(); } } /* Output: Cookie constructor ChocolateChip2 constructor bite *///:~ Access Control 157 Note that, although bite( ) also has package access, it is not public. Exercise 4: (2) Show that protected methods have package access but are not public. Exercise 5: (2) Create a class with public, private, protected, and package-access fields and method members. Create an object of this class and see what kind of compiler messages you get when you try to access all the class members. Be aware that classes in the same directory are part of the “default” package. Exercise 6: (1) Create a class with protected data. Create a second class in the same file with a method that manipulates the protected data in the first class. Interface and implementation Access control is often referred to as implementation hiding. Wrapping data and methods within classes in combination with implementation hiding is often called encapsulation.5 The result is a data type with characteristics and behaviors. Access control puts boundaries within a data type for two important reasons. The first is to establish what the client programmers can and can’t use. You can build your internal mechanisms into the structure without worrying that the client programmers will accidentally treat the internals as part of the interface that they should be using. This feeds directly into the second reason, which is to separate the interface from the implementation. If the structure is used in a set of programs, but client programmers can’t do anything but send messages to the public interface, then you are free to change anything that’s not public (e.g., package access, protected, or private) without breaking client code. For clarity, you might prefer a style of creating classes that puts the public members at the beginning, followed by the protected, package-access, and private members. The advantage is that the user of the class can then read down from the top and see first what’s important to them (the public members, because they can be accessed outside the file), and stop reading when they encounter the non-public members, which are part of the internal implementation: //: access/OrganizedByAccess.java public class OrganizedByAccess { public void pub1() { /* ... */ } public void pub2() { /* ... */ } public void pub3() { /* ... */ } private void priv1() { /* ... */ } private void priv2() { /* ... */ } private void priv3() { /* ... */ } private int i; // ... } ///:~ This will make it only partially easier to read, because the interface and implementation are still mixed together. That is, you still see the source code—the implementation—because it’s right there in the class. In addition, the comment documentation supported by Javadoc lessens the importance of code readability by the client programmer. Displaying the interface to the consumer of a class is really the job of the class browser, a tool whose job is to look at all the available classes and show you what you can do with them (i.e., what members are 5 However, people often refer to implementation hiding alone as encapsulation. 158 Thinking in Java Bruce Eckel available) in a useful fashion. In Java, viewing the JDK documentation with a Web browser gives you the same effect as a class browser. Class access In Java, the access specifiers can also be used to determine which classes within a library will be available to the users of that library. If you want a class to be available to a client programmer, you use the public keyword on the entire class definition. This controls whether the client programmer can even create an object of the class. To control the access of a class, the specifier must appear before the keyword class. Thus you can say: public class Widget { Now if the name of your library is access, any client programmer can access Widget by saying import access.Widget; or import access.*; However, there’s an extra set of constraints: 1. There can be only one public class per compilation unit (file). The idea is that each compilation unit has a single public interface represented by that public class. It can have as many supporting package-access classes as you want. If you have more than one public class inside a compilation unit, the compiler will give you an error message. 2. The name of the public class must exactly match the name of the file containing the compilation unit, including capitalization. So for Widget, the name of the file must be Widget.java, not widget.java or WIDGET.java. Again, you’ll get a compile-time error if they don’t agree. 3. It is possible, though not typical, to have a compilation unit with no public class at all. In this case, you can name the file whatever you like (although naming it arbitrarily will be confusing to people reading and maintaining the code). What if you’ve got a class inside access that you’re only using to accomplish the tasks performed by Widget or some other public class in access? You don’t want to go to the bother of creating documentation for the client programmer, and you think that sometime later you might want to completely change things and rip out your class altogether, substituting a different one. To give you this flexibility, you need to ensure that no client programmers become dependent on your particular implementation details hidden inside access. To accomplish this, you just leave the public keyword off the class, in which case it has package access. (That class can be used only within that package.) Exercise 7: (1) Create the library according to the code fragments describing access and Widget. Create a Widget in a class that is not part of the access package. When you create a package-access class, it still makes sense to make the fields of the class private—you should always make fields as private as possible—but it’s generally reasonable to give the methods the same access as the class (package access). Since a package-access Access Control 159 class is usually used only within the package, you only need to make the methods of such a class public if you’re forced to, and in those cases, the compiler will tell you. Note that a class cannot be private (that would make it inaccessible to anyone but the class) or protected.6 So you have only two choices for class access: package access or public. If you don’t want anyone else to have access to that class, you can make all the constructors private, thereby preventing anyone but you, inside a static member of the class, from creating an object of that class. Here’s an example: //: access/Lunch.java // Demonstrates class access specifiers. Make a class // effectively private with private constructors: class Soup1 { private Soup1() {} // (1) Allow creation via static method: public static Soup1 makeSoup() { return new Soup1(); } } class Soup2 { private Soup2() {} // (2) Create a static object and return a reference // upon request.(The "Singleton" pattern): private static Soup2 ps1 = new Soup2(); public static Soup2 access() { return ps1; } public void f() {} } // Only one public class allowed per file: public class Lunch { void testPrivate() { // Can’t do this! Private constructor: //! Soup1 soup = new Soup1(); } void testStatic() { Soup1 soup = Soup1.makeSoup(); } void testSingleton() { Soup2.access().f(); } } ///:~ Up to now, most of the methods have been returning either void or a primitive type, so the definition: public static Soup1 makeSoup() { return new Soup1(); } might look a little confusing at first. The word Soup1 before the method name (makeSoup) tells what the method returns. So far in this book, this has usually been void, which means it returns nothing. But you can also return a reference to an object, which is what happens here. This method returns a reference to an object of class Soup1. 6 Actually, an inner class can be private or protected, but that’s a special case. These will be introduced in the Inner Classes chapter. 160 Thinking in Java Bruce Eckel The classes Soup1 and Soup2 show how to prevent direct creation of a class by making all the constructors private. Remember that if you don’t explicitly create at least one constructor, the default constructor (a constructor with no arguments) will be created for you. By writing the default constructor, it won’t be created automatically. By making it private, no one can create an object of that class. But now how does anyone use this class? The preceding example shows two options. In Soup1, a static method is created that creates a new Soup1 and returns a reference to it. This can be useful if you want to do some extra operations on the Soup1 before returning it, or if you want to keep count of how many Soup1 objects to create (perhaps to restrict their population). Soup2 uses what’s called a design pattern, which is covered in Thinking in Patterns (with Java) at www.MindView.net. This particular pattern is called a Singleton, because it allows only a single object to ever be created. The object of class Soup2 is created as a static private member of Soup2, so there’s one and only one, and you can’t get at it except through the public method access( ). As previously mentioned, if you don’t put an access specifier for class access, it defaults to package access. This means that an object of that class can be created by any other class in the package, but not outside the package. (Remember, all the files within the same directory that don’t have explicit package declarations are implicitly part of the default package for that directory.) However, if a static member of that class is public, the client programmer can still access that static member even though they cannot create an object of that class. Exercise 8: (4) Following the form of the example Lunch.java, create a class called ConnectionManager that manages a fixed array of Connection objects. The client programmer must not be able to explicitly create Connection objects, but can only get them via a static method in ConnectionManager. When the ConnectionManager runs out of objects, it returns a null reference. Test the classes in main( ). Exercise 9: (2) Create the following file in the access/local directory (presumably in your CLASSPATH): // access/local/PackagedClass.java package access.local; class PackagedClass { public PackagedClass() { System.out.println("Creating a packaged class"); } } Then create the following file in a directory other than access/local: // access/foreign/Foreign.java package access.foreign; import access.local.*; public class Foreign { public static void main(String[] args) { PackagedClass pc = new PackagedClass(); } } Explain why the compiler generates an error. Would making the Foreign class part of the access.local package change anything? Access Control 161 Summary In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the user of that library—the client programmer—who is another programmer, but one using your library to build an application or a bigger library. Without rules, client programmers can do anything they want with all the members of a class, even if you might prefer they don’t directly manipulate some of the members. Everything’s naked to the world. This chapter looked at how classes are built to form libraries: first, the way a group of classes is packaged within a library, and second, the way the class controls access to its members. It is estimated that a C programming project begins to break down somewhere between 50K and 100K lines of code because C has a single namespace, and names begin to collide, causing extra management overhead. In Java, the package keyword, the package naming scheme, and the import keyword give you complete control over names, so the issue of name collision is easily avoided. There are two reasons for controlling access to members. The first is to keep users’ hands off portions that they shouldn’t touch. These pieces are necessary for the internal operations of the class, but not part of the interface that the client programmer needs. So making methods and fields private is a service to client programmers, because they can easily see what’s important to them and what they can ignore. It simplifies their understanding of the class. The second and most important reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. You might, for example, build a class one way at first, and then discover that restructuring your code will provide much greater speed. If the interface and implementation are clearly separated and protected, you can accomplish this without forcing client programmers to rewrite their code. Access control ensures that no client programmer becomes dependent on any part of the underlying implementation of a class. When you have the ability to change the underlying implementation, you not only have the freedom to improve your design, you also have the freedom to make mistakes. No matter how carefully you plan and design, you’ll make mistakes. Knowing that it’s relatively safe to make these mistakes means you’ll be more experimental, you’ll learn more quickly, and you’ll finish your project sooner. The public interface to a class is what the user does see, so that is the most important part of the class to get “right” during analysis and design. Even that allows you some leeway for change. If you don’t get the interface right the first time, you can add more methods, as long as you don’t remove any that client programmers have already used in their code. Notice that access control focuses on a relationship—and a kind of communication—between a library creator and the external clients of that library. There are many situations where this is not the case. For example, you are writing all the code yourself, or you are working in close quarters with a small team and everything goes into the same package. These situations have a different kind of communication, and rigid adherence to access rules may not be optimal. Default (package) access may be just fine. 162 Thinking in Java Bruce Eckel Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Access Control 163 Reusing Classes One of the most compelling features about Java is code reuse. But to be revolutionary, you’ve got to be able to do a lot more than copy code and change it. That’s the approach used in procedural languages like C, and it hasn’t worked very well. Like everything in Java, the solution revolves around the class. You reuse code by creating new classes, but instead of creating them from scratch, you use existing classes that someone has already built and debugged. The trick is to use the classes without soiling the existing code. In this chapter you’ll see two ways to accomplish this. The first is quite straightforward: you simply create objects of your existing class inside the new class. This is called composition, because the new class is composed of objects of existing classes. You’re simply reusing the functionality of the code, not its form. The second approach is more subtle. It creates a new class as a type of an existing class. You literally take the form of the existing class and add code to it without modifying the existing class. This technique is called inheritance, and the compiler does most of the work. Inheritance is one of the cornerstones of object-oriented programming, and has additional implications that will be explored in the Polymorphism chapter. It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense because they are both ways of making new types from existing types). In this chapter, you’ll learn about these code reuse mechanisms. Composition syntax Composition has been used quite frequently up to this point in the book. You simply place object references inside new classes. For example, suppose you’d like an object that holds several String objects, a couple of primitives, and an object of another class. For the nonprimitive objects, you put references inside your new class, but you define the primitives directly: //: reusing/SprinklerSystem.java // Composition for code reuse. class WaterSource { private String s; WaterSource() { System.out.println("WaterSource()"); s = "Constructed"; } public String toString() { return s; } } public class SprinklerSystem { private String valve1, valve2, valve3, valve4; private WaterSource source = new WaterSource(); private int i; private float f; public String toString() { return "valve1 = " + valve1 + " " + "valve2 = " + valve2 + " " + "valve3 = " + valve3 + " " + "valve4 = " + valve4 + "\n" + "i = " + i + " " + "f = " + f + " " + "source = " + source; } public static void main(String[] args) { SprinklerSystem sprinklers = new SprinklerSystem(); System.out.println(sprinklers); } } /* Output: WaterSource() valve1 = null valve2 = null valve3 = null valve4 = null i = 0 f = 0.0 source = Constructed *///: One of the methods defined in both classes is special: toString( ). Every non-primitive object has a toString( ) method, and it’s called in special situations when the compiler wants a String but it has an object. So in the expression in SprinklerSystem.toString( ): "source = " + source; the compiler sees you trying to add a String object ("source = ") to a WaterSource. Because you can only “add” a String to another String, it says “I’ll turn source into a String by calling toString( )!” After doing this it can combine the two Strings and pass the resulting String to System.out.println( ) (or equivalently, this book’s print() and printnb( ) static methods). Any time you want to allow this behavior with a class you create, you need only write a toString( ) method. Primitives that are fields in a class are automatically initialized to zero, as noted in the Everything Is an Object chapter. But the object references are initialized to null, and if you try to call methods for any of them, you’ll get an exception-a runtime error. Conveniently, you can still print a null reference without throwing an exception. It makes sense that the compiler doesn’t just create a default object for every reference, because that would incur unnecessary overhead in many cases. If you want the references initialized, you can do it: 1. At the point the objects are defined. This means that they’ll always be initialized before the constructor is called. 2. In the constructor for that class. 3. Right before you actually need to use the object. This is often called lazy initialization. It can reduce overhead in situations where object creation is expensive and the object doesn’t need to be created every time. 4. Using instance initialization. All four approaches are shown here: //: reusing/Bath.java // Constructor initialization with composition. import static net.mindview.util.Print.*; class Soap { private String s; 166 Thinking in Java Bruce Eckel Soap() { print("Soap()"); s = "Constructed"; } public String toString() { return s; } } public class Bath { private String // Initializing at point of definition: s1 = "Happy", s2 = "Happy", s3, s4; private Soap castille; private int i; private float toy; public Bath() { print("Inside Bath()"); s3 = "Joy"; toy = 3.14f; castille = new Soap(); } // Instance initialization: { i = 47; } public String toString() { if(s4 == null) // Delayed initialization: s4 = "Joy"; return "s1 = " + s1 + "\n" + "s2 = " + s2 + "\n" + "s3 = " + s3 + "\n" + "s4 = " + s4 + "\n" + "i = " + i + "\n" + "toy = " + toy + "\n" + "castille = " + castille; } public static void main(String[] args) { Bath b = new Bath(); print(b); } } /* Output: Inside Bath() Soap() s1 = Happy s2 = Happy s3 = Joy s4 = Joy i = 47 toy = 3.14 castille = Constructed *///:~ Note that in the Bath constructor, a statement is executed before any of the initializations take place. When you don’t initialize at the point of definition, there’s still no guarantee that you’ll perform any initialization before you send a message to an object reference—except for the inevitable run-time exception. When toString( ) is called it fills in s4 so that all the fields are properly initialized by the time they are used. Exercise 1: (2) Create a simple class. Inside a second class, define a reference to an object of the first class. Use lazy initialization to instantiate this object. Reusing Classes 167 Inheritance syntax Inheritance is an integral part of Java (and all OOP languages). It turns out that you’re always doing inheritance when you create a class, because unless you explicitly inherit from some other class, you implicitly inherit from Java’s standard root class Object. The syntax for composition is obvious, but to perform inheritance there’s a distinctly different form. When you inherit, you say “This new class is like that old class.” You state this in code before the opening brace of the class body, using the keyword extends followed by the name of the base class. When you do this, you automatically get all the fields and methods in the base class. Here’s an example: //: reusing/Detergent.java // Inheritance syntax & properties. import static net.mindview.util.Print.*; class Cleanser { private String s = "Cleanser"; public void append(String a) { s += a; } public void dilute() { append(" dilute()"); } public void apply() { append(" apply()"); } public void scrub() { append(" scrub()"); } public String toString() { return s; } public static void main(String[] args) { Cleanser x = new Cleanser(); x.dilute(); x.apply(); x.scrub(); print(x); } } public class Detergent extends Cleanser { // Change a method: public void scrub() { append(" Detergent.scrub()"); super.scrub(); // Call base-class version } // Add methods to the interface: public void foam() { append(" foam()"); } // Test the new class: public static void main(String[] args) { Detergent x = new Detergent(); x.dilute(); x.apply(); x.scrub(); x.foam(); print(x); print("Testing base class:"); Cleanser.main(args); } } /* Output: Cleanser dilute() apply() Detergent.scrub() scrub() foam() Testing base class: Cleanser dilute() apply() scrub() *///:~ This demonstrates a number of features. First, in the Cleanser append( ) method, Strings are concatenated to s using the += operator, which is one of the operators (along with ‘+’) that the Java designers “overloaded” to work with Strings. 168 Thinking in Java Bruce Eckel Second, both Cleanser and Detergent contain a main( ) method. You can create a main( ) for each one of your classes; this technique of putting a main() in each class allows easy testing for each class. And you don’t need to remove the main() when you’re finished; you can leave it in for later testing. Even if you have a lot of classes in a program, only the main( ) for the class invoked on the command line will be called. So in this case, when you say java Detergent, Detergent.main( ) will be called. But you can also say java Cleanser to invoke Cleanser.main( ), even though Cleanser is not a public class. Even if a class has package access, a public main() is accessible. Here, you can see that Detergent.main( ) calls Cleanser.main( ) explicitly, passing it the same arguments from the command line (however, you could pass it any String array). It’s important that all of the methods in Cleanser are public. Remember that if you leave off any access specifier, the member defaults to package access, which allows access only to package members. Thus, within this package, anyone could use those methods if there were no access specifier. Detergent would have no trouble, for example. However, if a class from some other package were to inherit from Cleanser, it could access only public members. So to allow for inheritance, as a general rule make all fields private and all methods public. (protected members also allow access by derived classes; you’ll learn about this later.) Of course, in particular cases you must make adjustments, but this is a useful guideline. Cleanser has a set of methods in its interface: append( ), dilute( ), apply( ), scrub( ), and toString( ). Because Detergent is derived from Cleanser (via the extends keyword), it automatically gets all these methods in its interface, even though you don’t see them all explicitly defined in Detergent. You can think of inheritance, then, as reusing the class. As seen in scrub( ), it’s possible to take a method that’s been defined in the base class and modify it. In this case, you might want to call the method from the base class inside the new version. But inside scrub( ), you cannot simply call scrub( ), since that would produce a recursive call, which isn’t what you want. To solve this problem, Java has the keyword super that refers to the “superclass” that the current class inherits. Thus the expression super.scrub( ) calls the base-class version of the method scrub( ). When inheriting you’re not restricted to using the methods of the base class. You can also add new methods to the derived class exactly the way you put any method in a class: Just define it. The method foam( ) is an example of this. In Detergent.main( ) you can see that for a Detergent object, you can call all the methods that are available in Cleanser as well as in Detergent (i.e., foam( )). Exercise 2: (2) Inherit a new class from class Detergent. Override scrub( ) and add a new method called sterilize( ). Initializing the base class Since there are now two classes involved—the base class and the derived class—instead of just one, it can be a bit confusing to try to imagine the resulting object produced by a derived class. From the outside, it looks like the new class has the same interface as the base class and maybe some additional methods and fields. But inheritance doesn’t just copy the interface of the base class. When you create an object of the derived class, it contains within it a subobject of the base class. This subobject is the same as if you had created an object of the base class by itself. It’s just that from the outside, the subobject of the base class is wrapped within the derived-class object. Reusing Classes 169 Of course, it’s essential that the base-class subobject be initialized correctly, and there’s only one way to guarantee this: Perform the initialization in the constructor by calling the baseclass constructor, which has all the appropriate knowledge and privileges to perform the base-class initialization. Java automatically inserts calls to the base-class constructor in the derived-class constructor. The following example shows this working with three levels of inheritance: //: reusing/Cartoon.java // Constructor calls during inheritance. import static net.mindview.util.Print.*; class Art { Art() { print("Art constructor"); } } class Drawing extends Art { Drawing() { print("Drawing constructor"); } } public class Cartoon extends Drawing { public Cartoon() { print("Cartoon constructor"); } public static void main(String[] args) { Cartoon x = new Cartoon(); } } /* Output: Art constructor Drawing constructor Cartoon constructor *///:~ You can see that the construction happens from the base “outward,” so the base class is initialized before the derived-class constructors can access it. Even if you don’t create a constructor for Cartoon( ), the compiler will synthesize a default constructor for you that calls the base class constructor. Exercise 3: (2) Prove the previous sentence. Exercise 4: (2) Prove that the base-class constructors are (a) always called and (b) called before derived-class constructors. Exercise 5: (1) Create two classes, A and B, with default constructors (empty argument lists) that announce themselves. Inherit a new class called C from A, and create a member of class B inside C. Do not create a constructor for C. Create an object of class C and observe the results. Constructors with arguments The preceding example has default constructors; that is, they don’t have any arguments. It’s easy for the compiler to call these because there’s no question about what arguments to pass. If your class doesn’t have default arguments, or if you want to call a base-class constructor that has an argument, you must explicitly write the calls to the base-class constructor using the super keyword and the appropriate argument list: //: reusing/Chess.java // Inheritance, constructors and arguments. import static net.mindview.util.Print.*; class Game { Game(int i) { 170 Thinking in Java Bruce Eckel print("Game constructor"); } } class BoardGame extends Game { BoardGame(int i) { super(i); print("BoardGame constructor"); } } public class Chess extends BoardGame { Chess() { super(11); print("Chess constructor"); } public static void main(String[] args) { Chess x = new Chess(); } } /* Output: Game constructor BoardGame constructor Chess constructor *///:~ If you don’t call the base-class constructor in BoardGame( ), the compiler will complain that it can’t find a constructor of the form Game( ). In addition, the call to the base-class constructor must be the first thing you do in the derived-class constructor. (The compiler will remind you if you get it wrong.) Exercise 6: (1) Using Chess.java, prove the statements in the previous paragraph. Exercise 7: (1) Modify Exercise 5 so that A and B have constructors with arguments instead of default constructors. Write a constructor for C and perform all initialization within C’s constructor. Exercise 8: (1) Create a base class with only a non-default constructor, and a derived class with both a default (no-arg) and non-default constructor. In the derived-class constructors, call the base-class constructor. Exercise 9: (2) Create a class called Root that contains an instance of each of the classes (that you also create) named Component1, Component2, and Component3. Derive a class Stem from Root that also contains an instance of each “component.” All classes should have default constructors that print a message about that class. Exercise 10: (1) Modify the previous exercise so that each class only has non-default constructors. Delegation A third relationship, which is not directly supported by Java, is called delegation. This is midway between inheritance and composition, because you place a member object in the class you’re building (like composition), but at the same time you expose all the methods from the member object in your new class (like inheritance). For example, a spaceship needs a control module: //: reusing/SpaceShipControls.java Reusing Classes 171 public class SpaceShipControls { void up(int velocity) {} void down(int velocity) {} void left(int velocity) {} void right(int velocity) {} void forward(int velocity) {} void back(int velocity) {} void turboBoost() {} } ///:~ One way to build a spaceship is to use inheritance: //: reusing/SpaceShip.java public class SpaceShip extends SpaceShipControls { private String name; public SpaceShip(String name) { this.name = name; } public String toString() { return name; } public static void main(String[] args) { SpaceShip protector = new SpaceShip("NSEA Protector"); protector.forward(100); } } ///:~ However, a SpaceShip isn’t really “a type of” SpaceShipControls, even if, for example, you “tell” a SpaceShip to go forward( ). It’s more accurate to say that a SpaceShip contains SpaceShipControls, and at the same time all the methods in SpaceShipControls are exposed in a SpaceShip. Delegation solves the dilemma: //: reusing/SpaceShipDelegation.java public class SpaceShipDelegation { private String name; private SpaceShipControls controls = new SpaceShipControls(); public SpaceShipDelegation(String name) { this.name = name; } // Delegated methods: public void back(int velocity) { controls.back(velocity); } public void down(int velocity) { controls.down(velocity); } public void forward(int velocity) { controls.forward(velocity); } public void left(int velocity) { controls.left(velocity); } public void right(int velocity) { controls.right(velocity); } public void turboBoost() { controls.turboBoost(); } public void up(int velocity) { controls.up(velocity); } public static void main(String[] args) { 172 Thinking in Java Bruce Eckel SpaceShipDelegation protector = new SpaceShipDelegation("NSEA Protector"); protector.forward(100); } } ///:~ You can see how the methods are forwarded to the underlying controls object, and the interface is thus the same as it is with inheritance. However, you have more control with delegation because you can choose to provide only a subset of the methods in the member object. Although the Java language doesn’t support delegation, development tools often do. The above example, for instance, was automatically generated using the JetBrains Idea IDE. Exercise 11: (3) Modify Detergent.java so that it uses delegation. Combining composition and inheritance It is very common to use composition and inheritance together. The following example shows the creation of a more complex class, using both inheritance and composition, along with the necessary constructor initialization: //: reusing/PlaceSetting.java // Combining composition & inheritance. import static net.mindview.util.Print.*; class Plate { Plate(int i) { print("Plate constructor"); } } class DinnerPlate extends Plate { DinnerPlate(int i) { super(i); print("DinnerPlate constructor"); } } class Utensil { Utensil(int i) { print("Utensil constructor"); } } class Spoon extends Utensil { Spoon(int i) { super(i); print("Spoon constructor"); } } class Fork extends Utensil { Fork(int i) { super(i); print("Fork constructor"); } Reusing Classes 173 } class Knife extends Utensil { Knife(int i) { super(i); print("Knife constructor"); } } // A cultural way of doing something: class Custom { Custom(int i) { print("Custom constructor"); } } public class PlaceSetting extends Custom { private Spoon sp; private Fork frk; private Knife kn; private DinnerPlate pl; public PlaceSetting(int i) { super(i + 1); sp = new Spoon(i + 2); frk = new Fork(i + 3); kn = new Knife(i + 4); pl = new DinnerPlate(i + 5); print("PlaceSetting constructor"); } public static void main(String[] args) { PlaceSetting x = new PlaceSetting(9); } } /* Output: Custom constructor Utensil constructor Spoon constructor Utensil constructor Fork constructor Utensil constructor Knife constructor Plate constructor DinnerPlate constructor PlaceSetting constructor *///:~ Although the compiler forces you to initialize the base classes, and requires that you do it right at the beginning of the constructor, it doesn’t watch over you to make sure that you initialize the member objects, so you must remember to pay attention to that. It’s rather amazing how cleanly the classes are separated. You don’t even need the source code for the methods in order to reuse the code. At most, you just import a package. (This is true for both inheritance and composition.) Guaranteeing proper cleanup Java doesn’t have the C++ concept of a destructor, a method that is automatically called when an object is destroyed. The reason is probably that in Java, the practice is simply to forget about objects rather than to destroy them, allowing the garbage collector to reclaim the memory as necessary. 174 Thinking in Java Bruce Eckel Often this is fine, but there are times when your class might perform some activities during its lifetime that require cleanup. As mentioned in the Initialization & Cleanup chapter, you can’t know when the garbage collector will be called, or if it will be called. So if you want something cleaned up for a class, you must explicitly write a special method to do it, and make sure that the client programmer knows that they must call this method. On top of this—as described in the Error Handling with Exceptions chapter—you must guard against an exception by putting such cleanup in a finally clause. Consider an example of a computer-aided design system that draws pictures on the screen: //: reusing/CADSystem.java // Ensuring proper cleanup. package reusing; import static net.mindview.util.Print.*; class Shape { Shape(int i) { print("Shape constructor"); } void dispose() { print("Shape dispose"); } } class Circle extends Shape { Circle(int i) { super(i); print("Drawing Circle"); } void dispose() { print("Erasing Circle"); super.dispose(); } } class Triangle extends Shape { Triangle(int i) { super(i); print("Drawing Triangle"); } void dispose() { print("Erasing Triangle"); super.dispose(); } } class Line extends Shape { private int start, end; Line(int start, int end) { super(start); this.start = start; this.end = end; print("Drawing Line: " + start + ", " + end); } void dispose() { print("Erasing Line: " + start + ", " + end); super.dispose(); } } public class CADSystem extends Shape { private Circle c; private Triangle t; private Line[] lines = new Line[3]; public CADSystem(int i) { super(i + 1); Reusing Classes 175 for(int j = 0; j < lines.length; j++) lines[j] = new Line(j, j*j); c = new Circle(1); t = new Triangle(1); print("Combined constructor"); } public void dispose() { print("CADSystem.dispose()"); // The order of cleanup is the reverse // of the order of initialization: t.dispose(); c.dispose(); for(int i = lines.length - 1; i >= 0; i--) lines[i].dispose(); super.dispose(); } public static void main(String[] args) { CADSystem x = new CADSystem(47); try { // Code and exception handling... } finally { x.dispose(); } } } /* Output: Shape constructor Shape constructor Drawing Line: 0, 0 Shape constructor Drawing Line: 1, 1 Shape constructor Drawing Line: 2, 4 Shape constructor Drawing Circle Shape constructor Drawing Triangle Combined constructor CADSystem.dispose() Erasing Triangle Shape dispose Erasing Circle Shape dispose Erasing Line: 2, 4 Shape dispose Erasing Line: 1, 1 Shape dispose Erasing Line: 0, 0 Shape dispose Shape dispose *///:~ Everything in this system is some kind of Shape (which is itself a kind of Object, since it’s implicitly inherited from the root class). Each class overrides Shape’s dispose( ) method in addition to calling the base-class version of that method using super. The specific Shape classes—Circle, Triangle, and Line—all have constructors that “draw,” although any method called during the lifetime of the object could be responsible for doing something that needs cleanup. Each class has its own dispose( ) method to restore non-memory things back to the way they were before the object existed. In main( ), you can see two keywords that are new, and won’t be explained until the Error Handling with Exceptions chapter: try and finally. The try keyword indicates that the block that follows (delimited by curly braces) is a guarded region, which means that it is given 176 Thinking in Java Bruce Eckel special treatment. One of these special treatments is that the code in the finally clause following this guarded region is always executed, no matter how the try block exits. (With exception handling, it’s possible to leave a try block in a number of non-ordinary ways.) Here, the finally clause is saying “always call dispose( ) for x, no matter what happens.” Note that in your cleanup method, you must also pay attention to the calling order for the base-class and member-object cleanup methods in case one subobject depends on another. In general, you should follow the same form that is imposed by a C++ compiler on its destructors: First perform all of the cleanup work specific to your class, in the reverse order of creation. (In general, this requires that base-class elements still be viable.) Then call the base-class cleanup method, as demonstrated here. There can be many cases in which the cleanup issue is not a problem; you just let the garbage collector do the work. But when you must do it explicitly, diligence and attention are required, because there’s not much you can rely on when it comes to garbage collection. The garbage collector might never be called. If it is, it can reclaim objects in any order it wants. You can’t rely on garbage collection for anything but memory reclamation. If you want cleanup to take place, make your own cleanup methods and don’t use on finalize( ). Exercise 12: (3) Add a proper hierarchy of dispose( ) methods to all the classes in Exercise 9. Name hiding If a Java base class has a method name that’s overloaded several times, redefining that method name in the derived class will not hide any of the base-class versions (unlike C++). Thus overloading works regardless of whether the method was defined at this level or in a base class: //: reusing/Hide.java // Overloading a base-class method name in a derived // class does not hide the base-class versions. import static net.mindview.util.Print.*; class Homer { char doh(char c) { print("doh(char)"); return ‘d’; } float doh(float f) { print("doh(float)"); return 1.0f; } } class Milhouse {} class Bart extends Homer { void doh(Milhouse m) { print("doh(Milhouse)"); } } public class Hide { public static void main(String[] args) { Bart b = new Bart(); b.doh(1); b.doh(‘x’); b.doh(1.0f); Reusing Classes 177 b.doh(new Milhouse()); } } /* Output: doh(float) doh(char) doh(float) doh(Milhouse) *///:~ You can see that all the overloaded methods of Homer are available in Bart, even though Bart introduces a new overloaded method (in C++ doing this would hide the base-class methods). As you’ll see in the next chapter, it’s far more common to override methods of the same name, using exactly the same signature and return type as in the base class. It can be confusing otherwise (which is why C++ disallows it—to prevent you from making what is probably a mistake). Java SE5 has added the @Override annotation, which is not a keyword but can be used as if it were. When you mean to override a method, you can choose to add this annotation and the compiler will produce an error message if you accidentally overload instead of overriding. //: reusing/Lisa.java // {CompileTimeError} (Won’t compile) class Lisa extends Homer { @Override void doh(Milhouse m) { System.out.println("doh(Milhouse)"); } } ///:~ The {CompileTimeError} tag excludes the file from this book’s Ant build, but if you compile it by hand you’ll see the error message: method does not override a method from its superclass The @Override annotation will thus prevent you from accidentally overloading when you don’t mean to. Exercise 13: (2) Create a class with a method that is overloaded three times. Inherit a new class, add a new overloading of the method, and show that all four methods are available in the derived class. Choosing composition vs. inheritance Both composition and inheritance allow you to place subobjects inside your new class (composition explicitly does this—with inheritance it’s implicit). You might wonder about the difference between the two, and when to choose one over the other. Composition is generally used when you want the features of an existing class inside your new class, but not its interface. That is, you embed an object so that you can use it to implement features in your new class, but the user of your new class sees the interface you’ve defined for the new class rather than the interface from the embedded object. For this effect, you embed private objects of existing classes inside your new class. Sometimes it makes sense to allow the class user to directly access the composition of your new class; that is, to make the member objects public. The member objects use 178 Thinking in Java Bruce Eckel implementation hiding themselves, so this is a safe thing to do. When the user knows you’re assembling a bunch of parts, it makes the interface easier to understand. A car object is a good example: //: reusing/Car.java // Composition with public objects. class Engine { public void start() {} public void rev() {} public void stop() {} } class Wheel { public void inflate(int psi) {} } class Window { public void rollup() {} public void rolldown() {} } class Door { public Window window = new Window(); public void open() {} public void close() {} } public class Car { public Engine engine = new Engine(); public Wheel[] wheel = new Wheel[4]; public Door left = new Door(), right = new Door(); // 2-door public Car() { for(int i = 0; i < 4; i++) wheel[i] = new Wheel(); } public static void main(String[] args) { Car car = new Car(); car.left.window.rollup(); car.wheel[0].inflate(72); } } ///:~ Because in this case the composition of a car is part of the analysis of the problem (and not simply part of the underlying design), making the members public assists the client programmer’s understanding of how to use the class and requires less code complexity for the creator of the class. However, keep in mind that this is a special case, and that in general you should make fields private. When you inherit, you take an existing class and make a special version of it. In general, this means that you’re taking a general-purpose class and specializing it for a particular need. With a little thought, you’ll see that it would make no sense to compose a car using a vehicle object—a car doesn’t contain a vehicle, it is a vehicle. The is-a relationship is expressed with inheritance, and the has-a relationship is expressed with composition. Exercise 14: (1) In Car.java add a service( ) method to Engine and call this method in main( ). Reusing Classes 179 protected Now that you’ve been introduced to inheritance, the keyword protected finally has meaning. In an ideal world, the private keyword would be enough. In real projects, there are times when you want to make something hidden from the world at large and yet allow access for members of derived classes. The protected keyword is a nod to pragmatism. It says “This is private as far as the class user is concerned, but available to anyone who inherits from this class or anyone else in the same package.” (In Java, protected also provides package access.) Although it’s possible to create protected fields, the best approach is to leave the fields private; you should always preserve your right to change the underlying implementation. You can then allow controlled access to inheritors of your class through protected methods: //: reusing/Orc.java // The protected keyword. import static net.mindview.util.Print.*; class Villain { private String name; protected void set(String nm) { name = nm; } public Villain(String name) { this.name = name; } public String toString() { return "I’m a Villain and my name is " + name; } } public class Orc extends Villain { private int orcNumber; public Orc(String name, int orcNumber) { super(name); this.orcNumber = orcNumber; } public void change(String name, int orcNumber) { set(name); // Available because it’s protected this.orcNumber = orcNumber; } public String toString() { return "Orc " + orcNumber + ": " + super.toString(); } public static void main(String[] args) { Orc orc = new Orc("Limburger", 12); print(orc); orc.change("Bob", 19); print(orc); } } /* Output: Orc 12: I’m a Villain and my name is Limburger Orc 19: I’m a Villain and my name is Bob *///:~ You can see that change( ) has access to set( ) because it’s protected. Also note the way that Orc’s toString( ) method is defined in terms of the base-class version of toString( ). Exercise 15: (2) Create a class inside a package. Your class should contain a protected method. Outside of the package, try to call the protected method and explain the results. Now inherit from your class and call the protected method from inside a method of your derived class. 180 Thinking in Java Bruce Eckel Upcasting The most important aspect of inheritance is not that it provides methods for the new class. It’s the relationship expressed between the new class and the base class. This relationship can be summarized by saying, “The new class is a type of the existing class.” This description is not just a fanciful way of explaining inheritance—it’s supported directly by the language. As an example, consider a base class called Instrument that represents musical instruments, and a derived class called Wind. Because inheritance means that all of the methods in the base class are also available in the derived class, any message you can send to the base class can also be sent to the derived class. If the Instrument class has a play( ) method, so will Wind instruments. This means we can accurately say that a Wind object is also a type of Instrument. The following example shows how the compiler supports this notion: //: reusing/Wind.java // Inheritance & upcasting. class Instrument { public void play() {} static void tune(Instrument i) { // ... i.play(); } } // Wind objects are instruments // because they have the same interface: public class Wind extends Instrument { public static void main(String[] args) { Wind flute = new Wind(); Instrument.tune(flute); // Upcasting } } ///:~ What’s interesting in this example is the tune( ) method, which accepts an Instrument reference. However, in Wind.main( ) the tune( ) method is called by giving it a Wind reference. Given that Java is particular about type checking, it seems strange that a method that accepts one type will readily accept another type, until you realize that a Wind object is also an Instrument object, and there’s no method that tune( ) could call for an Instrument that isn’t also in Wind. Inside tune( ), the code works for Instrument and anything derived from Instrument, and the act of converting a Wind reference into an Instrument reference is called upcasting. Why “upcasting”? The term is based on the way that class inheritance diagrams have traditionally been drawn: with the root at the top of the page, growing downward. (Of course, you can draw your diagrams any way you find helpful.) The inheritance diagram for Wind.java is then: Reusing Classes 181 Casting from a derived type to a base type moves up on the inheritance diagram, so it’s commonly referred to as upcasting. Upcasting is always safe because you’re going from a more specific type to a more general type. That is, the derived class is a superset of the base class. It might contain more methods than the base class, but it must contain at least the methods in the base class. The only thing that can occur to the class interface during the upcast is that it can lose methods, not gain them. This is why the compiler allows upcasting without any explicit casts or other special notation. You can also perform the reverse of upcasting, called downcasting, but this involves a dilemma that will be examined further in the next chapter, and in the Type Information chapter. Composition vs. inheritance revisited In object-oriented programming, the most likely way that you’ll create and use code is by simply packaging data and methods together into a class, and using objects of that class. You’ll also use existing classes to build new classes with composition. Less frequently, you’ll use inheritance. So although inheritance gets a lot of emphasis while learning OOP, it doesn’t mean that you should use it everywhere you possibly can. On the contrary, you should use it sparingly, only when it’s clear that inheritance is useful. One of the clearest ways to determine whether you should use composition or inheritance is to ask whether you’ll ever need to upcast from your new class to the base class. If you must upcast, then inheritance is necessary, but if you don’t need to upcast, then you should look closely at whether you need inheritance. The Polymorphism chapter provides one of the most compelling reasons for upcasting, but if you remember to ask “Do I need to upcast?” you’ll have a good tool for deciding between composition and inheritance. Exercise 16: (2) Create a class called Amphibian. From this, inherit a class called Frog. Put appropriate methods in the base class. In main( ), create a Frog and upcast it to Amphibian and demonstrate that all the methods still work. Exercise 17: (1) Modify Exercise 16 so that Frog overrides the method definitions from the base class (provides new definitions using the same method signatures). Note what happens in main( ). The final keyword Java’s final keyword has slightly different meanings depending on the context, but in general it says “This cannot be changed.” You might want to prevent changes for two reasons: design or efficiency. Because these two reasons are quite different, it’s possible to misuse the final keyword. The following sections discuss the three places where final can be used: for data, methods, and classes. 182 Thinking in Java Bruce Eckel final data Many programming languages have a way to tell the compiler that a piece of data is “constant.” A constant is useful for two reasons: 1. It can be a compile-time constant that won’t ever change. 2. It can be a value initialized at run time that you don’t want changed. In the case of a compile-time constant, the compiler is allowed to “fold” the constant value into any calculations in which it’s used; that is, the calculation can be performed at compile time, eliminating some run-time overhead. In Java, these sorts of constants must be primitives and are expressed with the final keyword. A value must be given at the time of definition of such a constant. A field that is both static and final has only one piece of storage that cannot be changed. When final is used with object references rather than primitives, the meaning can be confusing. With a primitive, final makes the value a constant, but with an object reference, final makes the reference a constant. Once the reference is initialized to an object, it can never be changed to point to another object. However, the object itself can be modified; Java does not provide a way to make any arbitrary object a constant. (You can, however, write your class so that objects have the effect of being constant.) This restriction includes arrays, which are also objects. Here’s an example that demonstrates final fields. Note that by convention, fields that are both static and final (that is, compile-time constants) are capitalized and use underscores to separate words. //: reusing/FinalData.java // The effect of final on fields. import java.util.*; import static net.mindview.util.Print.*; class Value { int i; // Package access public Value(int i) { this.i = i; } } public class FinalData { private static Random rand = new Random(47); private String id; public FinalData(String id) { this.id = id; } // Can be compile-time constants: private final int valueOne = 9; private static final int VALUE_TWO = 99; // Typical public constant: public static final int VALUE_THREE = 39; // Cannot be compile-time constants: private final int i4 = rand.nextInt(20); static final int INT_5 = rand.nextInt(20); private Value v1 = new Value(11); private final Value v2 = new Value(22); private static final Value VAL_3 = new Value(33); // Arrays: private final int[] a = { 1, 2, 3, 4, 5, 6 }; public String toString() { return id + ": " + "i4 = " + i4 + ", INT_5 = " + INT_5; } Reusing Classes 183 public static void main(String[] args) { FinalData fd1 = new FinalData("fd1"); //! fd1.valueOne++; // Error: can’t change value fd1.v2.i++; // Object isn’t constant! fd1.v1 = new Value(9); // OK -- not final for(int i = 0; i < fd1.a.length; i++) fd1.a[i]++; // Object isn’t constant! //! fd1.v2 = new Value(0); // Error: Can’t //! fd1.VAL_3 = new Value(1); // change reference //! fd1.a = new int[3]; print(fd1); print("Creating new FinalData"); FinalData fd2 = new FinalData("fd2"); print(fd1); print(fd2); } } /* Output: fd1: i4 = 15, INT_5 = 18 Creating new FinalData fd1: i4 = 15, INT_5 = 18 fd2: i4 = 13, INT_5 = 18 *///:~ Since valueOne and VALUE_TWO are final primitives with compile-time values, they can both be used as compile-time constants and are not different in any important way. VALUE_THREE is the more typical way you’ll see such constants defined: public so they’re usable outside the package, static to emphasize that there’s only one, and final to say that it’s a constant. Note that final static primitives with constant initial values (that is, compile-time constants) are named with all capitals by convention, with words separated by underscores. (This is just like C constants, which is where the convention originated.) Just because something is final doesn’t mean that its value is known at compile time. This is demonstrated by initializing i4 and INT_5 at run time using randomly generated numbers. This portion of the example also shows the difference between making a final value static or non-static. This difference shows up only when the values are initialized at run time, since the compile-time values are treated the same by the compiler. (And presumably optimized out of existence.) The difference is shown when you run the program. Note that the values of i4 for fd1 and fd2 are unique, but the value for INT_5 is not changed by creating the second FinalData object. That’s because it’s static and is initialized once upon loading and not each time a new object is created. The variables v1 through VAL_3 demonstrate the meaning of a final reference. As you can see in main( ), just because v2 is final doesn’t mean that you can’t change its value. Because it’s a reference, final means that you cannot rebind v2 to a new object. You can also see that the same meaning holds true for an array, which is just another kind of reference. (There is no way that I know of to make the array references themselves final.) Making references final seems less useful than making primitives final. Exercise 18: (2) Create a class with a static final field and a final field and demonstrate the difference between the two. Blank finals Java allows the creation of blank finals, which are fields that are declared as final but are not given an initialization value. In all cases, the blank final must be initialized before it is used, and the compiler ensures this. However, blank finals provide much more flexibility in the use of the final keyword since, for example, a final field inside a class can now be different for each object, and yet it retains its immutable quality. Here’s an example: 184 Thinking in Java Bruce Eckel //: reusing/BlankFinal.java // "Blank" final fields. class Poppet { private int i; Poppet(int ii) { i = ii; } } public class BlankFinal { private final int i = 0; // Initialized final private final int j; // Blank final private final Poppet p; // Blank final reference // Blank finals MUST be initialized in the constructor: public BlankFinal() { j = 1; // Initialize blank final p = new Poppet(1); // Initialize blank final reference } public BlankFinal(int x) { j = x; // Initialize blank final p = new Poppet(x); // Initialize blank final reference } public static void main(String[] args) { new BlankFinal(); new BlankFinal(47); } } ///:~ You’re forced to perform assignments to finals either with an expression at the point of definition of the field or in every constructor. That way it’s guaranteed that the final field is always initialized before use. Exercise 19: (2) Create a class with a blank final reference to an object. Perform the initialization of the blank final inside all constructors. Demonstrate the guarantee that the final must be initialized before use, and that it cannot be changed once initialized. final arguments Java allows you to make arguments final by declaring them as such in the argument list. This means that inside the method you cannot change what the argument reference points to: //: reusing/FinalArguments.java // Using "final" with method arguments. class Gizmo { public void spin() {} } public class FinalArguments { void with(final Gizmo g) { //! g = new Gizmo(); // Illegal -- g is final } void without(Gizmo g) { g = new Gizmo(); // OK -- g not final g.spin(); } // void f(final int i) { i++; } // Can’t change // You can only read from a final primitive: int g(final int i) { return i + 1; } public static void main(String[] args) { FinalArguments bf = new FinalArguments(); Reusing Classes 185 bf.without(null); bf.with(null); } } ///:~ The methods f( ) and g( ) show what happens when primitive arguments are final: You can read the argument, but you can’t change it. This feature is primarily used to pass data to anonymous inner classes, which you’ll learn about in the Inner Classes chapter. final methods There are two reasons for final methods. The first is to put a “lock” on the method to prevent any inheriting class from changing its meaning. This is done for design reasons when you want to make sure that a method’s behavior is retained during inheritance and cannot be overridden. The second reason for final methods is efficiency. In earlier implementations of Java, if you made a method final, you allowed the compiler to turn any calls to that method into inline calls. When the compiler saw a final method call, it could (at its discretion) skip the normal approach of inserting code to perform the method call mechanism (push arguments on the stack, hop over to the method code and execute it, hop back and clean off the stack arguments, and deal with the return value) and instead replace the method call with a copy of the actual code in the method body. This eliminated the overhead of the method call. Of course, if a method is big, then your code begins to bloat, and you probably wouldn’t see any performance gains from inlining, since any improvements will be dwarfed by the amount of time spent inside the method. In more recent version of Java, the virtual machine (in particular, the hotspot technologies) can detect these situations and optimize away the extra indirection, so its no longer necessary-in fact, it is now generally discouraged-to use final to try to help the optimizer. With Java SE5/6, you should let the compiler and JVM handle efficiency issues and make a method final only if you want to explicitly prevent overriding. 1 final and private Any private methods in a class are implicitly final. Because you can’t access a private method, you can’t override it. You can add the final specifier to a private method, but it doesn’t give that method any extra meaning. This issue can cause confusion, because if you try to override a private method (which is implicitly final), it seems to work, and the compiler doesn’t give an error message: //: reusing/FinalOverridingIllusion.java // It only looks like you can override // a private or private final method. import static net.mindview.util.Print.*; class WithFinals { // Identical to "private" alone: private final void f() { print("WithFinals.f()"); } // Also automatically "final": private void g() { print("WithFinals.g()"); } } 1 Don’t fall prey to the urge to prematurely optimize. If you get your system working and it’s too slow, it’s doubtful that you can fix it with the final keyword. http://MindView.net/Books/BetterJava has information about profiling, which can be helpful in speeding up your program. 186 Thinking in Java Bruce Eckel class OverridingPrivate extends WithFinals { private final void f() { print("OverridingPrivate.f()"); } private void g() { print("OverridingPrivate.g()"); } } class OverridingPrivate2 extends OverridingPrivate { public final void f() { print("OverridingPrivate2.f()"); } public void g() { print("OverridingPrivate2.g()"); } } public class FinalOverridingIllusion { public static void main(String[] args) { OverridingPrivate2 op2 = new OverridingPrivate2(); op2.f(); op2.g(); // You can upcast: OverridingPrivate op = op2; // But you can’t call the methods: //! op.f(); //! op.g(); // Same here: WithFinals wf = op2; //! wf.f(); //! wf.g(); } } /* Output: OverridingPrivate2.f() OverridingPrivate2.g() *///:~ “Overriding” can only occur if something is part of the base-class interface. That is, you must be able to upcast an object to its base type and call the same method (the point of this will become clear in the next chapter). If a method is private, it isn’t part of the base-class interface. It is just some code that’s hidden away inside the class, and it just happens to have that name, but if you create a public, protected, or package-access method with the same name in the derived class, there’s no connection to the method that might happen to have that name in the base class. You haven’t overridden the method; you’ve just created a new method. Since a private method is unreachable and effectively invisible, it doesn’t factor into anything except for the code organization of the class for which it was defined. Exercise 20: (1) Show that @Override annotation solves the problem in this section. Exercise 21: (1) Create a class with a final method. Inherit from that class and attempt to overwrite that method. final classes When you say that an entire class is final (by preceding its definition with the final keyword), you state that you don’t want to inherit from this class or allow anyone else to do so. In other words, for some reason the design of your class is such that there is never a need to make any changes, or for safety or security reasons you don’t want subclassing. Reusing Classes 187 //: reusing/Jurassic.java // Making an entire class final. class SmallBrain {} final class Dinosaur { int i = 7; int j = 1; SmallBrain x = new SmallBrain(); void f() {} } //! class Further extends Dinosaur {} // error: Cannot extend final class ‘Dinosaur’ public class Jurassic { public static void main(String[] args) { Dinosaur n = new Dinosaur(); n.f(); n.i = 40; n.j++; } } ///:~ Note that the fields of a final class can be final or not, as you choose. The same rules apply to final for fields regardless of whether the class is defined as final. However, because it prevents inheritance, all methods in a final class are implicitly final, since there’s no way to override them. You can add the final specifier to a method in a final class, but it doesn’t add any meaning. Exercise 22: (1) Create a final class and attempt to inherit from it. final caution It can seem to be sensible to make a method final while you’re designing a class. You might feel that no one could possibly want to override your methods. Sometimes this is true. But be careful with your assumptions. In general, it’s difficult to anticipate how a class can be reused, especially a general-purpose class. If you define a method as final, you might prevent the possibility of reusing your class through inheritance in some other programmer’s project simply because you couldn’t imagine it being used that way. The standard Java library is a good example of this. In particular, the Java 1.0/1.1 Vector class was commonly used and might have been even more useful if, in the name of efficiency (which was almost certainly an illusion), all the methods hadn’t been made final. It’s easily conceivable that you might want to inherit and override with such a fundamentally useful class, but the designers somehow decided this wasn’t appropriate. This is ironic for two reasons. First, Stack is inherited from Vector, which says that a Stack is a Vector, which isn’t really true from a logical standpoint. Nonetheless, it’s a case where the Java designers themselves inherited Vector. At the point they created Stack this way, they should have realized that final methods were too restrictive. Second, many of the most important methods of Vector, such as addElement( ) and elementAt( ), are synchronized. As you will see in the Concurrency chapter, this imposes a significant performance overhead that probably wipes out any gains provided by final. This lends credence to the theory that programmers are consistently bad at guessing where optimizations should occur. It’s just too bad that such a clumsy design made it into the standard library, where everyone had to cope with it. (Fortunately, the modern Java 188 Thinking in Java Bruce Eckel container library replaces Vector with ArrayList, which behaves much more civilly. Unfortunately, there’s still new code being written that uses the old container library.) It’s also interesting to note that Hashtable, another important Java 1.0/1.1 standard library class, does not have any final methods. As mentioned elsewhere in this book, it’s quite obvious that some classes were designed by completely different people than others. (You’ll see that the method names in Hashtable are much briefer compared to those in Vector, another piece of evidence.) This is precisely the sort of thing that should not be obvious to consumers of a class library. When things are inconsistent, it just makes more work for the user—yet another paean to the value of design and code walkthroughs. (Note that the modern Java container library replaces Hashtable with HashMap.) Initialization and class loading In more traditional languages, programs are loaded all at once as part of the startup process. This is followed by initialization, and then the program begins. The process of initialization in these languages must be carefully controlled so that the order of initialization of statics doesn’t cause trouble. C++, for example, has problems if one static expects another static to be valid before the second one has been initialized. Java doesn’t have this problem because it takes a different approach to loading. This is one of the activities that become easier, because everything in Java is an object. Remember that the compiled code for each class exists in its own separate file. That file isn’t loaded until the code is needed. In general, you can say that “class code is loaded at the point of first use.” This is usually when the first object of that class is constructed, but loading also occurs when a static field or static method is accessed. 2 The point of first use is also where the static initialization takes place. All the static objects and the static code block will be initialized in textual order (that is, the order that you write them down in the class definition) at the point of loading. The statics, of course, are initialized only once. Initialization with inheritance It’s helpful to look at the whole initialization process, including inheritance, to get a full picture of what happens. Consider the following example: //: reusing/Beetle.java // The full process of initialization. import static net.mindview.util.Print.*; class Insect { private int i = 9; protected int j; Insect() { print("i = " + i + ", j = " + j); j = 39; } private static int x1 = printInit("static Insect.x1 initialized"); static int printInit(String s) { 2 The constructor is also a static method even though the static keyword is not explicit. So to be precise, a class is first loaded when any one of its static members is accessed. Reusing Classes 189 print(s); return 47; } } public class Beetle extends Insect { private int k = printInit("Beetle.k initialized"); public Beetle() { print("k = " + k); print("j = " + j); } private static int x2 = printInit("static Beetle.x2 initialized"); public static void main(String[] args) { print("Beetle constructor"); Beetle b = new Beetle(); } } /* Output: static Insect.x1 initialized static Beetle.x2 initialized Beetle constructor i = 9, j = 0 Beetle.k initialized k = 47 j = 39 *///:~ The first thing that happens when you run Java on Beetle is that you try to access Beetle.main( ) (a static method), so the loader goes out and finds the compiled code for the Beetle class (this happens to be in a file called Beetle.class). In the process of loading it, the loader notices that it has a base class (that’s what the extends keyword says), which it then loads. This will happen whether or not you’re going to make an object of that base class. (Try commenting out the object creation to prove it to yourself.) If the base class has a base class, that second base class would then be loaded, and so on. Next, the static initialization in the root base class (in this case, Insect) is performed, and then the next derived class, and so on. This is important because the derived-class static initialization might depend on the base class member being initialized properly. At this point, the necessary classes have all been loaded so the object can be created. First, all the primitives in this object are set to their default values and the object references are set to null—this happens in one fell swoop by setting the memory in the object to binary zero. Then the base-class constructor will be called. In this case the call is automatic, but you can also specify the base-class constructor call (as the first operation in the Beetle( ) constructor) by using super. The base class construction goes through the same process in the same order as the derived-class constructor. After the base-class constructor completes, the instance variables are initialized in textual order. Finally, the rest of the body of the constructor is executed. Exercise 23: (2) Prove that class loading takes place only once. Prove that loading may be caused by either the creation of the first instance of that class or by the access of a static member. Exercise 24: (2) In Beetle.java, inherit a specific type of beetle from class Beetle, following the same format as the existing classes. Trace and explain the output. 190 Thinking in Java Bruce Eckel Summary Both inheritance and composition allow you to create a new type from existing types. Composition reuses existing types as part of the underlying implementation of the new type, and inheritance reuses the interface. With Inheritance, the derived class has the base-class interface, so it can be upcast to the base, which is critical for polymorphism, as you’ll see in the next chapter. Despite the strong emphasis on inheritance in object-oriented programming, when you start a design you should generally prefer composition (or possibly delegation) during the first cut and use inheritance only when it is clearly necessary. Composition tends to be more flexible. In addition, by using the added artifice of inheritance with your member type, you can change the exact type, and thus the behavior, of those member objects at run time. Therefore, you can change the behavior of the composed object at run time. When designing a system, your goal is to find or create a set of classes in which each class has a specific use and is neither too big (encompassing so much functionality that it’s unwieldy to reuse) nor annoyingly small (you can’t use it by itself or without adding functionality). If your designs become too complex, it’s often helpful to add more objects by breaking down existing ones into smaller parts. When you set out to design a system, it’s important to realize that program development is an incremental process, just like human learning. It relies on experimentation; you can do as much analysis as you want, but you still won’t know all the answers when you set out on a project. You’ll have much more success-and more immediate feedback-if you start out to “grow” your project as an organic, evolutionary creature, rather than constructing it all at once like a glass-box skyscraper. Inheritance and composition are two of the most fundamental tools in object-oriented programming that allow you to perform such experiments. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Reusing Classes 191 Polymorphism ”I have been asked, ‘Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?’ I am not able to rightly apprehend the kind of confusion of ideas that could provoke such a question.” Charles Babbage (1791-1871) Polymorphism is the third essential feature of an object-oriented programming language, after data abstraction and inheritance. It provides another dimension of separation of interface from implementation, to decouple what from how. Polymorphism allows improved code organization and readability as well as the creation of extensible programs that can be “grown” not only during the original creation of the project, but also when new features are desired. Encapsulation creates new data types by combining characteristics and behaviors. Implementation hiding separates the interface from the implementation by making the details private. This sort of mechanical organization makes ready sense to someone with a procedural programming background. But polymorphism deals with decoupling in terms of types. In the last chapter, you saw how inheritance allows the treatment of an object as its own type or its base type. This ability is critical because it allows many types (derived from the same base type) to be treated as if they were one type, and a single piece of code to work on all those different types equally. The polymorphic method call allows one type to express its distinction from another, similar type, as long as they’re both derived from the same base type. This distinction is expressed through differences in behavior of the methods that you can call through the base class. In this chapter, you’ll learn about polymorphism (also called dynamic binding or late binding or run-time binding) starting from the basics, with simple examples that strip away everything but the polymorphic behavior of the program. Upcasting revisited In the last chapter you saw how an object can be used as its own type or as an object of its base type. Taking an object reference and treating it as a reference to its base type is called upcasting because of the way inheritance trees are drawn with the base class at the top. You also saw a problem arise, which is embodied in the following example about musical instruments. First, since several examples play Notes, we should create a separate Note enumeration, in a package: //: polymorphism/music/Note.java // Notes to play on musical instruments. package polymorphism.music; public enum Note { MIDDLE_C, C_SHARP, B_FLAT; // Etc. } ///:~ enums were introduced in the Initialization & Cleanup chapter. Here, Wind is a type of Instrument; therefore, Wind is inherited from Instrument: //: polymorphism/music/Instrument.java package polymorphism.music; import static net.mindview.util.Print.*; class Instrument { public void play(Note n) { print("Instrument.play()"); } } ///:~ //: polymorphism/music/Wind.java package polymorphism.music; // Wind objects are instruments // because they have the same interface: public class Wind extends Instrument { // Redefine interface method: public void play(Note n) { System.out.println("Wind.play() " + n); } } ///:~ //: polymorphism/music/Music.java // Inheritance & upcasting. package polymorphism.music; public class Music { public static void tune(Instrument i) { // ... i.play(Note.MIDDLE_C); } public static void main(String[] args) { Wind flute = new Wind(); tune(flute); // Upcasting } } /* Output: Wind.play() MIDDLE_C *///:~ The method Music.tune( ) accepts an Instrument reference, but also anything derived from Instrument. In main( ), you can see this happening as a Wind reference is passed to tune( ), with no cast necessary. This is acceptable—the interface in Instrument must exist in Wind, because Wind is inherited from Instrument. Upcasting from Wind to Instrument may “narrow” that interface, but it cannot make it anything less than the full interface to Instrument. Forgetting the object type Music.java might seem strange to you. Why should anyone intentionally forget the type of an object? This is what happens when you upcast, and it seems like it could be much more straightforward if tune( ) simply takes a Wind reference as its argument. This brings up an essential point: If you did that, you’d need to write a new tune( ) for every type of Instrument in your system. Suppose we follow this reasoning and add Stringed and Brass instruments: //: polymorphism/music/Music2.java // Overloading instead of upcasting. 194 Thinking in Java Bruce Eckel package polymorphism.music; import static net.mindview.util.Print.*; class Stringed extends Instrument { public void play(Note n) { print("Stringed.play() " + n); } } class Brass extends Instrument { public void play(Note n) { print("Brass.play() " + n); } } public class Music2 { public static void tune(Wind i) { i.play(Note.MIDDLE_C); } public static void tune(Stringed i) { i.play(Note.MIDDLE_C); } public static void tune(Brass i) { i.play(Note.MIDDLE_C); } public static void main(String[] args) { Wind flute = new Wind(); Stringed violin = new Stringed(); Brass frenchHorn = new Brass(); tune(flute); // No upcasting tune(violin); tune(frenchHorn); } } /* Output: Wind.play() MIDDLE_C Stringed.play() MIDDLE_C Brass.play() MIDDLE_C *///:~ This works, but there’s a major drawback: you must write type-specific methods for each new Instrument class you add. This means more programming in the first place, but it also means that if you want to add a new method like tune( ) or a new type of Instrument, you’ve got a lot of work to do. Add the fact that the compiler won’t give you any error messages if you forget to overload one of your methods and the whole process of working with types becomes unmanageable. Wouldn’t it be much nicer if you could just write a single method that takes the base class as its argument, and not any of the specific derived classes? That is, wouldn’t it be nice if you could forget that there are derived classes, and write your code to talk only to the base class? That’s exactly what polymorphism allows you to do. However, most programmers who come from a procedural programming background have a bit of trouble with the way polymorphism works. Exercise 1: (2) Create a Cycle class, with subclasses Unicycle, Bicycle and Tricycle. Demonstrate that an instance of each type can be upcast to Cycle via a ride( ) method. Polymorphism 195 The twist The difficulty with Music.java can be seen by running the program. The output is Wind.play( ). This is clearly the desired output, but it doesn’t seem to make sense that it would work that way. Look at the tune( ) method: public static void tune(Instrument i) { // ... i.play(Note.MIDDLE_C); } It receives an Instrument reference. So how can the compiler possibly know that this Instrument reference points to a Wind in this case and not a Brass or Stringed? The compiler can’t. To get a deeper understanding of the issue, it’s helpful to examine the subject of binding. Method-call binding Connecting a method call to a method body is called binding. When binding is performed before the program is run (by the compiler and linker, if there is one), it’s called early binding. You might not have heard the term before because it has never been an option with procedural languages. C compilers have only one kind of method call, and that’s early binding. The confusing part of the preceding program revolves around early binding, because the compiler cannot know the correct method to call when it has only an Instrument reference. The solution is called late binding, which means that the binding occurs at run time, based on the type of object. Late binding is also called dynamic binding or runtime binding. When a language implements late binding, there must be some mechanism to determine the type of the object at run time and to call the appropriate method. That is, the compiler still doesn’t know the object type, but the method-call mechanism finds out and calls the correct method body. The late-binding mechanism varies from language to language, but you can imagine that some sort of type information must be installed in the objects. All method binding in Java uses late binding unless the method is static or final (private methods are implicitly final). This means that ordinarily you don’t need to make any decisions about whether late binding will occur—it happens automatically. Why would you declare a method final? As noted in the last chapter, it prevents anyone from overriding that method. Perhaps more important, it effectively “turns off” dynamic binding, or rather it tells the compiler that dynamic binding isn’t necessary. This allows the compiler to generate slightly more efficient code for final method calls. However, in most cases it won’t make any overall performance difference in your program, so it’s best to only use final as a design decision, and not as an attempt to improve performance. Producing the right behavior Once you know that all method binding in Java happens polymorphically via late binding, you can write your code to talk to the base class and know that all the derived-class cases will work correctly using the same code. Or to put it another way, you “send a message to an object and let the object figure out the right thing to do.” 196 Thinking in Java Bruce Eckel The classic example in OOP is the “shape” example. This is commonly used because it is easy to visualize, but unfortunately it can confuse novice programmers into thinking that OOP is just for graphics programming, which is of course not the case. The shape example has a base class called Shape and various derived types: Circle, Square, Triangle, etc. The reason the example works so well is that it’s easy to say “a circle is a type of shape” and be understood. The inheritance diagram shows the relationships: The upcast could occur in a statement as simple as: Shape s = new Circle(); Here, a Circle object is created, and the resulting reference is immediately assigned to a Shape, which would seem to be an error (assigning one type to another); and yet it’s fine because a Circle is a Shape by inheritance. So the compiler agrees with the statement and doesn’t issue an error message. Suppose you call one of the base-class methods (that have been overridden in the derived classes): s.draw(); Again, you might expect that Shape’s draw( ) is called because this is, after all, a Shape reference—so how could the compiler know to do anything else? And yet the proper Circle.draw( ) is called because of late binding (polymorphism). The following example puts it a slightly different way. First, let’s create a reusable library of Shape types: //: polymorphism/shape/Shape.java package polymorphism.shape; public class Shape { public void draw() {} public void erase() {} } ///:~ //: polymorphism/shape/Circle.java package polymorphism.shape; import static net.mindview.util.Print.*; public class Circle extends Shape { Polymorphism 197 public void draw() { print("Circle.draw()"); } public void erase() { print("Circle.erase()"); } } ///:~ //: polymorphism/shape/Square.java package polymorphism.shape; import static net.mindview.util.Print.*; public class Square extends Shape { public void draw() { print("Square.draw()"); } public void erase() { print("Square.erase()"); } } ///:~ //: polymorphism/shape/Triangle.java package polymorphism.shape; import static net.mindview.util.Print.*; public class Triangle extends Shape { public void draw() { print("Triangle.draw()"); } public void erase() { print("Triangle.erase()"); } } ///:~ //: polymorphism/shape/RandomShapeGenerator.java // A "factory" that randomly creates shapes. package polymorphism.shape; import java.util.*; public class RandomShapeGenerator { private Random rand = new Random(47); public Shape next() { switch(rand.nextInt(3)) { default: case 0: return new Circle(); case 1: return new Square(); case 2: return new Triangle(); } } } ///:~ //: polymorphism/Shapes.java // Polymorphism in Java. import polymorphism.shape.*; public class Shapes { private static RandomShapeGenerator gen = new RandomShapeGenerator(); public static void main(String[] args) { Shape[] s = new Shape[9]; // Fill up the array with shapes: for(int i = 0; i < s.length; i++) s[i] = gen.next(); // Make polymorphic method calls: for(Shape shp : s) shp.draw(); } } /* Output: Triangle.draw() Triangle.draw() Square.draw() Triangle.draw() Square.draw() Triangle.draw() Square.draw() 198 Thinking in Java Bruce Eckel Triangle.draw() Circle.draw() *///:~ The base class Shape establishes the common interface to anything inherited from Shape— that is, all shapes can be drawn and erased. The derived classes override these definitions to provide unique behavior for each specific type of shape. RandomShapeGenerator is a kind of “factory” that produces a reference to a randomlyselected Shape object each time you call its next( ) method. Note that the upcasting happens in the return statements, each of which takes a reference to a Circle, Square, or Triangle and sends it out of next( ) as the return type, Shape. So whenever you call next( ), you never get a chance to see what specific type it is, since you always get back a plain Shape reference. main( ) contains an array of Shape references filled through calls to RandomShapeGenerator.next( ). At this point you know you have Shapes, but you don’t know anything more specific than that (and neither does the compiler). However, when you step through this array and call draw( ) for each one, the correct type-specific behavior magically occurs, as you can see from the output when you run the program. The point of creating the shapes randomly is to drive home the understanding that the compiler can have no special knowledge that allows it to make the correct calls at compile time. All the calls to draw( ) must be made through dynamic binding. Exercise 2: (1) Add the @Override annotation to the shapes example. Exercise 3: (1) Add a new method in the base class of Shapes.java that prints a message, but don’t override it in the derived classes. Explain what happens. Now override it in one of the derived classes but not the others, and see what happens. Finally, override it in all the derived classes. Exercise 4: (2) Add a new type of Shape to Shapes.java and verify in main( ) that polymorphism works for your new type as it does in the old types. Exercise 5: (1) Starting from Exercise 1, add a wheels( ) method in Cycle, which returns the number of wheels. Modify ride( ) to call wheels( ) and verify that polymorphism works. Extensibility Now let’s return to the musical instrument example. Because of polymorphism, you can add as many new types as you want to the system without changing the tune( ) method. In a well-designed OOP program, most or all of your methods will follow the model of tune( ) and communicate only with the base-class interface. Such a program is extensible because you can add new functionality by inheriting new data types from the common base class. The methods that manipulate the base-class interface will not need to be changed at all to accommodate the new classes. Consider what happens if you take the instrument example and add more methods in the base class and a number of new classes. Here’s the diagram: Polymorphism 199 All these new classes work correctly with the old, unchanged tune( ) method. Even if tune( ) is in a separate file and new methods are added to the interface of Instrument, tune( ) will still work correctly, even without recompiling it. Here is the implementation of the diagram: //: polymorphism/music3/Music3.java // An extensible program. package polymorphism.music3; import polymorphism.music.Note; import static net.mindview.util.Print.*; class Instrument { void play(Note n) { print("Instrument.play() " + n); } String what() { return "Instrument"; } void adjust() { print("Adjusting Instrument"); } } class Wind extends Instrument { void play(Note n) { print("Wind.play() " + n); } String what() { return "Wind"; } void adjust() { print("Adjusting Wind"); } } class Percussion extends Instrument { void play(Note n) { print("Percussion.play() " + n); } String what() { return "Percussion"; } void adjust() { print("Adjusting Percussion"); } } class Stringed extends Instrument { 200 Thinking in Java Bruce Eckel void play(Note n) { print("Stringed.play() " + n); } String what() { return "Stringed"; } void adjust() { print("Adjusting Stringed"); } } class Brass extends Wind { void play(Note n) { print("Brass.play() " + n); } void adjust() { print("Adjusting Brass"); } } class Woodwind extends Wind { void play(Note n) { print("Woodwind.play() " + n); } String what() { return "Woodwind"; } } public class Music3 { // Doesn’t care about type, so new types // added to the system still work right: public static void tune(Instrument i) { // ... i.play(Note.MIDDLE_C); } public static void tuneAll(Instrument[] e) { for(Instrument i : e) tune(i); } public static void main(String[] args) { // Upcasting during addition to the array: Instrument[] orchestra = { new Wind(), new Percussion(), new Stringed(), new Brass(), new Woodwind() }; tuneAll(orchestra); } } /* Output: Wind.play() MIDDLE_C Percussion.play() MIDDLE_C Stringed.play() MIDDLE_C Brass.play() MIDDLE_C Woodwind.play() MIDDLE_C *///:~ The new methods are what( ), which returns a String reference with a description of the class, and adjust( ), which provides some way to adjust each instrument. In main( ), when you place something inside the orchestra array, you automatically upcast to Instrument. You can see that the tune( ) method is blissfully ignorant of all the code changes that have happened around it, and yet it works correctly. This is exactly what polymorphism is supposed to provide. Changes in your code don’t cause damage to parts of the program that should not be affected. Put another way, polymorphism is an important technique for the programmer to “separate the things that change from the things that stay the same.” Polymorphism 201 Exercise 6: (1) Change Music3.java so that what( ) becomes the root Object method toString( ). Try printing the Instrument objects using System.out.println( ) (without any casting). Exercise 7: (2) Add a new type of Instrument to Music3.java and verify that polymorphism works for your new type. Exercise 8: (2) Modify Music3.java so that it randomly creates Instrument objects the way Shapes.java does. Exercise 9: (3) Create an inheritance hierarchy of Rodent: Mouse, Gerbil, Hamster, etc. In the base class, provide methods that are common to all Rodents, and override these in the derived classes to perform different behaviors depending on the specific type of Rodent. Create an array of Rodent, fill it with different specific types of Rodents, and call your base-class methods to see what happens. Exercise 10: (3) Create a base class with two methods. In the first method, call the second method. Inherit a class and override the second method. Create an object of the derived class, upcast it to the base type, and call the first method. Explain what happens. Pitfall: “overriding” private methods Here’s something you might innocently try to do: //: polymorphism/PrivateOverride.java // Trying to override a private method. package polymorphism; import static net.mindview.util.Print.*; public class PrivateOverride { private void f() { print("private f()"); } public static void main(String[] args) { PrivateOverride po = new Derived(); po.f(); } } class Derived extends PrivateOverride { public void f() { print("public f()"); } } /* Output: private f() *///:~ You might reasonably expect the output to be “public f( )”, but a private method is automatically final, and is also hidden from the derived class. So Derived’s f( ) in this case is a brand new method; it’s not even overloaded, since the base-class version of f( ) isn’t visible in Derived. The result of this is that only non-private methods may be overridden, but you should watch out for the appearance of overriding private methods, which generates no compiler warnings, but doesn’t do what you might expect. To be clear, you should use a different name from a private base-class method in your derived class. 202 Thinking in Java Bruce Eckel Pitfall: fields and static methods Once you learn about polymorphism, you can begin to think that everything happens polymorphically. However, only ordinary method calls can be polymorphic. For example, if you access a field directly, that access will be resolved at compile time, as the following example demonstrates: 1 //: polymorphism/FieldAccess.java // Direct field access is determined at compile time. class Super { public int field = 0; public int getField() { return field; } } class Sub extends Super { public int field = 1; public int getField() { return field; } public int getSuperField() { return super.field; } } public class FieldAccess { public static void main(String[] args) { Super sup = new Sub(); // Upcast System.out.println("sup.field = " + sup.field + ", sup.getField() = " + sup.getField()); Sub sub = new Sub(); System.out.println("sub.field = " + sub.field + ", sub.getField() = " + sub.getField() + ", sub.getSuperField() = " + sub.getSuperField()); } } /* Output: sup.field = 0, sup.getField() = 1 sub.field = 1, sub.getField() = 1, sub.getSuperField() = 0 *///:~ When a Sub object is upcast to a Super reference, any field accesses are resolved by the compiler, and are thus not polymorphic. In this example, different storage is allocated for Super.field and Sub.field. Thus, Sub actually contains two fields called field: its own and the one that it gets from Super. However, the Super version is not the default that is produced when you refer to field in Sub; in order to get the Super field you must explicitly say super.field. Although this seems like it could be a confusing issue, in practice it virtually never comes up. For one thing, you’ll generally make all fields private and so you won’t access them directly, but only as side effects of calling methods. In addition, you probably won’t give the same name to a base-class field and a derived-class field, because its confusing. If a method is static, it doesn’t behave polymorphically: //: polymorphism/StaticPolymorphism.java // Static methods are not polymorphic. class StaticSuper { public static String staticGet() { 1 Thanks to Randy Nichols for asking this question. Polymorphism 203 return "Base staticGet()"; } public String dynamicGet() { return "Base dynamicGet()"; } } class StaticSub extends StaticSuper { public static String staticGet() { return "Derived staticGet()"; } public String dynamicGet() { return "Derived dynamicGet()"; } } public class StaticPolymorphism { public static void main(String[] args) { StaticSuper sup = new StaticSub(); // Upcast System.out.println(sup.staticGet()); System.out.println(sup.dynamicGet()); } } /* Output: Base staticGet() Derived dynamicGet() *///:~ static methods are associated with the class, and not the individual objects. Constructors and polymorphism As usual, constructors are different from other kinds of methods. This is also true when polymorphism is involved. Even though constructors are not polymorphic (they’re actually static methods, but the static declaration is implicit), it’s important to understand the way constructors work in complex hierarchies and with polymorphism. This understanding will help you avoid unpleasant entanglements. Order of constructor calls The order of constructor calls was briefly discussed in the Initialization & Cleanup chapter and again in the Reusing Classes chapter, but that was before polymorphism was introduced. A constructor for the base class is always called during the construction process for a derived class, chaining up the inheritance hierarchy so that a constructor for every base class is called. This makes sense because the constructor has a special job: to see that the object is built properly. A derived class has access to its own members only, and not to those of the base class (whose members are typically private). Only the base-class constructor has the proper knowledge and access to initialize its own elements. Therefore, it’s essential that all constructors get called; otherwise the entire object wouldn’t be constructed. That’s why the compiler enforces a constructor call for every portion of a derived class. It will silently call the default constructor if you don’t explicitly call a base-class constructor in the derived-class constructor body. If there is no default constructor, the compiler will complain. (In the case where a class has no constructors, the compiler will automatically synthesize a default constructor.) 204 Thinking in Java Bruce Eckel Let’s take a look at an example that shows the effects of composition, inheritance, and polymorphism on the order of construction: //: polymorphism/Sandwich.java // Order of constructor calls. package polymorphism; import static net.mindview.util.Print.*; class Meal { Meal() { print("Meal()"); } } class Bread { Bread() { print("Bread()"); } } class Cheese { Cheese() { print("Cheese()"); } } class Lettuce { Lettuce() { print("Lettuce()"); } } class Lunch extends Meal { Lunch() { print("Lunch()"); } } class PortableLunch extends Lunch { PortableLunch() { print("PortableLunch()");} } public class Sandwich extends PortableLunch { private Bread b = new Bread(); private Cheese c = new Cheese(); private Lettuce l = new Lettuce(); public Sandwich() { print("Sandwich()"); } public static void main(String[] args) { new Sandwich(); } } /* Output: Meal() Lunch() PortableLunch() Bread() Cheese() Lettuce() Sandwich() *///:~ This example creates a complex class out of other classes, and each class has a constructor that announces itself. The important class is Sandwich, which reflects three levels of inheritance (four, if you count the implicit inheritance from Object) and three member objects. You can see the output when a Sandwich object is created in main( ). This means that the order of constructor calls for a complex object is as follows: 1. The base-class constructor is called. This step is repeated recursively such that the root of the hierarchy is constructed first, followed by the next-derived class, etc., until the most-derived class is reached. 2. Member initializers are called in the order of declaration. Polymorphism 205 3. The body of the derived-class constructor is called. The order of the constructor calls is important. When you inherit, you know all about the base class and can access any public and protected members of the base class. This means that you must be able to assume that all the members of the base class are valid when you’re in the derived class. In a normal method, construction has already taken place, so all the members of all parts of the object have been built. Inside the constructor, however, you must be able to assume that all members that you use have been built. The only way to guarantee this is for the base-class constructor to be called first. Then when you’re in the derived-class constructor, all the members you can access in the base class have been initialized. Knowing that all members are valid inside the constructor is also the reason that, whenever possible, you should initialize all member objects (that is, objects placed in the class using composition) at their point of definition in the class (e.g., b, c, and l in the preceding example). If you follow this practice, you will help ensure that all base class members and member objects of the current object have been initialized. Unfortunately, this doesn’t handle every case, as you will see in the next section. Exercise 11: (1) Add class Pickle to Sandwich.java. Inheritance and cleanup When using composition and inheritance to create a new class, most of the time you won’t have to worry about cleaning up; subobjects can usually be left to the garbage collector. If you do have cleanup issues, you must be diligent and create a dispose( ) method (the name I have chosen to use here; you may come up with something better) for your new class. And with inheritance, you must override dispose( ) in the derived class if you have any special cleanup that must happen as part of garbage collection. When you override dispose( ) in an inherited class, it’s important to remember to call the base-class version of dispose( ), since otherwise the base-class cleanup will not happen. The following example demonstrates this: //: polymorphism/Frog.java // Cleanup and inheritance. package polymorphism; import static net.mindview.util.Print.*; class Characteristic { private String s; Characteristic(String s) { this.s = s; print("Creating Characteristic " + s); } protected void dispose() { print("disposing Characteristic " + s); } } class Description { private String s; Description(String s) { this.s = s; print("Creating Description " + s); } protected void dispose() { print("disposing Description " + s); } } 206 Thinking in Java Bruce Eckel class LivingCreature { private Characteristic p = new Characteristic("is alive"); private Description t = new Description("Basic Living Creature"); LivingCreature() { print("LivingCreature()"); } protected void dispose() { print("LivingCreature dispose"); t.dispose(); p.dispose(); } } class Animal extends LivingCreature { private Characteristic p = new Characteristic("has heart"); private Description t = new Description("Animal not Vegetable"); Animal() { print("Animal()"); } protected void dispose() { print("Animal dispose"); t.dispose(); p.dispose(); super.dispose(); } } class Amphibian extends Animal { private Characteristic p = new Characteristic("can live in water"); private Description t = new Description("Both water and land"); Amphibian() { print("Amphibian()"); } protected void dispose() { print("Amphibian dispose"); t.dispose(); p.dispose(); super.dispose(); } } public class Frog extends Amphibian { private Characteristic p = new Characteristic("Croaks"); private Description t = new Description("Eats Bugs"); public Frog() { print("Frog()"); } protected void dispose() { print("Frog dispose"); t.dispose(); p.dispose(); super.dispose(); } public static void main(String[] args) { Frog frog = new Frog(); print("Bye!"); frog.dispose(); } } /* Output: Creating Characteristic is alive Creating Description Basic Living Creature Polymorphism 207 LivingCreature() Creating Characteristic has heart Creating Description Animal not Vegetable Animal() Creating Characteristic can live in water Creating Description Both water and land Amphibian() Creating Characteristic Croaks Creating Description Eats Bugs Frog() Bye! Frog dispose disposing Description Eats Bugs disposing Characteristic Croaks Amphibian dispose disposing Description Both water and land disposing Characteristic can live in water Animal dispose disposing Description Animal not Vegetable disposing Characteristic has heart LivingCreature dispose disposing Description Basic Living Creature disposing Characteristic is alive *///:~ Each class in the hierarchy also contains a member objects of types Characteristic and Description, which must also be disposed. The order of disposal should be the reverse of the order of initialization, in case one subobject is dependent on another. For fields, this means the reverse of the order of declaration (since fields are initialized in declaration order). For base classes (following the form that’s used in C++ for destructors), you should perform the derived-class cleanup first, then the base-class cleanup. That’s because the derived-class cleanup could call some methods in the base class that require the base-class components to be alive, so you must not destroy them prematurely. From the output you can see that all parts of the Frog object are disposed in reverse order of creation. From this example, you can see that although you don’t always need to perform cleanup, when you do, the process requires care and awareness. Exercise 12: (3) Modify Exercise 9 so that it demonstrates the order of initialization of the base classes and derived classes. Now add member objects to both the base and derived classes and show the order in which their initialization occurs during construction. Also note that in the above example, a Frog object “owns” its member objects. It creates them, and it knows how long they should live (as long as the Frog does), so it knows when to dispose( ) the member objects. However, if one of these member objects is shared with one or more other objects, the problem becomes more complex and you cannot simply assume that you can call dispose( ). In this case, reference counting may be necessary to keep track of the number of objects that are still accessing a shared object. Here’s what it looks like: //: polymorphism/ReferenceCounting.java // Cleaning up shared member objects. import static net.mindview.util.Print.*; class Shared { private int refcount = 0; private static long counter = 0; private final long id = counter++; public Shared() { print("Creating " + this); } 208 Thinking in Java Bruce Eckel public void addRef() { refcount++; } protected void dispose() { if(--refcount == 0) print("Disposing " + this); } public String toString() { return "Shared " + id; } } class Composing { private Shared shared; private static long counter = 0; private final long id = counter++; public Composing(Shared shared) { print("Creating " + this); this.shared = shared; this.shared.addRef(); } protected void dispose() { print("disposing " + this); shared.dispose(); } public String toString() { return "Composing " + id; } } public class ReferenceCounting { public static void main(String[] args) { Shared shared = new Shared(); Composing[] composing = { new Composing(shared), new Composing(shared), new Composing(shared), new Composing(shared), new Composing(shared) }; for(Composing c : composing) c.dispose(); } } /* Output: Creating Shared 0 Creating Composing 0 Creating Composing 1 Creating Composing 2 Creating Composing 3 Creating Composing 4 disposing Composing 0 disposing Composing 1 disposing Composing 2 disposing Composing 3 disposing Composing 4 Disposing Shared 0 *///:~ The static long counter keeps track of the number of instances of Shared that are created, and it also provides a value for id. The type of counter is long rather than int, to prevent overflow (this is just good practice; overflowing such a counter is not likely to happen in any of the examples in this book). The id is final because we do not expect it to change its value during the lifetime of the object. When you attach a shared object to your class, you must remember to call addRef( ), but the dispose( ) method will keep track of the reference count and decide when to actually perform the cleanup. This technique requires extra diligence to use, but if you are sharing objects that require cleanup you don’t have much choice. Exercise 13: (3) Add a finalize( ) method to ReferenceCounting.java to verify the termination condition (see the Initialization & Cleanup chapter). Polymorphism 209 Exercise 14: (4) Modify Exercise 12 so that one of the member objects is a shared object with reference counting, and demonstrate that it works properly. Behavior of polymorphic methods inside constructors The hierarchy of constructor calls brings up an interesting dilemma. What happens if you’re inside a constructor and you call a dynamically-bound method of the object being constructed? Inside an ordinary method, the dynamically-bound call is resolved at run time, because the object cannot know whether it belongs to the class that the method is in or some class derived from it. If you call a dynamically-bound method inside a constructor, the overridden definition for that method is used. However, the effect of this call can be rather unexpected because the overridden method will be called before the object is fully constructed. This can conceal some difficult-to-find bugs. Conceptually, the constructor’s job is to bring the object into existence (which is hardly an ordinary feat). Inside any constructor, the entire object might be only partially formed—you can only know that the base-class objects have been initialized. If the constructor is only one step in building an object of a class that’s been derived from that constructor’s class, the derived parts have not yet been initialized at the time that the current constructor is being called. A dynamically bound method call, however, reaches “outward” into the inheritance hierarchy. It calls a method in a derived class. If you do this inside a constructor, you call a method that might manipulate members that haven’t been initialized yet—a sure recipe for disaster. You can see the problem in the following example: //: polymorphism/PolyConstructors.java // Constructors and polymorphism // don’t produce what you might expect. import static net.mindview.util.Print.*; class Glyph { void draw() { print("Glyph.draw()"); } Glyph() { print("Glyph() before draw()"); draw(); print("Glyph() after draw()"); } } class RoundGlyph extends Glyph { private int radius = 1; RoundGlyph(int r) { radius = r; print("RoundGlyph.RoundGlyph(), radius = " + radius); } void draw() { print("RoundGlyph.draw(), radius = " + radius); } } public class PolyConstructors { public static void main(String[] args) { 210 Thinking in Java Bruce Eckel new RoundGlyph(5); } } /* Output: Glyph() before draw() RoundGlyph.draw(), radius = 0 Glyph() after draw() RoundGlyph.RoundGlyph(), radius = 5 *///:~ Glyph. draw( ) is designed to be overridden, which happens in RoundGlyph. But the Glyph constructor calls this method, and the call ends up in RoundGlyph.draw( ), which would seem to be the intent. But if you look at the output, you can see that when Glyph’s constructor calls draw( ), the value of radius isn’t even the default initial value 1. It’s 0. This would probably result in either a dot or nothing at all being drawn on the screen, and you’d be left staring, trying to figure out why the program won’t work. The order of initialization described in the earlier section isn’t quite complete, and that’s the key to solving the mystery. The actual process of initialization is: 1. The storage allocated for the object is initialized to binary zero before anything else happens. 2. The base-class constructors are called as described previously. At this point, the overridden draw( ) method is called (yes, before the RoundGlyph constructor is called), which discovers a radius value of zero, due to Step 1. 3. Member initializers are called in the order of declaration. 4. The body of the derived-class constructor is called. There’s an upside to this, which is that everything is at least initialized to zero (or whatever zero means for that particular data type) and not just left as garbage. This includes object references that are embedded inside a class via composition, which become null. So if you forget to initialize that reference, you’ll get an exception at run time. Everything else gets zero, which is usually a telltale value when looking at output. On the other hand, you should be pretty horrified at the outcome of this program. You’ve done a perfectly logical thing, and yet the behavior is mysteriously wrong, with no complaints from the compiler. (C++ produces more rational behavior in this situation.) Bugs like this could easily be buried and take a long time to discover. As a result, a good guideline for constructors is, “Do as little as possible to set the object into a good state, and if you can possibly avoid it, don’t call any other methods in this class.” The only safe methods to call inside a constructor are those that are final in the base class. (This also applies to private methods, which are automatically final.) These cannot be overridden and thus cannot produce this kind of surprise. You may not always be able to follow this guideline, but it’s something to strive towards. Exercise 15: (2) Add a RectangularGlyph to PolyConstructors.java and demonstrate the problem described in this section. Covariant return types Java SE5 adds covariant return types, which means that an overridden method in a derived class can return a type derived from the type returned by the base-class method: //: polymorphism/CovariantReturn.java Polymorphism 211 class Grain { public String toString() { return "Grain"; } } class Wheat extends Grain { public String toString() { return "Wheat"; } } class Mill { Grain process() { return new Grain(); } } class WheatMill extends Mill { Wheat process() { return new Wheat(); } } public class CovariantReturn { public static void main(String[] args) { Mill m = new Mill(); Grain g = m.process(); System.out.println(g); m = new WheatMill(); g = m.process(); System.out.println(g); } } /* Output: Grain Wheat *///:~ The key difference between Java SE5 and earlier versions of java is that the earlier versions would force the overridden version of process( ) to return Grain, rather than Wheat, even though Wheat is derived from Grain and thus is still a legitimate return type. Covariant return types allow the more specific Wheat return type. Designing with inheritance Once you learn about polymorphism, it can seem that everything ought to be inherited, because polymorphism is such a clever tool. This can burden your designs; in fact, if you choose inheritance first when you’re using an existing class to make a new class, things can become needlessly complicated. A better approach is to choose composition first, especially when it’s not obvious which one you should use. Composition does not force a design into an inheritance hierarchy. But composition is also more flexible since it’s possible to dynamically choose a type (and thus behavior) when using composition, whereas inheritance requires an exact type to be known at compile time. The following example illustrates this: //: polymorphism/Transmogrify.java // Dynamically changing the behavior of an object // via composition (the "State" design pattern). import static net.mindview.util.Print.*; class Actor { public void act() {} } class HappyActor extends Actor { public void act() { print("HappyActor"); } 212 Thinking in Java Bruce Eckel } class SadActor extends Actor { public void act() { print("SadActor"); } } class Stage { private Actor actor = new HappyActor(); public void change() { actor = new SadActor(); } public void performPlay() { actor.act(); } } public class Transmogrify { public static void main(String[] args) { Stage stage = new Stage(); stage.performPlay(); stage.change(); stage.performPlay(); } } /* Output: HappyActor SadActor *///:~ A Stage object contains a reference to an Actor, which is initialized to a HappyActor object. This means performPlay( ) produces a particular behavior. But since a reference can be rebound to a different object at run time, a reference for a SadActor object can be substituted in actor, and then the behavior produced by performPlay( ) changes. Thus you gain dynamic flexibility at run time. (This is also called the State Pattern. See Thinking in Patterns (with Java) at www.MindView.com.) In contrast, you can’t decide to inherit differently at run time; that must be completely determined at compile time. A general guideline is “Use inheritance to express differences in behavior, and fields to express variations in state.” In the preceding example, both are used; two different classes are inherited to express the difference in the act( ) method, and Stage uses composition to allow its state to be changed. In this case, that change in state happens to produce a change in behavior. Exercise 16: (3) Following the example in Transmogrify.java, create a Starship class containing an AlertStatus reference that can indicate three different states. Include methods to change the states. Substitution vs. extension It would seem that the cleanest way to create an inheritance hierarchy is to take the “pure” approach. That is, only methods that have been established in the base class are overridden in the derived class, as seen in this diagram: Polymorphism 213 This can be called a pure “is-a” relationship because the interface of a class establishes what it is. Inheritance guarantees that any derived class will have the interface of the base class and nothing less. If you follow this diagram, derived classes will also have no more than the base-class interface. This can be thought of as pure substitution, because derived class objects can be perfectly substituted for the base class, and you never need to know any extra information about the subclasses when you’re using them: That is, the base class can receive any message you can send to the derived class because the two have exactly the same interface. All you need to do is upcast from the derived class and never look back to see what exact type of object you’re dealing with. Everything is handled through polymorphism. When you see it this way, it seems like a pure is-a relationship is the only sensible way to do things, and any other design indicates muddled thinking and is by definition broken. This too is a trap. As soon as you start thinking this way, you’ll turn around and discover that extending the interface (which, unfortunately, the keyword extends seems to encourage) is the perfect solution to a particular problem. This can be termed an “is-like-a” relationship, because the derived class is like the base class—it has the same fundamental interface—but it has other features that require additional methods to implement: 214 Thinking in Java Bruce Eckel While this is also a useful and sensible approach (depending on the situation), it has a drawback. The extended part of the interface in the derived class is not available from the base class, so once you upcast, you can’t call the new methods: If you’re not upcasting in this case, it won’t bother you, but often you’ll get into a situation in which you need to rediscover the exact type of the object so you can access the extended methods of that type. The following section shows how this is done. Downcasting and runtime type information Since you lose the specific type information via an upcast (moving up the inheritance hierarchy), it makes sense that to retrieve the type information—that is, to move back down the inheritance hierarchy—you use a downcast. However, you know an upcast is always safe because the base class cannot have a bigger interface than the derived class. Therefore, every message you send through the base class interface is guaranteed to be accepted. But with a downcast, you don’t really know that a shape (for example) is actually a circle. It could instead be a triangle or square or some other type. To solve this problem, there must be some way to guarantee that a downcast is correct, so that you won’t accidentally cast to the wrong type and then send a message that the object can’t accept. This would be quite unsafe. In some languages (like C++) you must perform a special operation in order to get a type-safe downcast, but in Java, every cast is checked! So even though it looks like you’re just performing an ordinary parenthesized cast, at run time this cast is checked to ensure that it is in fact the type you think it is. If it isn’t, you get a ClassCastException. This act of checking Polymorphism 215 types at run time is called runtime type identification (RTTI). The following example demonstrates the behavior of RTTI: //: polymorphism/RTTI.java // Downcasting & Runtime type information (RTTI). // {ThrowsException} class Useful { public void f() {} public void g() {} } class MoreUseful extends Useful { public void f() {} public void g() {} public void u() {} public void v() {} public void w() {} } public class RTTI { public static void main(String[] args) { Useful[] x = { new Useful(), new MoreUseful() }; x[0].f(); x[1].g(); // Compile time: method not found in Useful: //! x[1].u(); ((MoreUseful)x[1]).u(); // Downcast/RTTI ((MoreUseful)x[0]).u(); // Exception thrown } } ///:~ As in the previous diagram, MoreUseful extends the interface of Useful. But since it’s inherited, it can also be upcast to a Useful. You can see this happening in the initialization of the array x in main( ). Since both objects in the array are of class Useful, you can send the f( ) and g( ) methods to both, and if you try to call u( ) (which exists only in MoreUseful), you’ll get a compile-time error message. If you want to access the extended interface of a MoreUseful object, you can try to downcast. If it’s the correct type, it will be successful. Otherwise, you’ll get a ClassCastException. You don’t need to write any special code for this exception, since it indicates a programmer error that could happen anywhere in a program. The {ThrowsException} comment tag tells this book’s build system to expect this program to throw an exception when it executes. There’s more to RTTI than a simple cast. For example, there’s a way to see what type you’re dealing with before you try to downcast it. All of the Type Information chapter is devoted to the study of different aspects of Java run-time type identification. Exercise 17: (2) Using the Cycle hierarchy from Exercise 1, add a balance( ) method to Unicycle and Bicycle, but not to Tricycle. Create instances of all three types and upcast them to an array of Cycle. Try to call balance( ) on each element of the array and observe the results. Downcast and call balance( ) and observe what happens. 216 Thinking in Java Bruce Eckel Summary Polymorphism means “different forms.” In object-oriented programming, you have the same interface from the base class, and different forms using that interface: the different versions of the dynamically bound methods. You’ve seen in this chapter that it’s impossible to understand, or even create, an example of polymorphism without using data abstraction and inheritance. Polymorphism is a feature that cannot be viewed in isolation (like a switch statement can, for example), but instead works only in concert, as part of the larger picture of class relationships. To use polymorphism—and thus object-oriented techniques—effectively in your programs, you must expand your view of programming to include not just members and messages of an individual class, but also the commonality among classes and their relationships with each other. Although this requires significant effort, it’s a worthy struggle. The results are faster program development, better code organization, extensible programs, and easier code maintenance. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Polymorphism 217 Interfaces Interfaces and abstract classes provide more structured way to separate interface from implementation. Such mechanisms are not that common in programming languages. C++, for example, only has indirect support for these concepts. The fact that language keywords exist in Java indicates that these ideas were considered important enough to provide direct support. First, we’ll look at the abstract class, which is a kind of midway step between an ordinary class and an interface. Although your first impulse will be to create an interface, the abstract class is an important and necessary tool for building classes that have some unimplemented methods. You can’t always use a pure interface. Abstract classes and methods In all the “instrument” examples in the previous chapter, the methods in the base class Instrument were always “dummy” methods. If these methods are ever called, you’ve done something wrong. That’s because the intent of Instrument is to create a common interface for all the classes derived from it. In those examples, the only reason to establish this common interface is so that it can be expressed differently for each different subtype. It establishes a basic form, so that you can say what’s common for all the derived classes. Another way of saying this is to call Instrument an abstract base class, or simply an abstract class. If you have an abstract class like Instrument, objects of that specific class almost always have no meaning. You create an abstract class when you want to manipulate a set of classes through its common interface. Thus, Instrument is meant to express only the interface, and not a particular implementation, so creating an Instrument object makes no sense, and you’ll probably want to prevent the user from doing it. This can be accomplished by making all methods in Instrument generate errors, but that delays the information until run time and requires reliable exhaustive testing on the user’s part. It’s usually better to catch problems at compile time. Java provides a mechanism for doing this called the abstract method. 1 This is a method that is incomplete; it has only a declaration and no method body. Here is the syntax for an abstract method declaration. abstract void f( ); A class containing abstract methods is called an abstract class. If a class contains one or more abstract methods, the class itself must be qualified as abstract. (Otherwise, the compiler gives you an error message.) If an abstract class is incomplete, what is the compiler supposed to do when someone tries to make an object of that class? It cannot safely create an object of an abstract class, so you get 1 For C++ programmers, this ist he analogue of C++’s pure virtual function. an error message from the compiler. This way, the compiler ensures the purity of the abstract class, and you don’t need to worry about misusing it. If you inherit from an abstract class and you want to make objects of the new type, you must provide method definitions for all the abstract methods in the base class. If you don’t (and you may choose not to), then the derived class is also abstract, and the compiler will force you to qualify that class with the abstract keyword. It’s possible to make a class abstract without including any abstract methods. This is useful when you’ve got a class in which it doesn’t make sense to have any abstract methods, and yet you want to prevent any instances of that class. The Instrument class from the previous chapter can easily be turned into an abstract class. Only some of the methods will be abstract, since making a class abstract doesn’t force you to make all the methods abstract. Here’s what it looks like: Here’s the orchestra example modified to use abstract classes and methods: //: interfaces/music4/Music4.java // Abstract classes and methods. package interfaces.music4; import polymorphism.music.Note; import static net.mindview.util.Print.*; abstract class Instrument { private int i; // Storage allocated for each 220 Thinking in Java Bruce Eckel public abstract void play(Note n); public String what() { return "Instrument"; } public abstract void adjust(); } class Wind extends Instrument { public void play(Note n) { print("Wind.play() " + n); } public String what() { return "Wind"; } public void adjust() {} } class Percussion extends Instrument { public void play(Note n) { print("Percussion.play() " + n); } public String what() { return "Percussion"; } public void adjust() {} } class Stringed extends Instrument { public void play(Note n) { print("Stringed.play() " + n); } public String what() { return "Stringed"; } public void adjust() {} } class Brass extends Wind { public void play(Note n) { print("Brass.play() " + n); } public void adjust() { print("Brass.adjust()"); } } class Woodwind extends Wind { public void play(Note n) { print("Woodwind.play() " + n); } public String what() { return "Woodwind"; } } public class Music4 { // Doesn’t care about type, so new types // added to the system still work right: static void tune(Instrument i) { // ... i.play(Note.MIDDLE_C); } static void tuneAll(Instrument[] e) { for(Instrument i : e) tune(i); } public static void main(String[] args) { // Upcasting during addition to the array: Instrument[] orchestra = { new Wind(), new Percussion(), new Stringed(), new Brass(), new Woodwind() }; Interfaces 221 tuneAll(orchestra); } } /* Output: Wind.play() MIDDLE_C Percussion.play() MIDDLE_C Stringed.play() MIDDLE_C Brass.play() MIDDLE_C Woodwind.play() MIDDLE_C *///:~ You can see that there’s really no change except in the base class. It’s helpful to create abstract classes and methods because they make the abstractness of a class explicit, and tell both the user and the compiler how it was intended to be used. Abstract classes are also useful refactoring tolls, since they allow you to easily move common methods up the inheritance hierarchy. Exercise 1: (1) Modify Exercise 9 in the previous chapter so that Rodent is an abstract class. Make the methods of Rodent abstract whenever possible. Exercise 2: (1) Create a class as abstract without including any abstract methods and verify that you cannot create any instances of that class. Exercise 3: (2) Create a base class with an abstract print( ) method that is overridden in a derived class. The overridden version of the method prints the value of an int variable defined in the derived class. At the point of definition of this variable, give it a nonzero value. In the base-class constructor, call this method. In main( ), create an object of the derived type, and then call its print( ) method. Explain the results. Exercise 4: (3) Create an abstract class with no methods. Derive a class and add a method. Create a static method that takes a reference to the base class, downcasts it to the derived class, and calls the method. In main( ), demonstrate that it works. Now put the abstract declaration for the method in the base class, thus eliminating the need for the downcast. Interfaces The interface keyword takes the concept of abstractness one step further. The abstract keyword allows you to create one or more undefined methods in a class—you provide part of the interface without providing a corresponding implementation. The implementation is provided by inheritors. The interface keyword produces a completely abstract class, one that provides no implementation at all. It allows the creator to determine method names, argument lists, and return types, but no method bodies. An interface provides only a form, but no implementation. An interface says, "All classes that implement this particular interface will look like this." Thus, any code that uses a particular interface knows what methods might be called for that interface, and that’s all. So the interface is used to establish a "protocol" between classes. (Some object-oriented programming languages have a keyword called protocol to do the same thing.) However, an interface is more than just an abstract class taken to the extreme, since it allows you to perform a variation of "multiple inheritance" by creating a class that can be upcast to more than one base type. 222 Thinking in Java Bruce Eckel To create an interface, use the interface keyword instead of the class keyword. As with a class, you can add the public keyword before the interface keyword (but only if that interface is defined in a file of the same name). If you leave off the public keyword, you get package access, so the interface is only usable within the same package. An interface can also contain fields, but these are implicitly static and final. To make a class that conforms to a particular interface (or group of interfaces), use the implements keyword, which says, "The interface is what it looks like, but now I’m going to say how it works." Other than that, it looks like inheritance. The diagram for the instrument example shows this: You can see from the Woodwind and Brass classes that once you’ve implemented an interface, that implementation becomes an ordinary class that can be extended in the regular way. You can choose to explicitly declare the methods in an interface as public, but they are public even if you don’t say it. So when you implement an interface, the methods from the interface must be defined as public. Otherwise, they would default to package access, and you’d be reducing the accessibility of a method during inheritance, which is not allowed by the Java compiler. You can see this in the modified version of the Instrument example. Note that every method in the interface is strictly a declaration, which is the only thing the compiler allows. In addition, none of the methods in Instrument are declared as public, but they’re automatically public anyway: Interfaces 223 //: interfaces/music5/Music5.java // Interfaces. package interfaces.music5; import polymorphism.music.Note; import static net.mindview.util.Print.*; interface Instrument { // Compile-time constant: int VALUE = 5; // static & final // Cannot have method definitions: void play(Note n); // Automatically public void adjust(); } class Wind implements Instrument { public void play(Note n) { print(this + ".play() " + n); } public String toString() { return "Wind"; } public void adjust() { print(this + ".adjust()"); } } class Percussion implements Instrument { public void play(Note n) { print(this + ".play() " + n); } public String toString() { return "Percussion"; } public void adjust() { print(this + ".adjust()"); } } class Stringed implements Instrument { public void play(Note n) { print(this + ".play() " + n); } public String toString() { return "Stringed"; } public void adjust() { print(this + ".adjust()"); } } class Brass extends Wind { public String toString() { return "Brass"; } } class Woodwind extends Wind { public String toString() { return "Woodwind"; } } public class Music5 { // Doesn’t care about type, so new types // added to the system still work right: static void tune(Instrument i) { // ... i.play(Note.MIDDLE_C); } static void tuneAll(Instrument[] e) { for(Instrument i : e) tune(i); } public static void main(String[] args) { // Upcasting during addition to the array: Instrument[] orchestra = { new Wind(), new Percussion(), new Stringed(), 224 Thinking in Java Bruce Eckel new Brass(), new Woodwind() }; tuneAll(orchestra); } } /* Output: Wind.play() MIDDLE_C Percussion.play() MIDDLE_C Stringed.play() MIDDLE_C Brass.play() MIDDLE_C Woodwind.play() MIDDLE_C *///:~ One other change has been made to this version of the example: The what( ) method has been changed to toString( ), since that was how the method was being used. Since toString( ) is part of the root class Object, it doesn’t need to appear in the interface. The rest of the code works the same. Notice that it doesn’t matter if you are upcasting to a "regular" class called Instrument, an abstract class called Instrument, or to an interface called Instrument. The behavior is the same. In fact, you can see in the tune( ) method that there isn’t any evidence about whether Instrument is a "regular" class, an abstract class, or an interface. Exercise 5: (2) Create an interface containing three methods, in its own package. Implement the interface in a different package. Exercise 6: (2) Prove that all the methods in an interface are automatically public. Exercise 7: (1) Change Exercise 9 in the Polymorphism chapter so that Rodent is an interface. Exercise 8: (2) In polymorphism.Sandwich.java, create an interface called FastFoo d (with appropriate methods) and change Sandwic h so that it also implements FastFood. Exercise 9: (3) Refactor Musics.java by moving the common methods in Wind, Percussion and Stringed into an abstract class. Exercise 10: (3) Modify Musics.java by adding a Playable interface. Move the play( ) declaration from Instrument to Playable. Add Playable to the derived classes by including it in the implement s list. Change tune( ) so that it takes a Playable instead of an Instrument. Complete decoupling Whenever a method works with a class instead of an interface, you are limited to using that class or its subclasses. If you would like to apply the method to a class that isn’t in that hierarchy, you’re out of luck. An interface relaxes this constraint considerably. As a result, it allows you to write more reusable code. For example, suppose you have a Processor class that has a name( ) and a process( ) method that takes input, modifies it and produces output. The base class is extended to create different types of Processor. In this case, the Processor subtypes modify String objects (note that the return types can be covariant, but not the argument types): //: interfaces/classprocessor/Apply.java Interfaces 225 package interfaces.classprocessor; import java.util.*; import static net.mindview.util.Print.*; class Processor { public String name() { return getClass().getSimpleName(); } Object process(Object input) { return input; } } class Upcase extends Processor { String process(Object input) { // Covariant return return ((String)input).toUpperCase(); } } class Downcase extends Processor { String process(Object input) { return ((String)input).toLowerCase(); } } class Splitter extends Processor { String process(Object input) { // The split() argument divides a String into pieces: return Arrays.toString(((String)input).split(" ")); } } public class Apply { public static void process(Processor p, Object s) { print("Using Processor " + p.name()); print(p.process(s)); } public static String s = "Disagreement with beliefs is by definition incorrect"; public static void main(String[] args) { process(new Upcase(), s); process(new Downcase(), s); process(new Splitter(), s); } } /* Output: Using Processor Upcase DISAGREEMENT WITH BELIEFS IS BY DEFINITION INCORRECT Using Processor Downcase disagreement with beliefs is by definition incorrect Using Processor Splitter [Disagreement, with, beliefs, is, by, definition, incorrect] *///:~ The Apply.process( ) method takes any kind of Processor and applies it to an Object, then prints the results. Creating a method that behaves differently depending on the argument object that you pass it is called the Strategy design pattern. The method contains the fixed part of the algorithm to be performed, and the Strategy contains the part that varies. The Strategy is the object that you pass in, and it contains code to be executed. Here, the Processor object is the Strategy, and in main( ) you can see three different Strategies applied to the String s. The split( ) method is part of the String class. It takes the String object and splits it using the argument as a boundary, and returns a String[]. It is used here as a shorter way of creating an array of String. 226 Thinking in Java Bruce Eckel Now suppose you discover a set of electronic filters that seem like they could fit into your Apply.process( ) method: //: interfaces/filters/Waveform.java package interfaces.filters; public class Waveform { private static long counter; private final long id = counter++; public String toString() { return "Waveform " + id; } } ///:~ //: interfaces/filters/Filter.java package interfaces.filters; public class Filter { public String name() { return getClass().getSimpleName(); } public Waveform process(Waveform input) { return input; } } ///:~ //: interfaces/filters/LowPass.java package interfaces.filters; public class LowPass extends Filter { double cutoff; public LowPass(double cutoff) { this.cutoff = cutoff; } public Waveform process(Waveform input) { return input; // Dummy processing } } ///:~ //: interfaces/filters/HighPass.java package interfaces.filters; public class HighPass extends Filter { double cutoff; public HighPass(double cutoff) { this.cutoff = cutoff; } public Waveform process(Waveform input) { return input; } } ///:~ //: interfaces/filters/BandPass.java package interfaces.filters; public class BandPass extends Filter { double lowCutoff, highCutoff; public BandPass(double lowCut, double highCut) { lowCutoff = lowCut; highCutoff = highCut; } public Waveform process(Waveform input) { return input; } } ///:~ Filter has the same interface elements as Processor, but because it isn’t inherited from Processor—because the creator of the Filter class had no clue you might want to use it as a Processor—you can’t use a Filter with the Apply.process( ) method, even though it would work fine. Basically, the coupling between Apply.process( ) and Processor is stronger than it needs to be, and this prevents the Apply.process( ) code from being reused when it ought to be. Also notice that the inputs and outputs are both Waveforms. Interfaces 227 If Processor is an interface, however, the constraints are loosened enough that you can reuse an Apply.process( ) that takes that interface. Here are the modified versions of Processor and Apply: //: interfaces/interfaceprocessor/Processor.java package interfaces.interfaceprocessor; public interface Processor { String name(); Object process(Object input); } ///:~ //: interfaces/interfaceprocessor/Apply.java package interfaces.interfaceprocessor; import static net.mindview.util.Print.*; public class Apply { public static void process(Processor p, Object s) { print("Using Processor " + p.name()); print(p.process(s)); } } ///:~ The first way you can reuse code is if client programmers can write their classes to conform to the interface, like this: //: interfaces/interfaceprocessor/StringProcessor.java package interfaces.interfaceprocessor; import java.util.*; public abstract class StringProcessor implements Processor{ public String name() { return getClass().getSimpleName(); } public abstract String process(Object input); public static String s = "If she weighs the same as a duck, she’s made of wood"; public static void main(String[] args) { Apply.process(new Upcase(), s); Apply.process(new Downcase(), s); Apply.process(new Splitter(), s); } } class Upcase extends StringProcessor { public String process(Object input) { // Covariant return return ((String)input).toUpperCase(); } } class Downcase extends StringProcessor { public String process(Object input) { return ((String)input).toLowerCase(); } } class Splitter extends StringProcessor { public String process(Object input) { return Arrays.toString(((String)input).split(" ")); } } /* Output: 228 Thinking in Java Bruce Eckel Using Processor Upcase IF SHE WEIGHS THE SAME AS A DUCK, SHE’S MADE OF WOOD Using Processor Downcase if she weighs the same as a duck, she’s made of wood Using Processor Splitter [If, she, weighs, the, same, as, a, duck,, she’s, made, of, wood] *///:~ However, you are often in the situation of not being able to modify the classes that you want to use. In the case of the electronic filters, for example, the library was discovered rather than created. In these cases, you can use the Adapter design pattern. In Adapter, you write code to take the interface that you have and produce the interface that you need, like this: //: interfaces/interfaceprocessor/FilterProcessor.java package interfaces.interfaceprocessor; import interfaces.filters.*; class FilterAdapter implements Processor { Filter filter; public FilterAdapter(Filter filter) { this.filter = filter; } public String name() { return filter.name(); } public Waveform process(Object input) { return filter.process((Waveform)input); } } public class FilterProcessor { public static void main(String[] args) { Waveform w = new Waveform(); Apply.process(new FilterAdapter(new LowPass(1.0)), w); Apply.process(new FilterAdapter(new HighPass(2.0)), w); Apply.process( new FilterAdapter(new BandPass(3.0, 4.0)), w); } } /* Output: Using Processor LowPass Waveform 0 Using Processor HighPass Waveform 0 Using Processor BandPass Waveform 0 *///:~ In this approach to Adapter, the FilterAdapter constructor takes the interface that you have—Filter—and produces an object that has the Processor interface that you need. You may also notice delegation in the FilterAdapter class. Decoupling interface from implementation allows an interface to be applied to multiple different implementations, and thus your code is more reusable. Exercise 11: (4) Create a class with a method that takes a String argument and produces a result that swaps each pair of characters in that argument. Adapt the class so that it works with interfaceprocessor.Apply.process( ). Interfaces 229 “Multiple inheritance” in Java Because an interface has no implementation at all—that is, there is no storage associated with an interface—there’s nothing to prevent many interfaces from being combined. This is valuable because there are times when you need to say, "An x is an a and a b and a c." In C++, this act of combining multiple class interfaces is called multiple inheritance, and it carries some rather sticky baggage because each class can have an implementation. In Java, you can perform the same act, but only one of the classes can have an implementation, so the C++ problems do not occur with Java when combining multiple interfaces: In a derived class, you aren’t forced to have a base class that is either abstract or "concrete" (one with no abstract methods). If you do inherit from a non-interface, you can inherit from only one. All the rest of the base elements must be interfaces. You place all the interface names after the implements keyword and separate them with commas. You can have as many interfaces as you want. You can upcast to each interface, because each interface is an independent type. The following example shows a concrete class combined with several interfaces to produce a new class: //: interfaces/Adventure.java // Multiple interfaces. interface CanFight { void fight(); } interface CanSwim { void swim(); } interface CanFly { void fly(); } class ActionCharacter { public void fight() {} } class Hero extends ActionCharacter implements CanFight, CanSwim, CanFly { public void swim() {} public void fly() {} } public class Adventure { public static void t(CanFight x) { x.fight(); } public static void u(CanSwim x) { x.swim(); } 230 Thinking in Java Bruce Eckel public static void v(CanFly x) { x.fly(); } public static void w(ActionCharacter x) { x.fight(); } public static void main(String[] args) { Hero h = new Hero(); t(h); // Treat it as a CanFight u(h); // Treat it as a CanSwim v(h); // Treat it as a CanFly w(h); // Treat it as an ActionCharacter } } ///:~ You can see that Hero combines the concrete class ActionCharacter with the interfaces CanFight, CanSwim, and CanFly. When you combine a concrete class with interfaces this way, the concrete class must come first, then the interfaces. (The compiler gives an error otherwise.) The signature for fight( ) is the same in the interface CanFight and the class ActionCharacter, and that fight( ) is not provided with a definition in Hero. You can extend an interface, but then you’ve got another interface. When you want to create an object, all the definitions must first exist. Even though Hero does not explicitly provide a definition for fight( ), the definition comes along with ActionCharacter; thus, it’s possible to create Hero objects. In class Adventure, you can see that there are four methods that take arguments of the various interfaces and of the concrete class. When a Hero object is created, it can be passed to any of these methods, which means it is being upcast to each interface in turn. Because of the way interfaces are designed in Java, this works without any particular effort on the part of the programmer. Keep in mind that one of the core reasons for interfaces is shown in the preceding example: to upcast to more than one base type (and the flexibility that this provides). However, a second reason for using interfaces is the same as using an abstract base class: to prevent the client programmer from making an object of this class and to establish that it is only an interface. This brings up a question: Should you use an interface or an abstract class? If it’s possible to create your base class without any method definitions or member variables, you should always prefer interfaces to abstract classes. In fact, if you know something is going to be a base class, you can consider making it an interface (this subject will be revisited in the chapter summary). Exercise 12: (2) In Adventure.java, add an interface called CanClimb, following the form of the other interfaces. Exercise 13: (2) Create an interface, and inherit two new interfaces from that interface. Multiply inherit a third interface from the second two. 2 Extending an interface with inheritance You can easily add new method declarations to an interface by using inheritance, and you can also combine several interfaces into a new interface with inheritance. In both cases you get a new interface, as seen in this example: 2 This shows how interfaces prevent the "diamond problem" that occurs with C++ multiple inheritance. Interfaces 231 //: interfaces/HorrorShow.java // Extending an interface with inheritance. interface Monster { void menace(); } interface DangerousMonster extends Monster { void destroy(); } interface Lethal { void kill(); } class DragonZilla implements DangerousMonster { public void menace() {} public void destroy() {} } interface Vampire extends DangerousMonster, Lethal { void drinkBlood(); } class VeryBadVampire implements Vampire { public void menace() {} public void destroy() {} public void kill() {} public void drinkBlood() {} } public class HorrorShow { static void u(Monster b) { b.menace(); } static void v(DangerousMonster d) { d.menace(); d.destroy(); } static void w(Lethal l) { l.kill(); } public static void main(String[] args) { DangerousMonster barney = new DragonZilla(); u(barney); v(barney); Vampire vlad = new VeryBadVampire(); u(vlad); v(vlad); w(vlad); } } ///:~ DangerousMonster is a simple extension to Monster that produces a new interface. This is implemented in DragonZilla. The syntax used in Vampire works only when inheriting interfaces. Normally, you can use extends with only a single class, but extends can refer to multiple base interfaces when building a new interface. As you can see, the interface names are simply separated with commas. Exercise 14: (2) Create three interfaces, each with two methods. Inherit a new interface that combines the three, adding a new method. Create a class by implementing the new interface and also inheriting from a concrete class. Now write four methods, each of which 232 Thinking in Java Bruce Eckel takes one of the four interfaces as an argument. In main( ), create an object of your class and pass it to each of the methods. Exercise 15: (2) Modify the previous exercise by creating an abstract class and inheriting that into the derived class. Name collisions when combining Interfaces You can encounter a small pitfall when implementing multiple interfaces. In the preceding example, both CanFight and ActionCharacter have identical void fight( ) methods. An identical method is not a problem, but what if the method differs by signature or return type? Here’s an example: //: interfaces/InterfaceCollision.java package interfaces; interface I1 { void f(); } interface I2 { int f(int i); } interface I3 { int f(); } class C { public int f() { return 1; } } class C2 implements I1, I2 { public void f() {} public int f(int i) { return 1; } // overloaded } class C3 extends C implements I2 { public int f(int i) { return 1; } // overloaded } class C4 extends C implements I3 { // Identical, no problem: public int f() { return 1; } } // Methods differ only by return type: //! class C5 extends C implements I1 {} //! interface I4 extends I1, I3 {} ///:~ The difficulty occurs because overriding, implementation, and overloading get unpleasantly mixed together. Also, overloaded methods cannot differ only by return type. When the last two lines are uncommented, the error messages say it all: InterfaceCollision.java:23: f() in C cannot implementf() in It; attempting to use incompatible return type found: int required: void InterfaceCollision.java:24: Interfaces I3 andh are incompatible; both define f(), but with different return type Using the same method names in different interfaces that are intended to be combined generally causes confusion in the readability of the code, as well. Strive to avoid it. Interfaces 233 Adapting to an interface One of the most compelling reasons for interfaces is to allow multiple implementations for the same interface. In simple cases this is in the form of a method that accepts an interface, leaving it up to you to implement that interface and pass your object to the method. Thus, a common use for interfaces is the aforementioned Strategy design pattern. You write a method that performs certain operations, and that method takes an interface that you also specify. You’re basically saying, "You can use my method with any object you like, as long as your object conforms to my interface." This makes your method more flexible, general and reusable. For example, the constructor for the Java SE5 Scanner class (which you’ll learn more about in the Strings chapter) takes a Readable interface. You’ll find that Readable is not an argument for any other method in the Java standard library—it was created solely for Scanner, so that Scanner doesn’t have to constrain its argument to be a particular class. This way, Scanner can be made to work with more types. If you create a new class and you want it to be usable with Scanner, you make it Readable, like this: //: interfaces/RandomWords.java // Implementing an interface to conform to a method. import java.nio.*; import java.util.*; public class RandomWords implements Readable { private static Random rand = new Random(47); private static final char[] capitals = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray(); private static final char[] lowers = "abcdefghijklmnopqrstuvwxyz".toCharArray(); private static final char[] vowels = "aeiou".toCharArray(); private int count; public RandomWords(int count) { this.count = count; } public int read(CharBuffer cb) { if(count-- == 0) return -1; // Indicates end of input cb.append(capitals[rand.nextInt(capitals.length)]); for(int i = 0; i < 4; i++) { cb.append(vowels[rand.nextInt(vowels.length)]); cb.append(lowers[rand.nextInt(lowers.length)]); } cb.append(" "); return 10; // Number of characters appended } public static void main(String[] args) { Scanner s = new Scanner(new RandomWords(10)); while(s.hasNext()) System.out.println(s.next()); } } /* Output: Yazeruyac Fowenucor Goeazimom Raeuuacio Nuoadesiw Hageaikux Ruqicibui Numasetih Kuuuuozog 234 Thinking in Java Bruce Eckel Waqizeyoy *///:~ The Readable interface only requires the implementation of a read( ) method. Inside read( ), you add to the CharBuffer argument (there are several ways to do this; see the CharBuffer documentation), or return -l when you have no more input. Suppose you have a class that does not already implement Readable—how do you make it work with Scanner? Here’s an example of a class that produces random floating point numbers: //: interfaces/RandomDoubles.java import java.util.*; public class RandomDoubles { private static Random rand = new Random(47); public double next() { return rand.nextDouble(); } public static void main(String[] args) { RandomDoubles rd = new RandomDoubles(); for(int i = 0; i < 7; i ++) System.out.print(rd.next() + " "); } } /* Output: 0.7271157860730044 0.5309454508634242 0.16020656493302599 0.18847866977771732 0.5166020801268457 0.2678662084200585 0.2613610344283964 *///:~ Because you can add an interface onto any existing class in this way, it means that a method that takes an interface provides a way for any class to be adapted to work with that method. This is the power of using interfaces instead of classes. Exercise 16: (3) Create a class that produces a sequence of chars. Adapt this class so that it can be an input to a Scanner object. Fields in interfaces Because any fields you put into an interface are automatically static and final, the interface is a convenient tool for creating groups of constant values. Before Java SE5, this was the only way to produce the same effect as an enum in C or C++. So you will see pre-Java SE5 code like this: //: interfaces/Months.java // Using interfaces to create groups of constants. package interfaces; public interface Months { int JANUARY = 1, FEBRUARY = 2, MARCH = 3, APRIL = 4, MAY = 5, JUNE = 6, JULY = 7, AUGUST = 8, SEPTEMBER = 9, OCTOBER = 10, NOVEMBER = 11, DECEMBER = 12; } ///:~ Notice the Java style of using all uppercase letters (with underscores to separate multiple words in a single identifier) for static finals that have constant initializers. The fields in an interface are automatically public, so that is not explicitly specified. Interfaces 235 With Java SE5, you now have the much more powerful and flexible enum keyword, so it rarely makes sense to use interfaces for constants anymore. However, you will probably run across the old idiom on many occasions when reading legacy code (the supplements for this book at www.MindView.net contain a complete description of the pre-Java SE5 approach to producing enumerated types using interfaces). You can find more details about using enums in the Enumerated Types chapter. Exercise 17: (2) Prove that the fields in an interface are implicitly static and final. Initializing fields in interfaces Fields defined in interfaces cannot be "blank finals," but they can be initialized with nonconstant expressions. For example: //: interfaces/RandVals.java // Initializing interface fields with // non-constant initializers. import java.util.*; public interface RandVals { Random RAND = new Random(47); int RANDOM_INT = RAND.nextInt(10); long RANDOM_LONG = RAND.nextLong() * 10; float RANDOM_FLOAT = RAND.nextLong() * 10; double RANDOM_DOUBLE = RAND.nextDouble() * 10; } ///:~ Since the fields are static, they are initialized when the class is first loaded, which happens when any of the fields are accessed for the first time. Here’s a simple test: //: interfaces/TestRandVals.java import static net.mindview.util.Print.*; public class TestRandVals { public static void main(String[] args) { print(RandVals.RANDOM_INT); print(RandVals.RANDOM_LONG); print(RandVals.RANDOM_FLOAT); print(RandVals.RANDOM_DOUBLE); } } /* Output: 8 -32032247016559954 -8.5939291E18 5.779976127815049 *///:~ The fields, of course, are not part of the interface. The values are stored in the static storage area for that interface. 236 Thinking in Java Bruce Eckel Nesting interfaces Interfaces may be nested within classes and within other interfaces. 3 This reveals a number of interesting features: //: interfaces/nesting/NestingInterfaces.java package interfaces.nesting; class A { interface B { void f(); } public class BImp implements B { public void f() {} } private class BImp2 implements B { public void f() {} } public interface C { void f(); } class CImp implements C { public void f() {} } private class CImp2 implements C { public void f() {} } private interface D { void f(); } private class DImp implements D { public void f() {} } public class DImp2 implements D { public void f() {} } public D getD() { return new DImp2(); } private D dRef; public void receiveD(D d) { dRef = d; dRef.f(); } } interface E { interface G { void f(); } // Redundant "public": public interface H { void f(); } void g(); // Cannot be private within an interface: //! private interface I {} } public class NestingInterfaces { 3 Thanks to Martin Danner for asking about this during a seminar. Interfaces 237 public class BImp implements A.B { public void f() {} } class CImp implements A.C { public void f() {} } // Cannot implement a private interface except // within that interface’s defining class: //! class DImp implements A.D { //! public void f() {} //! } class EImp implements E { public void g() {} } class EGImp implements E.G { public void f() {} } class EImp2 implements E { public void g() {} class EG implements E.G { public void f() {} } } public static void main(String[] args) { A a = new A(); // Can’t access A.D: //! A.D ad = a.getD(); // Doesn’t return anything but A.D: //! A.DImp2 di2 = a.getD(); // Cannot access a member of the interface: //! a.getD().f(); // Only another A can do anything with getD(): A a2 = new A(); a2.receiveD(a.getD()); } } ///:~ The syntax for nesting an interface within a class is reasonably obvious. Just like non-nested interfaces, these can have public or package-access visibility. As an added twist, interfaces can also be private, as seen in A.D (the same qualification syntax is used for nested interfaces as for nested classes). What good is a private nested interface? You might guess that it can only be implemented as a private inner class as in DImp, but A.DImp2 shows that it can also be implemented as a public class. However, A.DImp2 can only be used as itself. You are not allowed to mention the fact that it implements the private interface D, so implementing a private interface is a way to force the definition of the methods in that interface without adding any type information (that is, without allowing any upcasting). The method getD( ) produces a further quandary concerning the private interface: It’s a public method that returns a reference to a private interface. What can you do with the return value of this method? In main( ), you can see several attempts to use the return value, all of which fail. The only thing that works is if the return value is handed to an object that has permission to use it—in this case, another A, via the receiveD( ) method. Interface E shows that interfaces can be nested within each other. However, the rules about interfaces—in particular, that all interface elements must be public—are strictly enforced here, so an interface nested within another interface is automatically public and cannot be made private. 238 Thinking in Java Bruce Eckel Nestinglnterfaces shows the various ways that nested interfaces can be implemented. In particular, notice that when you implement an interface, you are not required to implement any interfaces nested within. Also, private interfaces cannot be implemented outside of their defining classes. Initially, these features may seem like they are added strictly for syntactic consistency, but I generally find that once you know about a feature, you often discover places where it is useful. Interfaces and factories An interface is intended to be a gateway to multiple implementations, and a typical way to produce objects that fit the interface is the Factory Method design pattern. Instead of calling a constructor directly, you call a creation method on a factory object which produces an implementation of the interface—this way, in theory, your code is completely isolated from the implementation of the interface, thus making it possible to transparently swap one implementation for another. Here’s a demonstration showing the structure of the Factory Method: //: interfaces/Factories.java import static net.mindview.util.Print.*; interface Service { void method1(); void method2(); } interface ServiceFactory { Service getService(); } class Implementation1 implements Service { Implementation1() {} // Package access public void method1() {print("Implementation1 method1");} public void method2() {print("Implementation1 method2");} } class Implementation1Factory implements ServiceFactory { public Service getService() { return new Implementation1(); } } class Implementation2 implements Service { Implementation2() {} // Package access public void method1() {print("Implementation2 method1");} public void method2() {print("Implementation2 method2");} } class Implementation2Factory implements ServiceFactory { public Service getService() { return new Implementation2(); } } public class Factories { public static void serviceConsumer(ServiceFactory fact) { Service s = fact.getService(); s.method1(); s.method2(); Interfaces 239 } public static void main(String[] args) { serviceConsumer(new Implementation1Factory()); // Implementations are completely interchangeable: serviceConsumer(new Implementation2Factory()); } } /* Output: Implementation1 method1 Implementation1 method2 Implementation2 method1 Implementation2 method2 *///:~ Without the Factory Method, your code would somewhere have to specify the exact type of Service being created, so that it could call the appropriate constructor. Why would you want to add this extra level of indirection? One common reason is to create a framework. Suppose you are creating a system to play games; for example, to play both chess and checkers on the same board: //: interfaces/Games.java // A Game framework using Factory Methods. import static net.mindview.util.Print.*; interface Game { boolean move(); } interface GameFactory { Game getGame(); } class Checkers implements Game { private int moves = 0; private static final int MOVES = 3; public boolean move() { print("Checkers move " + moves); return ++moves != MOVES; } } class CheckersFactory implements GameFactory { public Game getGame() { return new Checkers(); } } class Chess implements Game { private int moves = 0; private static final int MOVES = 4; public boolean move() { print("Chess move " + moves); return ++moves != MOVES; } } class ChessFactory implements GameFactory { public Game getGame() { return new Chess(); } } public class Games { public static void playGame(GameFactory factory) { Game s = factory.getGame(); while(s.move()) ; } public static void main(String[] args) { playGame(new CheckersFactory()); playGame(new ChessFactory()); 240 Thinking in Java Bruce Eckel } } /* Output: Checkers move 0 Checkers move 1 Checkers move 2 Chess move 0 Chess move 1 Chess move 2 Chess move 3 *///:~ If the Games class represents a complex piece of code, this approach allows you to reuse that code with different types of games. You can imagine more elaborate games that can benefit from this pattern. In the next chapter, you’ll see a more elegant way to implement the factories using anonymous inner classes. Exercise 18: (2) Create a Cycle interface, with implementations Unicycle, Bicycle and Tricycle. Create factories for each type of Cycle, and code that uses these factories. Exercise 19: (3) Create a framework using Factory Methods that performs both coin tossing and dice tossing. Summary It is tempting to decide that interfaces are good, and therefore you should always choose interfaces over concrete classes. Of course, almost anytime you create a class, you could instead create an interface and a factory. Many people have fallen to this temptation, creating interfaces and factories wherever it’s possible. The logic seems to be that you might need to use a different implementation, so you should always add that abstraction. It has become a kind of premature design optimization. Any abstraction should be motivated by a real need. Interfaces should be something you refactor to when necessary, rather than installing the extra level of indirection everywhere, along with the extra complexity. That extra complexity is significant, and if you make someone work through that complexity only to realize that you’ve added interfaces "just in case" and for no compelling reason—well, if I see such a thing I begin to question all the designs that this particular person has done. An appropriate guideline is to prefer classes to interfaces. Start with classes, and if it becomes clear that interfaces are necessary, then refactor. Interfaces are a great tool, but they can easily be overused. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Interfaces 241 Inner Classes It’s possible to place a class definition within another class definition. This is called an inner class. The inner class is a valuable feature because it allows you to group classes that logically belong together and to control the visibility of one within the other. However, it’s important to understand that inner classes are distinctly different from composition. At first, inner classes look like a simple code-hiding mechanism: You place classes inside other classes. You’ll learn, however, that the inner class does more than that—it knows about and can communicate with the surrounding class—and the kind of code you can write with inner classes is more elegant and clear, although there’s certainly no guarantee of this. Initially, inner classes may seem odd, and it will take some time to become comfortable using them in your designs. The need for inner classes isn’t always obvious, but after the basic syntax and semantics of inner classes have been described, the section "Why inner classes?" should begin to make clear the benefits of inner classes. After that section, the remainder of the chapter contains more detailed explorations of the syntax of inner classes. These features are provided for language completeness, but you might not need to use them, at least not at first. So the initial parts of the chapter might be all you need for now, and you can leave the more detailed explorations as reference material. Creating inner classes You create an inner class just as you’d expect—by placing the class definition inside a surrounding class: //: innerclasses/Parcel1.java // Creating inner classes. public class Parcel1 { class Contents { private int i = 11; public int value() { return i; } } class Destination { private String label; Destination(String whereTo) { label = whereTo; } String readLabel() { return label; } } // Using inner classes looks just like // using any other class, within Parcel1: public void ship(String dest) { Contents c = new Contents(); Destination d = new Destination(dest); System.out.println(d.readLabel()); } public static void main(String[] args) { Parcel1 p = new Parcel1(); p.ship("Tasmania"); } } /* Output: Tasmania *///:~ The inner classes used inside ship( ) look just like ordinary classes. Here, the only practical difference is that the names are nested within Parceli. You’ll see in a while that this isn’t the only difference. More typically, an outer class will have a method that returns a reference to an inner class, as you can see in the to( ) and contents( ) methods: //: innerclasses/Parcel2.java // Returning a reference to an inner class. public class Parcel2 { class Contents { private int i = 11; public int value() { return i; } } class Destination { private String label; Destination(String whereTo) { label = whereTo; } String readLabel() { return label; } } public Destination to(String s) { return new Destination(s); } public Contents contents() { return new Contents(); } public void ship(String dest) { Contents c = contents(); Destination d = to(dest); System.out.println(d.readLabel()); } public static void main(String[] args) { Parcel2 p = new Parcel2(); p.ship("Tasmania"); Parcel2 q = new Parcel2(); // Defining references to inner classes: Parcel2.Contents c = q.contents(); Parcel2.Destination d = q.to("Borneo"); } } /* Output: Tasmania *///:~ If you want to make an object of the inner class anywhere except from within a non-static method of the outer class, you must specify the type of that object as OuterClassName.InnerClassName, as seen in main( ). Exercise 1: (1) Write a class named Outer that contains an inner class named Inner. Add a method to Outer that returns an object of type Inner. In main( ), create and initialize a reference to an Inner. The link to the outer class So far, it appears that inner classes are just a name-hiding and code organization scheme, which is helpful but not totally compelling. However, there’s another twist. When you create 244 Thinking in Java Bruce Eckel an inner class, an object of that inner class has a link to the enclosing object that made it, and so it can access the members of that enclosing object—without any special qualifications. In addition, inner classes have access rights to all the elements in the enclosing class. 1 The following example demonstrates this: //: innerclasses/Sequence.java // Holds a sequence of Objects. interface Selector { boolean end(); Object current(); void next(); } public class Sequence { private Object[] items; private int next = 0; public Sequence(int size) { items = new Object[size]; } public void add(Object x) { if(next < items.length) items[next++] = x; } private class SequenceSelector implements Selector { private int i = 0; public boolean end() { return i == items.length; } public Object current() { return items[i]; } public void next() { if(i < items.length) i++; } } public Selector selector() { return new SequenceSelector(); } public static void main(String[] args) { Sequence sequence = new Sequence(10); for(int i = 0; i < 10; i++) sequence.add(Integer.toString(i)); Selector selector = sequence.selector(); while(!selector.end()) { System.out.print(selector.current() + " "); selector.next(); } } } /* Output: 0123456789 *///:~ The Sequence is simply a fixed-sized array of Object with a class wrapped around it. You call add( ) to add a new Object to the end of the sequence (if there’s room left). To fetch each of the objects in a Sequence, there’s an interface called Selector. This is an example of the Iterator design pattern that you shall learn more about later in the book. A Selector allows you to see if you’re at the end( ), to access the current( ) Object, and to move to the next( ) Object in the Sequence. Because Selector is an interface, other classes can implement the interface in their own ways, and other methods can take the interface as an argument, in order to create more general-purpose code. Here, the SequenceSelector is a private class that provides Selector functionality. In main( ), you can see the creation of a Sequence, followed by the addition of a number of String objects. Then, a Selector is produced with a call to selector( ), and this is used to move through the Sequence and select each item. 1 This is very different from the design of nested classes in C++, which is simply a namehiding mechanism. There is no link to an enclosing object and no implied permissions in C++. Inner Classes 245 At first, the creation of SequenceSelector looks like just another inner class. But examine it closely. Note that each of the methods—end( ), current( ), and next( )—refers to items, which is a reference that isn’t part of SequenceSelector, but is instead a private field in the enclosing class. However, the inner class can access methods and fields from the enclosing class as if it owned them. This turns out to be very convenient, as you can see in the preceding example. So an inner class has automatic access to the members of the enclosing class. How can this happen? The inner class secretly captures a reference to the particular object of the enclosing class that was responsible for creating it. Then, when you refer to a member of the enclosing class, that reference is used to select that member. Fortunately, the compiler takes care of all these details for you, but now you can see that an object of an inner class can be created only in association with an object of the enclosing class (when, as you shall see, the inner class is non-static). Construction of the inner-class object requires the reference to the object of the enclosing class, and the compiler will complain if it cannot access that reference. Most of the time this occurs without any intervention on the part of the programmer. Exercise 2: (1) Create a class that holds a String, and has a toString( ) method that displays this String. Add several instances of your new class to a Sequence object, then display them. Exercise 3: (1) Modify Exercise 1 so that Outer has a private String field (initialized by the constructor), and Inner has a toString( ) that displays this field. Create an object of type Inner and display it. Using .this and .new If you need to produce the reference to the outer-class object, you name the outer class followed by a dot and this. The resulting reference is automatically the correct type, which is known and checked at compile time, so there is no runtime overhead. Here’s an example that shows how to use .this: //: innerclasses/DotThis.java // Qualifying access to the outer-class object. public class DotThis { void f() { System.out.println("DotThis.f()"); } public class Inner { public DotThis outer() { return DotThis.this; // A plain "this" would be Inner’s "this" } } public Inner inner() { return new Inner(); } public static void main(String[] args) { DotThis dt = new DotThis(); DotThis.Inner dti = dt.inner(); dti.outer().f(); } } /* Output: DotThis.f() *///:~ Sometimes you want to tell some other object to create an object of one of its inner classes. To do this you must provide a reference to the other outer-class object in the new expression, using the .new syntax, like this: //: innerclasses/DotNew.java 246 Thinking in Java Bruce Eckel // Creating an inner class directly using the .new syntax. public class DotNew { public class Inner {} public static void main(String[] args) { DotNew dn = new DotNew(); DotNew.Inner dni = dn.new Inner(); } } ///:~ To create an object of the inner class directly, you don’t follow the same form and refer to the outer class name DotNew as you might expect, but instead you must use an object of the outer class to make an object of the inner class, as you can see above. This also resolves the name scoping issues for the inner class, so you don’t say (indeed, you can’t say) dn.new DotNew.Inner( ). It’s not possible to create an object of the inner class unless you already have an object of the outer class. This is because the object of the inner class is quietly connected to the object of the outer class that it was made from. However, if you make a nested class (a static inner class), then it doesn’t need a reference to the outer-class object. Here, you see the use of .new applied to the "Parcel" example: //: innerclasses/Parcel3.java // Using .new to create instances of inner classes. public class Parcel3 { class Contents { private int i = 11; public int value() { return i; } } class Destination { private String label; Destination(String whereTo) { label = whereTo; } String readLabel() { return label; } } public static void main(String[] args) { Parcel3 p = new Parcel3(); // Must use instance of outer class // to create an instance of the inner class: Parcel3.Contents c = p.new Contents(); Parcel3.Destination d = p.new Destination("Tasmania"); } } ///:~ Exercise 4: (2) Add a method to the class Sequence.SequenceSelector that produces the reference to the outer class Sequence. Exercise 5: (1) Create a class with an inner class. In a separate class, make an instance of the inner class. Inner classes and upcasting Inner classes really come into their own when you start upcasting to a base class, and in particular to an interface. (The effect of producing an interface reference from an object that implements it is essentially the same as upcasting to a base class.) That’s because the inner class—the implementation of the interface—can then be unseen and unavailable, which is Inner Classes 247 convenient for hiding the implementation. All you get back is a reference to the base class or the interface. We can create interfaces for the previous examples: //: innerclasses/Destination.java public interface Destination { String readLabel(); } ///:~ Now Contents and Destination represent interfaces available to the client programmer. Remember that an interface automatically makes all of its members public. When you get a reference to the base class or the interface, it’s possible that you can’t even find out the exact type, as shown here: //: innerclasses/TestParcel.java class Parcel4 { private class PContents implements Contents { private int i = 11; public int value() { return i; } } protected class PDestination implements Destination { private String label; private PDestination(String whereTo) { label = whereTo; } public String readLabel() { return label; } } public Destination destination(String s) { return new PDestination(s); } public Contents contents() { return new PContents(); } } public class TestParcel { public static void main(String[] args) { Parcel4 p = new Parcel4(); Contents c = p.contents(); Destination d = p.destination("Tasmania"); // Illegal -- can’t access private class: //! Parcel4.PContents pc = p.new PContents(); } } ///:~ In Parcel4, something new has been added: The inner class PContents is private, so nothing but Parcel4 can access it. Normal (non-inner) classes cannot be made private or protected; they may only be given public or package access. PDestination is protected, so nothing but Parcel4, classes in the same package (since protected also gives package access), and the inheritors of Parcel4 can access PDestination. This means that the client programmer has restricted knowledge and access to these members. In fact, you can’t even downcast to a private inner class (or a protected inner class unless you’re an inheritor), because you can’t access the name, as you can see in class TestParcel. Thus, the private inner class provides a way for the class designer to completely prevent any type-coding dependencies and to completely hide details about implementation. In addition, extension of an interface is useless from the client programmer’s perspective since the client programmer 248 Thinking in Java Bruce Eckel cannot access any additional methods that aren’t part of the public interface. This also provides an opportunity for the Java compiler to generate more efficient code. Exercise 6: (2) Create an interface with at least one method, in its own package. Create a class in a separate package. Add a protected inner class that implements the interface. In a third package, inherit from your class and, inside a method, return an object of the protected inner class, upcasting to the interface during the return. Exercise 7: (2) Create a class with a private field and a private method. Create an inner class with a method that modifies the outer-class field and calls the outer-class method. In a second outer-class method, create an object of the inner class and call its method, then show the effect on the outer-class object. Exercise 8: (2) Determine whether an outer class has access to the private elements of its inner class. Inner classes in methods and scopes What you’ve seen so far encompasses the typical use for inner classes. In general, the code that you’ll write and read involving inner classes will be "plain" inner classes that are simple and easy to understand. However, the syntax for inner classes covers a number of other, more obscure techniques. Inner classes can be created within a method or even an arbitrary scope. There are two reasons for doing this: 1. As shown previously, you’re implementing an interface of some kind so that you can create and return a reference. 2. You’re solving a complicated problem and you want to create a class to aid in your solution, but you don’t want it publicly available. In the following examples, the previous code will be modified to use: 1. A class defined within a method 2. A class defined within a scope inside a method 3. An anonymous class implementing an interface 4. An anonymous class extending a class that has a non-default constructor 5. An anonymous class that performs field initialization 6. An anonymous class that performs construction using instance initialization (anonymous inner classes cannot have constructors) The first example shows the creation of an entire class within the scope of a method (instead of the scope of another class). This is called a local inner class: //: innerclasses/Parcel5.java // Nesting a class within a method. public class Parcel5 { public Destination destination(String s) { class PDestination implements Destination { Inner Classes 249 private String label; private PDestination(String whereTo) { label = whereTo; } public String readLabel() { return label; } } return new PDestination(s); } public static void main(String[] args) { Parcel5 p = new Parcel5(); Destination d = p.destination("Tasmania"); } } ///:~ The class PDestination is part of destination( ) rather than being part of Parcels. Therefore, PDestination cannot be accessed outside of destination( ). Notice the upcasting that occurs in the return statementnothing comes out of destination( ) except a reference to Destination, the base class. Of course, the fact that the name of the class PDestination is placed inside destination( ) doesn’t mean that PDestination is not a valid object once destination( ) returns. You could use the class identifier PDestination for an inner class inside each class in the same subdirectory without a name clash. The next example shows how you can nest an inner class within any arbitrary scope: //: innerclasses/Parcel6.java // Nesting a class within a scope. public class Parcel6 { private void internalTracking(boolean b) { if(b) { class TrackingSlip { private String id; TrackingSlip(String s) { id = s; } String getSlip() { return id; } } TrackingSlip ts = new TrackingSlip("slip"); String s = ts.getSlip(); } // Can’t use it here! Out of scope: //! TrackingSlip ts = new TrackingSlip("x"); } public void track() { internalTracking(true); } public static void main(String[] args) { Parcel6 p = new Parcel6(); p.track(); } } ///:~ The class TrackingSlip is nested inside the scope of an if statement. This does not mean that the class is conditionally created—it gets compiled along with everything else. However, it’s not available outside the scope in which it is defined. Other than that, it looks just like an ordinary class. Exercise 9: (1) Create an interface with at least one method, and implement that interface by defining an inner class within a method, which returns a reference to your interface. 250 Thinking in Java Bruce Eckel Exercise 10: (1) Repeat the previous exercise but define the inner class within a scope within a method. Exercise 11: (2) Create a private inner class that implements a public interface. Write a method that returns a reference to an instance of the private inner class, upcast to the interface. Show that the inner class is completely hidden by trying to downcast to it. Anonymous inner classes The next example looks a little odd: //: innerclasses/Parcel7.java // Returning an instance of an anonymous inner class. public class Parcel7 { public Contents contents() { return new Contents() { // Insert a class definition private int i = 11; public int value() { return i; } }; // Semicolon required in this case } public static void main(String[] args) { Parcel7 p = new Parcel7(); Contents c = p.contents(); } } ///:~ The contents( ) method combines the creation of the return value with the definition of the class that represents that return value! In addition, the class is anonymous; it has no name. To make matters a bit worse, it looks like you’re starting out to create a Contents object, But then, before you get to the semicolon, you say, "But wait, I think I’ll slip in a class definition." What this strange syntax means is "Create an object of an anonymous class that’s inherited from Contents." The reference returned by the new expression is automatically upcast to a Contents reference. The anonymous inner-class syntax is a shorthand for: //: innerclasses/Parcel7b.java // Expanded version of Parcel7.java public class Parcel7b { class MyContents implements Contents { private int i = 11; public int value() { return i; } } public Contents contents() { return new MyContents(); } public static void main(String[] args) { Parcel7b p = new Parcel7b(); Contents c = p.contents(); } } ///:~ In the anonymous inner class, Contents is created by using a default constructor. The following code shows what to do if your base class needs a constructor with an argument: //: innerclasses/Parcel8.java // Calling the base-class constructor. Inner Classes 251 public class Parcel8 { public Wrapping wrapping(int x) { // Base constructor call: return new Wrapping(x) { // Pass constructor argument. public int value() { return super.value() * 47; } }; // Semicolon required } public static void main(String[] args) { Parcel8 p = new Parcel8(); Wrapping w = p.wrapping(10); } } ///:~ That is, you simply pass the appropriate argument to the base-class constructor, seen here as the x passed in new Wrapping(x). Although it’s an ordinary class with an implementation, Wrapping is also being used as a common "interface" to its derived classes: //: innerclasses/Wrapping.java public class Wrapping { private int i; public Wrapping(int x) { i = x; } public int value() { return i; } } ///:~ You’ll notice that Wrapping has a constructor that requires an argument, to make things a bit more interesting. The semicolon at the end of the anonymous inner class doesn’t mark the end of the class body. Instead, it marks the end of the expression that happens to contain the anonymous class. Thus, it’s identical to the use of the semicolon everywhere else. You can also perform initialization when you define fields in an anonymous class: //: innerclasses/Parcel9.java // An anonymous inner class that performs // initialization. A briefer version of Parcel5.java. public class Parcel9 { // Argument must be final to use inside // anonymous inner class: public Destination destination(final String dest) { return new Destination() { private String label = dest; public String readLabel() { return label; } }; } public static void main(String[] args) { Parcel9 p = new Parcel9(); Destination d = p.destination("Tasmania"); } } ///:~ If you’re defining an anonymous inner class and want to use an object that’s defined outside the anonymous inner class, the compiler requires that the argument reference be final, as you see in the argument to destination( ). If you forget, you’ll get a compile-time error message. 252 Thinking in Java Bruce Eckel As long as you’re simply assigning a field, the approach in this example is fine. But what if you need to perform some constructor-like activity? You can’t have a named constructor in an anonymous class (since there’s no name!), but with instance initialization, you can, in effect, create a constructor for an anonymous inner class, like this: //: innerclasses/AnonymousConstructor.java // Creating a constructor for an anonymous inner class. import static net.mindview.util.Print.*; abstract class Base { public Base(int i) { print("Base constructor, i = " + i); } public abstract void f(); } public class AnonymousConstructor { public static Base getBase(int i) { return new Base(i) { { print("Inside instance initializer"); } public void f() { print("In anonymous f()"); } }; } public static void main(String[] args) { Base base = getBase(47); base.f(); } } /* Output: Base constructor, i = 47 Inside instance initializer In anonymous f() *///:~ In this case, the variable i did nor have to be final. While i is passed to the base constructor of the anonymous class, it is never directly used inside the anonymous class. Here’s the "parcel" theme with instance initialization. Note that the arguments to destination( ) must be final since they are used within the anonymous class: //: innerclasses/Parcel10.java // Using "instance initialization" to perform // construction on an anonymous inner class. public class Parcel10 { public Destination destination(final String dest, final float price) { return new Destination() { private int cost; // Instance initialization for each object: { cost = Math.round(price); if(cost > 100) System.out.println("Over budget!"); } private String label = dest; public String readLabel() { return label; } }; } public static void main(String[] args) { Parcel10 p = new Parcel10(); Inner Classes 253 Destination d = p.destination("Tasmania", 101.395F); } } /* Output: Over budget! *///:~ Inside the instance initializer you can see code that couldn’t be executed as part of a field initializer (that is, the if statement). So in effect, an instance initializer is the constructor for an anonymous inner class. Of course, it’s limited; you can’t overload instance initializers, so you can have only one of these constructors. Anonymous inner classes are somewhat limited compared to regular inheritance, because they can either extend a class or implement an interface, but not both. And if you do implement an interface, you can only implement one. Exercise 12: (1) Repeat Exercise 7 using an anonymous inner class. Exercise 13: (1) Repeat Exercise 9 using an anonymous inner class. Exercise 14: (1) Modify interfaces/HorrorShow.java to implement DangerousMonster and Vampire using anonymous classes. Exercise 15: (2) Create a class with a non-default constructor (one with arguments) and no default constructor (no "no-arg" constructor). Create a second class that has a method that returns a reference to an object of the first class. Create the object that you return by making an anonymous inner class that inherits from the first class. Factory Method revisited Look at how much nicer the interfaces/Factories.java example comes out when you use anonymous inner classes: //: innerclasses/Factories.java import static net.mindview.util.Print.*; interface Service { void method1(); void method2(); } interface ServiceFactory { Service getService(); } class Implementation1 implements Service { private Implementation1() {} public void method1() {print("Implementation1 method1");} public void method2() {print("Implementation1 method2");} public static ServiceFactory factory = new ServiceFactory() { public Service getService() { return new Implementation1(); } }; } class Implementation2 implements Service { private Implementation2() {} 254 Thinking in Java Bruce Eckel public void method1() {print("Implementation2 method1");} public void method2() {print("Implementation2 method2");} public static ServiceFactory factory = new ServiceFactory() { public Service getService() { return new Implementation2(); } }; } public class Factories { public static void serviceConsumer(ServiceFactory fact) { Service s = fact.getService(); s.method1(); s.method2(); } public static void main(String[] args) { serviceConsumer(Implementation1.factory); // Implementations are completely interchangeable: serviceConsumer(Implementation2.factory); } } /* Output: Implementation1 method1 Implementation1 method2 Implementation2 method1 Implementation2 method2 *///:~ Now the constructors for Implementation1 and Implementation2 can be private, and there’s no need to create a named class as the factory. In addition, you often only need a single factory object, and so here it has been created as a static field in the Service implementation. The resulting syntax is more meaningful, as well. The interfaces/Games.java example can also be improved with anonymous inner classes: //: innerclasses/Games.java // Using anonymous inner classes with the Game framework. import static net.mindview.util.Print.*; interface Game { boolean move(); } interface GameFactory { Game getGame(); } class Checkers implements Game { private Checkers() {} private int moves = 0; private static final int MOVES = 3; public boolean move() { print("Checkers move " + moves); return ++moves != MOVES; } public static GameFactory factory = new GameFactory() { public Game getGame() { return new Checkers(); } }; } class Chess implements Game { private Chess() {} private int moves = 0; private static final int MOVES = 4; public boolean move() { print("Chess move " + moves); return ++moves != MOVES; Inner Classes 255 } public static GameFactory factory = new GameFactory() { public Game getGame() { return new Chess(); } }; } public class Games { public static void playGame(GameFactory factory) { Game s = factory.getGame(); while(s.move()) ; } public static void main(String[] args) { playGame(Checkers.factory); playGame(Chess.factory); } } /* Output: Checkers move 0 Checkers move 1 Checkers move 2 Chess move 0 Chess move 1 Chess move 2 Chess move 3 *///:~ Remember the advice given at the end of the last chapter: Prefer classes to interfaces. If your design demands an interface, you’ll know it. Otherwise, don’t put it in until you are forced to. Exercise 16: (1) Modify the solution to Exercise 18 from the Interfaces chapter to use anonymous inner classes. Exercise 17: (1) Modify the solution to Exercise 19 from the Interfaces chapter to use anonymous inner classes. Nested classes If you don’t need a connection between the inner-class object and the outerclass object, then you can make the inner class static. This is commonly called a nested class. 2 To understand the meaning of static when applied to inner classes, you must remember that the object of an ordinary inner class implicitly keeps a reference to the object of the enclosing class that created it. This is not true, however, when you say an inner class is static. A nested class means: 1. You don’t need an outer-class object in order to create an object of a nested class. 2. You can’t access a non-static outer-class object from an object of a nested class. Nested classes are different from ordinary inner classes in another way, as well. Fields and methods in ordinary inner classes can only be at the outer level of a class, so ordinary inner classes cannot have static data, static fields, or nested classes. However, nested classes can have all of these: //: innerclasses/Parcel11.java // Nested classes (static inner classes). 2 Roughly similar to nested classes in C++, except that those classes cannot access private members as they can in Java. 256 Thinking in Java Bruce Eckel public class Parcel11 { private static class ParcelContents implements Contents { private int i = 11; public int value() { return i; } } protected static class ParcelDestination implements Destination { private String label; private ParcelDestination(String whereTo) { label = whereTo; } public String readLabel() { return label; } // Nested classes can contain other static elements: public static void f() {} static int x = 10; static class AnotherLevel { public static void f() {} static int x = 10; } } public static Destination destination(String s) { return new ParcelDestination(s); } public static Contents contents() { return new ParcelContents(); } public static void main(String[] args) { Contents c = contents(); Destination d = destination("Tasmania"); } } ///:~ In main( ), no object of Parcel11 is necessary; instead, you use the normal syntax for selecting a static member to call the methods that return references to Contents and Destination. As you’ve seen earlier in this chapter, in an ordinary (non-static) inner class, the link to the outer-class object is achieved with a special this reference. A nested class does not have a special this reference, which makes it analogous to a static method. Exercise 18: (1) Create a class containing a nested class. In main( ), create an instance of the nested class. Exercise 19: (2) Create a class containing an inner class that itself contains an inner class. Repeat this using nested classes. Note the names of the .class files produced by the compiler. Classes inside interfaces Normally, you can’t put any code inside an interface, but a nested class can be part of an interface. Any class you put inside an interface is automatically public and static. Since the class is static, it doesn’t violate the rules for interfaces—the nested class is only placed inside the namespace of the interface. You can even implement the surrounding interface in the inner class, like this: //: innerclasses/ClassInInterface.java // {main: ClassInInterface$Test} Inner Classes 257 public interface ClassInInterface { void howdy(); class Test implements ClassInInterface { public void howdy() { System.out.println("Howdy!"); } public static void main(String[] args) { new Test().howdy(); } } } /* Output: Howdy! *///:~ It’s convenient to nest a class inside an interface when you want to create some common code to be used with all different implementations of that interface. Earlier in this book I suggested putting a main( ) in every class to act as a test bed for that class. One drawback to this is the amount of extra compiled code you must carry around. If this is a problem, you can use a nested class to hold your test code: //: innerclasses/TestBed.java // Putting test code in a nested class. // {main: TestBed$Tester} public class TestBed { public void f() { System.out.println("f()"); } public static class Tester { public static void main(String[] args) { TestBed t = new TestBed(); t.f(); } } } /* Output: f() *///:~ This generates a separate class called TestBed$Tester (to run the program, you say Java TestBed$Tester, but you must escape the ‘$’ under Unix/Linux systems). You can use this class for testing, but you don’t need to include it in your shipping product; you can simply delete TestBed$Tester.class before packaging things up. Exercise 20: (1) Create an interface containing a nested class. Implement this interface and create an instance of the nested class. Exercise 21: (2) Create an interface that contains a nested class that has a static method that calls the methods of your interface and displays the results. Implement your interface and pass an instance of your implementation to the method. 258 Thinking in Java Bruce Eckel Reaching outward from a multiply nested class It doesn’t matter how deeply an inner class may be nested—it can transparently access all of the members of all the classes it is nested within, as seen here: 3 //: innerclasses/MultiNestingAccess.java // Nested classes can access all members of all // levels of the classes they are nested within. class MNA { private void f() {} class A { private void g() {} public class B { void h() { g(); f(); } } } } public class MultiNestingAccess { public static void main(String[] args) { MNA mna = new MNA(); MNA.A mnaa = mna.new A(); MNA.A.B mnaab = mnaa.new B(); mnaab.h(); } } ///:~ You can see that in MNAAB, the methods g( ) and f( ) are callable without any qualification (despite the fact that they are private). This example also demonstrates the syntax necessary to create objects of multiply nested inner classes when you create the objects in a different class. The ".new" syntax produces the correct scope, so you do not have to qualify the class name in the constructor call. Why inner classes? At this point you’ve seen a lot of syntax and semantics describing the way inner classes work, but this doesn’t answer the question of why they exist. Why did the Java designers go to so much trouble to add this fundamental language feature? Typically, the inner class inherits from a class or implements an interface, and the code in the inner class manipulates the outer-class object that it was created within. So you could say that an inner class provides a kind of window into the outer class. A question that cuts to the heart of inner classes is this: If I just need a reference to an interface, why don’t I just make the outer class implement that interface? The answer is "If that’s all you need, then that’s how you should do it." So what is it that distinguishes an inner class implementing an interface from an outer class implementing the same interface? The answer is that you can’t always have the convenience of interfaces—sometimes you’re working with implementations. So the most compelling reason for inner classes is: 3 Thanks again to Martin Danner. Inner Classes 259 Each inner class can independently inherit from an implementation. Thus, the inner class is not limited by whether the outer class is already inheriting from an implementation. Without the ability that inner classes provide to inherit—in effect—from more than one concrete or abstract class, some design and programming problems would be intractable. So one way to look at the inner class is as the rest of the solution of the multiple-inheritance problem. Interfaces solve part of the problem, but inner classes effectively allow "multiple implementation inheritance." That is, inner classes effectively allow you to inherit from more than one non-interface. To see this in more detail, consider a situation in which you have two interfaces that must somehow be implemented within a class. Because of the flexibility of interfaces, you have two choices: a single class or an inner class. //: innerclasses/MultiInterfaces.java // Two ways that a class can implement multiple interfaces. package innerclasses; interface A {} interface B {} class X implements A, B {} class Y implements A { B makeB() { // Anonymous inner class: return new B() {}; } } public class MultiInterfaces { static void takesA(A a) {} static void takesB(B b) {} public static void main(String[] args) { X x = new X(); Y y = new Y(); takesA(x); takesA(y); takesB(x); takesB(y.makeB()); } } ///:~ Of course, this assumes that the structure of your code makes logical sense either way. However, you’ll ordinarily have some kind of guidance from the nature of the problem about whether to use a single class or an inner class. But without any other constraints, the approach in the preceding example doesn’t really make much difference from an implementation standpoint. Both of them work. However, if you have abstract or concrete classes instead of interfaces, you are suddenly limited to using inner classes if your class must somehow implement both of the others: //: innerclasses/MultiImplementation.java // With concrete or abstract classes, inner // classes are the only way to produce the effect // of "multiple implementation inheritance." package innerclasses; class D {} 260 Thinking in Java Bruce Eckel abstract class E {} class Z extends D { E makeE() { return new E() {}; } } public class MultiImplementation { static void takesD(D d) {} static void takesE(E e) {} public static void main(String[] args) { Z z = new Z(); takesD(z); takesE(z.makeE()); } } ///:~ If you didn’t need to solve the "multiple implementation inheritance" problem, you could conceivably code around everything else without the need for inner classes. But with inner classes you have these additional features: 1. The inner class can have multiple instances, each with its own state information that is independent of the information in the outer-class object. 2. In a single outer class you can have several inner classes, each of which implements the same interface or inherits from the same class in a different way. An example of this will be shown shortly. 3. The point of creation of the inner-class object is not tied to the creation of the outerclass object. 4. There is no potentially confusing "is-a" relationship with the inner class; it’s a separate entity. As an example, if Sequence.java did not use inner classes, you’d have to say, "A Sequence is a Selector," and you’d only be able to have one Selector in existence for a particular Sequence. You can easily have a second method, reverseSelector( ), that produces a Selector that moves backward through the sequence. This kind of flexibility is only available with inner classes. Exercise 22: (2) Implement reverseSelector( ) in Sequence.java. Exercise 23: (4) Create an interface U with three methods. Create a class A with a method that produces a reference to a U by building an anonymous inner class. Create a second class B that contains an array of U. B should have one method that accepts and stores a reference to a U in the array, a second method that sets a reference in the array (specified by the method argument) to null, and a third method that moves through the array and calls the methods in U. In main( ), create a group of A objects and a single B. Fill the B with U references produced by the A objects. Use the B to call back into all the A objects. Remove some of the U references from the B. Closures & callbacks A closure is a callable object that retains information from the scope in which it was created. From this definition, you can see that an inner class is an object-oriented closure, because it doesn’t just contain each piece of information from the outer-class object ("the scope in which it was created"), but it automatically holds a reference back to the whole outer-class object, where it has permission to manipulate all the members, even private ones. Inner Classes 261 One of the most compelling arguments made to include some kind of pointer mechanism in Java was to allow callbacks. With a callback, some other object is given a piece of information that allows it to call back into the originating object at some later point. This is a very powerful concept, as you will see later in the book. If a callback is implemented using a pointer, however, you must rely on the programmer to behave properly and not misuse the pointer. As you’ve seen by now, Java tends to be more careful than that, so pointers were not included in the language. The closure provided by the inner class is a good solution—more flexible and far safer than a pointer. Here’s an example: //: innerclasses/Callbacks.java // Using inner classes for callbacks package innerclasses; import static net.mindview.util.Print.*; interface Incrementable { void increment(); } // Very simple to just implement the interface: class Callee1 implements Incrementable { private int i = 0; public void increment() { i++; print(i); } } class MyIncrement { public void increment() { print("Other operation"); } static void f(MyIncrement mi) { mi.increment(); } } // If your class must implement increment() in // some other way, you must use an inner class: class Callee2 extends MyIncrement { private int i = 0; public void increment() { super.increment(); i++; print(i); } private class Closure implements Incrementable { public void increment() { // Specify outer-class method, otherwise // you’d get an infinite recursion: Callee2.this.increment(); } } Incrementable getCallbackReference() { return new Closure(); } } class Caller { private Incrementable callbackReference; Caller(Incrementable cbh) { callbackReference = cbh; } void go() { callbackReference.increment(); } } public class Callbacks { 262 Thinking in Java Bruce Eckel public static void main(String[] args) { Callee1 c1 = new Callee1(); Callee2 c2 = new Callee2(); MyIncrement.f(c2); Caller caller1 = new Caller(c1); Caller caller2 = new Caller(c2.getCallbackReference()); caller1.go(); caller1.go(); caller2.go(); caller2.go(); } } /* Output: Other operation 1 1 2 Other operation 2 Other operation 3 *///:~ This also shows a further distinction between implementing an interface in an outer class versus doing so in an inner class. Callee1 is clearly the simpler solution in terms of the code. Callee2 inherits from Mylncrement, which already has a different increment( ) method that does something unrelated to the one expected by the Incrementable interface. When Mylncrement is inherited into Callee2, increment( ) can’t be overridden for use by Incrementable, so you’re forced to provide a separate implementation using an inner class. Also note that when you create an inner class, you do not add to or modify the interface of the outer class. Everything except getCallbackReference( ) in Callee2 is private. To allow any connection to the outside world, the interface Incrementable is essential. Here you can see how interfaces allow for a complete separation of interface from implementation. The inner class Closure implements Incrementable to provide a hook back into Callee2— but a safe hook. Whoever gets the Incrementable reference can, of course, only call increment( ) and has no other abilities (unlike a pointer, which would allow you to run wild). Caller takes an Incrementable reference in its constructor (although the capturing of the callback reference could happen at any time) and then, sometime later, uses the reference to "call back" into the Callee class. The value of the callback is in its flexibility; you can dynamically decide what methods will be called at run time. The benefit of this will become more evident in the Graphical User Interfaces chapter, where callbacks are used everywhere to implement GUI functionality. Inner classes & control frameworks A more concrete example of the use of inner classes can be found in something that I will refer to here as a control framework. An application framework is a class or a set of classes that’s designed to solve a particular type of problem. To apply an application framework, you typically inherit from one or more classes and override some of the methods. The code that you write in the overridden methods customizes the general solution provided by that application framework in order to solve your specific problem. This is an example of the Template Method design pattern (see Inner Classes 263 Thinking in Patterns (with Java) at www.MindView.net). The Template Method contains the basic structure of the algorithm, and it calls one or more overrideable methods to complete the action of that algorithm. A design pattern separates things that change from things that stay the same, and in this case the Template Method is the part that stays the same, and the overrideable methods are the things that change. A control framework is a particular type of application framework dominated by the need to respond to events. A system that primarily responds to events is called an event-driven system. A common problem in application programming is the graphical user interface (GUI), which is almost entirely event-driven. As you will see in the Graphical User Interfaces chapter, the Java Swing library is a control framework that elegantly solves the GUI problem and that heavily uses inner classes. To see how inner classes allow the simple creation and use of control frameworks, consider a control framework whose job is to execute events whenever those events are "ready." Although "ready" could mean anything, in this case it will be based on clock time. What follows is a control framework that contains no specific information about what it’s controlling. That information is supplied during inheritance, when the action( ) portion of the algorithm is implemented. First, here is the interface that describes any control event. It’s an abstract class instead of an actual interface because the default behavior is to perform the control based on time. Thus, some of the implementation is included here: //: innerclasses/controller/Event.java // The common methods for any control event. package innerclasses.controller; public abstract class Event { private long eventTime; protected final long delayTime; public Event(long delayTime) { this.delayTime = delayTime; start(); } public void start() { // Allows restarting eventTime = System.nanoTime() + delayTime; } public boolean ready() { return System.nanoTime() >= eventTime; } public abstract void action(); } ///:~ The constructor captures the time (measured from the time of creation of the object) when you want the Event to run, and then calls start( ), which takes the current time and adds the delay time to produce the time when the event will occur. Rather than being included in the constructor, start( ) is a separate method. This way, you can restart the timer after the event has run out, so the Event object can be reused. For example, if you want a repeating event, you can simply call start( ) inside your action( ) method. ready( ) tells you when it’s time to run the action( ) method. Of course, ready( ) can be overridden in a derived class to base the Event on something other than time. The following file contains the actual control framework that manages and fires events. The Event objects are held inside a container object of type List (pronounced "List of Event"), which you’ll learn more about in the Holding Your Objects chapter. For now, all you need to know is that add( ) will append an Event to the end of the List, size( ) produces 264 Thinking in Java Bruce Eckel the number of entries in the List, the foreach syntax fetches successive Events from the List, and remove( ) removes the specified Event from the List. //: innerclasses/controller/Controller.java // The reusable framework for control systems. package innerclasses.controller; import java.util.*; public class Controller { // A class from java.util to hold Event objects: private List eventList = new ArrayList(); public void addEvent(Event c) { eventList.add(c); } public void run() { while(eventList.size() > 0) // Make a copy so you’re not modifying the list // while you’re selecting the elements in it: for(Event e : new ArrayList(eventList)) if(e.ready()) { System.out.println(e); e.action(); eventList.remove(e); } } } ///:~ The run( ) method loops through a copy of the eventList, hunting for an Event object that’s ready( ) to run. For each one it finds ready( ), it prints information using the object’s toString( ) method, calls the action( ) method, and then removes the Event from the list. Note that so far in this design you know nothing about exactly what an Event does. And this is the crux of the design—how it "separates the things that change from the things that stay the same." Or, to use my term, the "vector of change" is the different actions of the various kinds of Event objects, and you express different actions by creating different Event subclasses. This is where inner classes come into play. They allow two things: 1. The entire implementation of a control framework is created in a single class, thereby encapsulating everything that’s unique about that implementation. Inner classes are used to express the many different kinds of action( ) necessary to solve the problem. 2. Inner classes keep this implementation from becoming awkward, since you’re able to easily access any of the members in the outer class. Without this ability the code might become unpleasant enough that you’d end up seeking an alternative. Consider a particular implementation of the control framework designed to control greenhouse functions. 4 Each action is entirely different: turning lights, water, and thermostats on and off, ringing bells, and restarting the system. But the control framework is designed to easily isolate this different code. Inner classes allow you to have multiple derived versions of the same base class, Event, within a single class. For each type of action, you inherit a new Event inner class, and write the control code in the action( ) implementation. As is typical with an application framework, the class GreenhouseControls is inherited from Controller: 4 For some reason this has always been a pleasing problem for me to solve; it came from my earlier book C++ Inside & Out, but Java allows a more elegant solution Inner Classes 265 //: innerclasses/GreenhouseControls.java // This produces a specific application of the // control system, all in a single class. Inner // classes allow you to encapsulate different // functionality for each type of event. import innerclasses.controller.*; public class GreenhouseControls extends Controller { private boolean light = false; public class LightOn extends Event { public LightOn(long delayTime) { super(delayTime); } public void action() { // Put hardware control code here to // physically turn on the light. light = true; } public String toString() { return "Light is on"; } } public class LightOff extends Event { public LightOff(long delayTime) { super(delayTime); } public void action() { // Put hardware control code here to // physically turn off the light. light = false; } public String toString() { return "Light is off"; } } private boolean water = false; public class WaterOn extends Event { public WaterOn(long delayTime) { super(delayTime); } public void action() { // Put hardware control code here. water = true; } public String toString() { return "Greenhouse water is on"; } } public class WaterOff extends Event { public WaterOff(long delayTime) { super(delayTime); } public void action() { // Put hardware control code here. water = false; } public String toString() { return "Greenhouse water is off"; } } private String thermostat = "Day"; public class ThermostatNight extends Event { public ThermostatNight(long delayTime) { super(delayTime); } public void action() { // Put hardware control code here. thermostat = "Night"; } public String toString() { return "Thermostat on night setting"; } } public class ThermostatDay extends Event { public ThermostatDay(long delayTime) { 266 Thinking in Java Bruce Eckel super(delayTime); } public void action() { // Put hardware control code here. thermostat = "Day"; } public String toString() { return "Thermostat on day setting"; } } // An example of an action() that inserts a // new one of itself into the event list: public class Bell extends Event { public Bell(long delayTime) { super(delayTime); } public void action() { addEvent(new Bell(delayTime)); } public String toString() { return "Bing!"; } } public class Restart extends Event { private Event[] eventList; public Restart(long delayTime, Event[] eventList) { super(delayTime); this.eventList = eventList; for(Event e : eventList) addEvent(e); } public void action() { for(Event e : eventList) { e.start(); // Rerun each event addEvent(e); } start(); // Rerun this Event addEvent(this); } public String toString() { return "Restarting system"; } } public static class Terminate extends Event { public Terminate(long delayTime) { super(delayTime); } public void action() { System.exit(0); } public String toString() { return "Terminating"; } } } ///:~ Note that light, water, and thermostat belong to the outer class GreenhouseControls, and yet the inner classes can access those fields without qualification or special permission. Also, the action( ) methods usually involve some sort of hardware control. Most of the Event classes look similar, but Bell and Restart are special. Bell rings and then adds a new Bell object to the event list, so it will ring again later. Notice how inner classes almost look like multiple inheritance: Bell and Restart have all the methods of Event and also appear to have all the methods of the outer class GreenhouseControls. Restart is given an array of Event objects that it adds to the controller. Since Restart( ) is just another Event object, you can also add a Restart object within Restart.action( ) so that the system regularly restarts itself. Inner Classes 267 The following class configures the system by creating a GreenhouseControls object and adding various kinds of Event objects. This is an example of the Command design pattern— each object in eventList is a request encapsulated as an object: //: innerclasses/GreenhouseController.java // Configure and execute the greenhouse system. // {Args: 5000} import innerclasses.controller.*; public class GreenhouseController { public static void main(String[] args) { GreenhouseControls gc = new GreenhouseControls(); // Instead of hard-wiring, you could parse // configuration information from a text file here: gc.addEvent(gc.new Bell(900)); Event[] eventList = { gc.new ThermostatNight(0), gc.new LightOn(200), gc.new LightOff(400), gc.new WaterOn(600), gc.new WaterOff(800), gc.new ThermostatDay(1400) }; gc.addEvent(gc.new Restart(2000, eventList)); if(args.length == 1) gc.addEvent( new GreenhouseControls.Terminate( new Integer(args[0]))); gc.run(); } } /* Output: Bing! Thermostat on night setting Light is on Light is off Greenhouse water is on Greenhouse water is off Thermostat on day setting Restarting system Terminating *///:~ This class initializes the system, so it adds all the appropriate events. The Restart event is repeatedly run, and it loads the eventList into the GreenhouseControls object each time. If you provide a command-line argument indicating milliseconds, it will terminate the program after that many milliseconds (this is used for testing). Of course, it’s more flexible to read the events from a file instead of hardcoding them. An exercise in the I/O chapter asks you to modify this example to do just that. This example should move you toward an appreciation of the value of inner classes, especially when used within a control framework. However, in the Graphical User Interfaces chapter you’ll see how elegantly inner classes are used to describe the actions of a graphical user interface. By the time you finish that chapter, you should be fully convinced. Exercise 24: (2) In GreenhouseControls.java, add Event inner classes that turn fans on and off. Configure GreenhouseController.java to use these new Event objects. 268 Thinking in Java Bruce Eckel Exercise 25: (3) Inherit from GreenhouseControls in GreenhouseControls.java to add Event inner classes that turn water mist generators on and off. Write a new version of GreenhouseController.java to use these new Event objects. Inheriting from inner classes Because the inner-class constructor must attach to a reference of the enclosing class object, things are slightly complicated when you inherit from an inner class. The problem is that the "secret" reference to the enclosing class object must be initialized, and yet in the derived class there’s no longer a default object to attach to. You must use a special syntax to make the association explicit: //: innerclasses/InheritInner.java // Inheriting an inner class. class WithInner { class Inner {} } public class InheritInner extends WithInner.Inner { //! InheritInner() {} // Won’t compile InheritInner(WithInner wi) { wi.super(); } public static void main(String[] args) { WithInner wi = new WithInner(); InheritInner ii = new InheritInner(wi); } } ///:~ You can see that InheritInner is extending only the inner class, not the outer one. But when it comes time to create a constructor, the default one is no good, and you can’t just pass a reference to an enclosing object. In addition, you must use the syntax enclosingClassReference.super(); inside the constructor. This provides the necessary reference, and the program will then compile. Exercise 26: (2) Create a class with an inner class that has a non-default constructor (one that takes arguments). Create a second class with an inner class that inherits from the first inner class. Can inner classes be overridden? What happens when you create an inner class, then inherit from the enclosing class and redefine the inner class? That is, is it possible to "override" the entire inner class? This seems like it would be a powerful concept, but "overriding" an inner class as if it were another method of the outer class doesn’t really do anything: //: innerclasses/BigEgg.java // An inner class cannot be overriden like a method. import static net.mindview.util.Print.*; class Egg { private Yolk y; Inner Classes 269 protected class Yolk { public Yolk() { print("Egg.Yolk()"); } } public Egg() { print("New Egg()"); y = new Yolk(); } } public class BigEgg extends Egg { public class Yolk { public Yolk() { print("BigEgg.Yolk()"); } } public static void main(String[] args) { new BigEgg(); } } /* Output: New Egg() Egg.Yolk() *///:~ The default constructor is synthesized automatically by the compiler, and this calls the baseclass default constructor. You might think that since a BigEgg is being created, the "overridden" version of Yolk would be used, but this is not the case, as you can see from the output. This example shows that there isn’t any extra inner-class magic going on when you inherit from the outer class. The two inner classes are completely separate entities, each in its own namespace. However, it’s still possible to explicitly inherit from the inner class: //: innerclasses/BigEgg2.java // Proper inheritance of an inner class. import static net.mindview.util.Print.*; class Egg2 { protected class Yolk { public Yolk() { print("Egg2.Yolk()"); } public void f() { print("Egg2.Yolk.f()");} } private Yolk y = new Yolk(); public Egg2() { print("New Egg2()"); } public void insertYolk(Yolk yy) { y = yy; } public void g() { y.f(); } } public class BigEgg2 extends Egg2 { public class Yolk extends Egg2.Yolk { public Yolk() { print("BigEgg2.Yolk()"); } public void f() { print("BigEgg2.Yolk.f()"); } } public BigEgg2() { insertYolk(new Yolk()); } public static void main(String[] args) { Egg2 e2 = new BigEgg2(); e2.g(); } } /* Output: Egg2.Yolk() New Egg2() Egg2.Yolk() BigEgg2.Yolk() BigEgg2.Yolk.f() *///:~ 270 Thinking in Java Bruce Eckel Now BigEgg2.Yolk explicitly extends Egg2.Yolk and overrides its methods. The method insertYolk( ) allows BigEgg2 to upcast one of its own Yolk objects into the y reference in Egg2, so when g( ) calls y.f( ), the overridden version of f( ) is used. The second call to Egg2.Yolk( ) is the base-class constructor call of the BigEgg2.Yolk constructor. You can see that the overridden version of f( ) is used when g( ) is called. Local inner classes As noted earlier, inner classes can also be created inside code blocks, typically inside the body of a method. A local inner class cannot have an access specifier because it isn’t part of the outer class, but it does have access to the final variables in the current code block and all the members of the enclosing class. Here’s an example comparing the creation of a local inner class with an anonymous inner class: //: innerclasses/LocalInnerClass.java // Holds a sequence of Objects. import static net.mindview.util.Print.*; interface Counter { int next(); } public class LocalInnerClass { private int count = 0; Counter getCounter(final String name) { // A local inner class: class LocalCounter implements Counter { public LocalCounter() { // Local inner class can have a constructor print("LocalCounter()"); } public int next() { printnb(name); // Access local final return count++; } } return new LocalCounter(); } // The same thing with an anonymous inner class: Counter getCounter2(final String name) { return new Counter() { // Anonymous inner class cannot have a named // constructor, only an instance initializer: { print("Counter()"); } public int next() { printnb(name); // Access local final return count++; } }; } public static void main(String[] args) { LocalInnerClass lic = new LocalInnerClass(); Counter c1 = lic.getCounter("Local inner "), c2 = lic.getCounter2("Anonymous inner "); for(int i = 0; i < 5; i++) print(c1.next()); for(int i = 0; i < 5; i++) Inner Classes 271 print(c2.next()); } } /* Output: LocalCounter() Counter() Local inner 0 Local inner 1 Local inner 2 Local inner 3 Local inner 4 Anonymous inner 5 Anonymous inner 6 Anonymous inner 7 Anonymous inner 8 Anonymous inner 9 *///:~ Counter returns the next value in a sequence. It is implemented as both a local class and an anonymous inner class, both of which have the same behaviors and capabilities. Since the name of the local inner class is not accessible outside the method, the only justification for using a local inner class instead of an anonymous inner class is if you need a named constructor and/or an overloaded constructor, since an anonymous inner class can only use instance initialization. Another reason to make a local inner class rather than an anonymous inner class is if you need to make more than one object of that class. Inner-class identifiers Since every class produces a .class file that holds all the information about how to create objects of this type (this information produces a "meta-class" called the Class object), you might guess that inner classes must also produce .class files to contain the information for their Class objects. The names of these files/classes have a strict formula: the name of the enclosing class, followed by a ‘$’, followed by the name of the inner class. For example, the .class files created by LocalInnerClass.java include: Counter.class LocalInnerClass$l.class LocallnnerClassSlLocalCounter.class LocallnnerClass.class If inner classes are anonymous, the compiler simply starts generating numbers as inner-class identifiers. If inner classes are nested within inner classes, their names are simply appended after a ‘$’ and the outer-class identifier (s). Although this scheme of generating internal names is simple and straightforward, it’s also robust and handles most situations. 5 Since it is the standard naming scheme for Java, the generated files are automatically platform-independent. (Note that the Java compiler is changing your inner classes in all sorts of other ways in order to make them work.) 5 On the other hand, ‘$’ is a meta-character to the Unix shell and so you’ll sometimes have trouble when listing the .class files. This is a bit strange coming from Sun, a Unix-based company. My guess is that they weren’t considering this issue, but instead thought you’d naturally focus on the source-code files. 272 Thinking in Java Bruce Eckel Summary Interfaces and inner classes are more sophisticated concepts than what you’ll find in many OOP languages; for example, there’s nothing like them in C++. Together, they solve the same problem that C++ attempts to solve with its multiple inheritance (MI) feature. However, MI in C++ turns out to be rather difficult to use, whereas Java interfaces and inner classes are, by comparison, much more accessible. Although the features themselves are reasonably straightforward, the use of these features is a design issue, much the same as polymorphism. Over time, you’ll become better at recognizing situations where you should use an interface, or an inner class, or both. But at this point in this book, you should at least be comfortable with the syntax and semantics. As you see these language features in use, you’ll eventually internalize them. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Inner Classes 273 Holding Your Objects It’s a fairly simple program that only has a fixed quantity of objects with known lifetimes. In general, your programs will always be creating new objects based on some criteria that will be known only at run time. Before then, you won’t know the quantity or even the exact type of the objects you need. To solve the general programming problem, you need to create any number of objects, anytime, anywhere. So you can’t rely on creating a named reference to hold each one of your objects: MyType aReference; since you’ll never know how many of these you’ll actually need. Most languages provide some way to solve this essential problem. Java has several ways to hold objects (or rather, references to objects). The compiler-supported type is the array, which has been discussed before. An array is the most efficient way to hold a group of objects, and you’re pointed towards this choice if you want to hold a group of primitives. But an array has a fixed size, and in the more general case, you won’t know at the time you’re writing the program how many objects you’re going to need, or whether you need a more sophisticated way to store your objects—so the fixed-sized constraint of an array is too limiting. The java.util library has a reasonably complete set of container classes to solve this problem, the basic types of which are List, Set, Queue, and Map. These types of objects are also known as collection classes, but because the Java library uses the name Collection to refer to a particular subset of the library, I shall use the more inclusive term "container." Containers provide sophisticated ways to hold your objects, and you can solve a surprising number of problems by using these tools. Among their other characteristics—Set, for example, holds only one object of each value, and Map is an associative array that lets you associate objects with other objects—the Java container classes will automatically resize themselves. So, unlike with arrays, you can put in any number of objects and you don’t need to worry about how big to make the container while you’re writing the program. Even though they don’t have direct keyword support in Java,1 container classes are fundamental tools that significantly increase your programming muscle. In this chapter you’ll get a basic working knowledge of the Java container library, with an emphasis on typical usage. Here, we’ll focus on the containers that you’ll use in day-to-day programming. Later, in the Containers in Depth chapter, you’ll learn about the rest of the containers and more details about their functionality and how to use them. 1 A number of languages, such as Perl, Python, and Ruby, have native support for containers. Generics and type-safe containers One of the problems of using pre-Java SE5 containers was that the compiler allowed you to insert an incorrect type into a container. For example, consider a container of Apple objects, using the basic workhorse container, ArrayList. For now, you can think of ArrayList as "an array that automatically expands itself." Using an ArrayList is straightforward: Create one, insert objects using add( ), and access them with get( ), using an index—just as you do with an array, but without the square brackets.2 ArrayList also has a method size( ) to let you know how many elements have been added, so that you don’t inadvertently index off the end and cause an error (by throwing a runtime exception; exceptions will be introduced in the chapter Error Handling with Exceptions). In this example, Apples and Oranges are placed into the container, then pulled out. Normally, the Java compiler will give you a warning because the example does not use generics. Here, a special Java SE5 annotation is used to suppress the warning. Annotations start with an ‘@’ sign, and can take an argument; this one is @SuppressWarnings and the argument indicates that "unchecked" warnings only should be suppressed: //: holding/ApplesAndOrangesWithoutGenerics.java // Simple container example (produces compiler warnings). // {ThrowsException} import java.util.*; class Apple { private static long counter; private final long id = counter++; public long id() { return id; } } class Orange {} public class ApplesAndOrangesWithoutGenerics { @SuppressWarnings("unchecked") public static void main(String[] args) { ArrayList apples = new ArrayList(); for(int i = 0; i < 3; i++) apples.add(new Apple()); // Not prevented from adding an Orange to apples: apples.add(new Orange()); for(int i = 0; i < apples.size(); i++) ((Apple)apples.get(i)).id(); // Orange is detected only at run time } } /* (Execute to see output) *///:~ You’ll learn more about Java SE5 annotations in the Annotations chapter. The classes Apple and Orange are distinct; they have nothing in common except that they are both Objects. (Remember that if you don’t explicitly say what class you’re inheriting from, you automatically inherit from Object.) Since ArrayList holds Objects, you can not only add Apple objects into this container using the ArrayList method add( ), but you can also add Orange objects without complaint at either compile time or run time. When you go to fetch out what you think are Apple objects using the ArrayList method get( ), you get back a reference to an Object that you must cast to an Apple. Then you need to surround the entire expression with parentheses to force the evaluation of the cast before calling the 2 This is a place where operator overloading would have been nice. C++ and C# container classes produce a cleaner syntax using operator overloading. 276 Thinking in Java Bruce Eckel id( ) method for Apple; otherwise, you’ll get a syntax error. At run time, when you try to cast the Orange object to an Apple, you’ll get an error in the form of the aforementioned exception. In the Generics chapter, you’ll learn that creating classes using Java generics can be complex. However, applying predefined generic classes is usually straightforward. For example, to define an ArrayList intended to hold Apple objects, you say ArrayList instead of just ArrayList. The angle brackets surround the type parameters (there may be more than one), which specify the type(s) that can be held by that instance of the container. With generics, you’re prevented, at compile time, from putting the wrong type of object into a container.3 Here’s the example again, using generics: //: holding/ApplesAndOrangesWithGenerics.java import java.util.*; public class ApplesAndOrangesWithGenerics { public static void main(String[] args) { ArrayList apples = new ArrayList(); for(int i = 0; i < 3; i++) apples.add(new Apple()); // Compile-time error: // apples.add(new Orange()); for(int i = 0; i < apples.size(); i++) System.out.println(apples.get(i).id()); // Using foreach: for(Apple c : apples) System.out.println(c.id()); } } /* Output: 0 1 2 0 1 2 *///:~ Now the compiler will prevent you from putting an Orange into apples, so it becomes a compile-time error rather than a runtime error. Also notice that the cast is no longer necessary when fetching items back out from the List. Since the List knows what type it holds, it does the cast for you when you call get( ). Thus, with generics you not only know that the compiler will check the type of object that you put into a container, but you also get cleaner syntax when using the objects in the container. The example also shows that, if you do not need to use the index of each element, you can use the foreach syntax to select each element in the List. You are not limited to putting the exact type of object into a container when you specify that type as a generic parameter. Upcasting works the same with generics as it does with other types: //: holding/GenericsAndUpcasting.java import java.util.*; class GrannySmith extends Apple {} class Gala extends Apple {} class Fuji extends Apple {} class Braeburn extends Apple {} public class GenericsAndUpcasting { public static void main(String[] args) { ArrayList apples = new ArrayList(); apples.add(new GrannySmith()); 3 At the end of the Generics chapter, you’ll find a discussion about whether this is such a bad problem. However, the Generics chapter will also show you that Java generics are useful for more than just type-safe containers. Holding Your Objects 277 apples.add(new Gala()); apples.add(new Fuji()); apples.add(new Braeburn()); for(Apple c : apples) System.out.println(c); } } /* Output: (Sample) GrannySmith@7d772e Gala@11b86e7 Fuji@35ce36 Braeburn@757aef *///:~ Thus, you can add a subtype of Apple to a container that is specified to hold Apple objects. The output is produced from the default toString( ) method of Object, which prints the class name followed by the unsigned hexadecimal representation of the hash code of the object (generated by the hashCode( ) method). You’ll learn about hash codes in detail in Containers in Depth. Exercise 1: (2) Create a new class called Gerbil with an int gerbilNumber that’s initialized in the constructor. Give it a method called hop( ) that displays which gerbil number this is, and that it’s hopping. Create an ArrayList and add Gerbil objects to the List. Now use the get( ) method to move through the List and call hop( ) for each Gerbil. Basic concepts The Java container library takes the idea of "holding your objects" and divides it into two distinct concepts, expressed as the basic interfaces of the library: 1. Collection: a sequence of individual elements with one or more rules applied to them. A List must hold the elements in the way that they were inserted, a Set cannot have duplicate elements, and a Queue produces the elements in the order determined by a queuing discipline (usually the same order in which they are inserted). 2. Map: a group of key-value object pairs, allowing you to look up a value using a key. An ArrayList allows you to look up an object using a number, so in a sense it associates numbers to objects. A map allows you to look up an object using another object. It’s also called an associative array, because it associates objects with other objects, or a dictionary, because you look up a value object using a key object just like you look up a definition using a word. Maps are powerful programming tools. Although it’s not always possible, ideally you’ll write most of your code to talk to these interfaces, and the only place where you’ll specify the precise type you’re using is at the point of creation. So you can create a List like this: List apples = new ArrayList(); Notice that the ArrayList has been upcast to a List, in contrast to the way it was handled in the previous examples. The intent of using the interface is that if you decide you want to change your implementation, all you need to do is change it at the point of creation, like this: List apples = new LinkedList(); Thus, you’ll typically make an object of a concrete class, upcast it to the corresponding interface, and then use the interface throughout the rest of your code. 278 Thinking in Java Bruce Eckel This approach won’t always work, because some classes have additional functionality. For example, LinkedList has additional methods that are not in the List interface, and a TreeMap has methods that are not in the Map interface. If you need to use those methods, you won’t be able to upcast to the more general interface. The Collection interface generalizes the idea of a sequence—a way of holding a group of objects. Here’s a simple example that fills a Collection (represented here with an ArrayList) with Integer objects and then prints each element in the resulting container: //: holding/SimpleCollection.java import java.util.*; public class SimpleCollection { public static void main(String[] args) { Collection c = new ArrayList(); for(int i = 0; i < 10; i++) c.add(i); // Autoboxing for(Integer i : c) System.out.print(i + ", "); } } /* Output: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, *///:~ Since this example only uses Collection methods, any object of a class inherited from Collection would work, but ArrayList is the most basic type of sequence. The name of the add( ) method suggests that it puts a new element in the Collection. However, the documentation carefully states that add( ) "ensures that this Collection contains the specified element." This is to allow for the meaning of Set, which adds the element only if it isn’t already there. With an ArrayList, or any sort of List, add( ) always means "put it in," because Lists don’t care if there are duplicates. All Collections can be traversed using the foreach syntax, as shown here. Later in this chapter you’ll learn about a more flexible concept called an Iterator. Exercise 2: (1) Modify SimpleCollection.java to use a Set for c. Exercise 3: (2) Modify innerclasses/Sequence.java so that you can add any number of elements to it. Adding groups of elements There are utility methods in both the Arrays and Collections classes in java.util that add groups of elements to a Collection. Arrays.asList( ) takes either an array or a commaseparated list of elements (using varargs) and turns it into a List object. Collections.addAll( ) takes a Collection object and either an array or a comma-separated list and adds the elements to the Collection. Here’s an example that shows both methods, as well as the more conventional addAll( ) method that’s part of all Collection types: //: holding/AddingGroups.java // Adding groups of elements to Collection objects. import java.util.*; public class AddingGroups { public static void main(String[] args) { Collection collection = Holding Your Objects 279 new ArrayList(Arrays.asList(1, 2, 3, 4, 5)); Integer[] moreInts = { 6, 7, 8, 9, 10 }; collection.addAll(Arrays.asList(moreInts)); // Runs significantly faster, but you can’t // construct a Collection this way: Collections.addAll(collection, 11, 12, 13, 14, 15); Collections.addAll(collection, moreInts); // Produces a list "backed by" an array: List list = Arrays.asList(16, 17, 18, 19, 20); list.set(1, 99); // OK -- modify an element // list.add(21); // Runtime error because the // underlying array cannot be resized. } } ///:~ The constructor for a Collection can accept another Collection which it uses for initializing itself, so you can use Arrays.asList( ) to produce input for the constructor. However, Collections.addAll( ) runs much faster, and it’s just as easy to construct the Collection with no elements and then call Collections.addAll( ), so this is the preferred approach. The Collection.addAll( ) member method can only take an argument of another Collection object, so it is not as flexible as Arrays.asList( ) or Collections.addAll( ), which use variable argument lists. It’s also possible to use the output of Arrays.asList( ) directly, as a List, but the underlying representation in this case is the array, which cannot be resized. If you try to add( ) or delete( ) elements in such a list, that would attempt to change the size of an array, so you’ll get an "Unsupported Operation" error at run time. A limitation of Arrays.asList( ) is that it takes a best guess about the resulting type of the List, and doesn’t pay attention to what you’re assigning it to. Sometimes this can cause a problem: //: holding/AsListInference.java // Arrays.asList() makes its best guess about type. import java.util.*; class Snow {} class Powder extends Snow {} class Light extends Powder {} class Heavy extends Powder {} class Crusty extends Snow {} class Slush extends Snow {} public class AsListInference { public static void main(String[] args) { List snow1 = Arrays.asList( new Crusty(), new Slush(), new Powder()); // Won’t compile: // List snow2 = Arrays.asList( // new Light(), new Heavy()); // Compiler says: // found : java.util.List // required: java.util.List // Collections.addAll() doesn’t get confused: List snow3 = new ArrayList(); Collections.addAll(snow3, new Light(), new Heavy()); 280 Thinking in Java Bruce Eckel // Give a hint using an // explicit type argument specification: List snow4 = Arrays.asList( new Light(), new Heavy()); } } ///:~ When trying to create snow2, Arrays.asList( ) only has types of Powder, so it creates a List rather than a List, whereas Collections.addAll( ) works fine because it knows from the first argument what the target type is. As you can see from the creation of snow4, it’s possible to insert a "hint" in the middle of Arrays.asList( ), to tell the compiler what the actual target type should be for the resulting List type produced by Arrays.asList( ). This is called an explicit type argument specification. Maps are more complex, as you’ll see, and the Java standard library does not provide any way to automatically initialize them, except from the contents of another Map. Printing containers You must use Arrays.toString( ) to produce a printable representation of an array, but the containers print nicely without any help. Here’s an example that also introduces you to the basic Java containers: //: holding/PrintingContainers.java // Containers print themselves automatically. import java.util.*; import static net.mindview.util.Print.*; public class PrintingContainers { static Collection fill(Collection collection) { collection.add("rat"); collection.add("cat"); collection.add("dog"); collection.add("dog"); return collection; } static Map fill(Map map) { map.put("rat", "Fuzzy"); map.put("cat", "Rags"); map.put("dog", "Bosco"); map.put("dog", "Spot"); return map; } public static void main(String[] args) { print(fill(new ArrayList())); print(fill(new LinkedList())); print(fill(new HashSet())); print(fill(new TreeSet())); print(fill(new LinkedHashSet())); print(fill(new HashMap())); print(fill(new TreeMap())); print(fill(new LinkedHashMap())); } } /* Output: [rat, cat, dog, dog] [rat, cat, dog, dog] [dog, cat, rat] Holding Your Objects 281 [cat, dog, rat] [rat, cat, dog] {dog=Spot, cat=Rags, rat=Fuzzy} {cat=Rags, dog=Spot, rat=Fuzzy} {rat=Fuzzy, cat=Rags, dog=Spot} *///:~ This shows the two primary categories in the Java container library. The distinction is based on the number of items that are held in each "slot" in the container. The Collection category only holds one item in each slot. It includes the List, which holds a group of items in a specified sequence, the Set, which only allows the addition of one identical item, and the Queue, which only allows you to insert objects at one "end" of the container and remove objects from the other "end" (for the purposes of this example, this is just another way of looking at a sequence and so it is not shown). A Map holds two objects, a key and an associated value, in each slot. In the output, you can see that the default printing behavior (provided via each container’s toString( ) method) produces reasonably readable results. A Collection is printed surrounded by square brackets, with each element separated by a comma. A Map is surrounded by curly braces, with each key and value associated with an equal sign (keys on the left, values on the right). The first fill( ) method works with all types of Collection, each of which implements the add( ) method to include new elements. ArrayList and LinkedList are both types of List, and you can see from the output that they both hold elements in the same order in which they are inserted. The difference between the two is not only performance for certain types of operations, but also that a LinkedList contains more operations than an ArrayList. These will be explored more fully later in this chapter. HashSet, TreeSet and LinkedHashSet are types of Set. The output shows that a Set will only hold one of each identical item, but it also shows that the different Set implementations store the elements differently. The HashSet stores elements using a rather complex approach that will be explored in the Containers in Depth chapter—all you need to know at this point is that this technique is the fastest way to retrieve elements, and as a result the storage order can seem nonsensical (often, you only care whether something is a member of the Set, not the order in which it appears). If storage order is important, you can use a TreeSet, which keeps the objects in ascending comparison order, or a LinkedHashSet, which keeps the objects in the order in which they were added. A Map (also called an associative array) allows you to look up an object using a key, like a simple database. The associated object is called a value. If you have a Map that associates states with their capitals and you want to know the capital of Ohio, you look it up using "Ohio" as the key—almost as if you were indexing into an array. Because of this behavior, a Map only accepts one of each key. Map.put(key, value) adds a value (the thing you want) and associates it with a key (the thing you look it up with). Map.get(key) produces the value associated with that key. The above example only adds key-value pairs, and does not perform lookups. That will be shown later. Notice that you don’t have to specify (or think about) the size of the Map because it resizes itself automatically. Also, Maps know how to print themselves, showing the association with keys and values. The order that the keys and values are held inside the Map is not the insertion order because the HashMap implementation uses a very fast algorithm that controls the order. 282 Thinking in Java Bruce Eckel The example uses the three basic flavors of Map: HashMap, TreeMap and LinkedHashMap. Like HashSet, HashMap provides the fastest lookup technique, and also doesn’t hold its elements in any apparent order. A TreeMap keeps the keys sorted by ascending comparison order, and a LinkedHashMap keeps the keys in insertion order while retaining the lookup speed of the HashMap. Exercise 4: (3) Create a generator class that produces character names (as String objects) from your favorite movie (you can use Snow White or Star Wars as a fallback) each time you call next( ), and loops around to the beginning of the character list when it runs out of names. Use this generator to fill an array, an ArrayList, a LinkedList, a HashSet, a LinkedHashSet, and a TreeSet, then print each container. List Lists promise to maintain elements in a particular sequence. The List interface adds a number of methods to Collection that allow insertion and removal of elements in the middle of a List. There are two types of List: • The basic ArrayList, which excels at randomly accessing elements, but is slower when inserting and removing elements in the middle of a List. • The LinkedList, which provides optimal sequential access, with inexpensive insertions and deletions from the middle of the List. A LinkedList is relatively slow for random access, but it has a larger feature set than the ArrayList. The following example reaches forward in the book to use a library from the Type Information chapter by importing typeinfo.pets. This is a library that contains a hierarchy of Pet classes along with some tools to randomly generate Pet objects. You don’t need to know the full details at this point, just that (1) there’s a Pet class and various subtypes of Pet and (2) the static Pets.arrayList( ) method will return an ArrayList filled with randomly selected Pet objects: //: holding/ListFeatures.java import typeinfo.pets.*; import java.util.*; import static net.mindview.util.Print.*; public class ListFeatures { public static void main(String[] args) { Random rand = new Random(47); List pets = Pets.arrayList(7); print("1: " + pets); Hamster h = new Hamster(); pets.add(h); // Automatically resizes print("2: " + pets); print("3: " + pets.contains(h)); pets.remove(h); // Remove by object Pet p = pets.get(2); print("4: " + p + " " + pets.indexOf(p)); Pet cymric = new Cymric(); print("5: " + pets.indexOf(cymric)); print("6: " + pets.remove(cymric)); // Must be the exact object: print("7: " + pets.remove(p)); print("8: " + pets); pets.add(3, new Mouse()); // Insert at an index Holding Your Objects 283 print("9: " + pets); List sub = pets.subList(1, 4); print("subList: " + sub); print("10: " + pets.containsAll(sub)); Collections.sort(sub); // In-place sort print("sorted subList: " + sub); // Order is not important in containsAll(): print("11: " + pets.containsAll(sub)); Collections.shuffle(sub, rand); // Mix it up print("shuffled subList: " + sub); print("12: " + pets.containsAll(sub)); List copy = new ArrayList(pets); sub = Arrays.asList(pets.get(1), pets.get(4)); print("sub: " + sub); copy.retainAll(sub); print("13: " + copy); copy = new ArrayList(pets); // Get a fresh copy copy.remove(2); // Remove by index print("14: " + copy); copy.removeAll(sub); // Only removes exact objects print("15: " + copy); copy.set(1, new Mouse()); // Replace an element print("16: " + copy); copy.addAll(2, sub); // Insert a list in the middle print("17: " + copy); print("18: " + pets.isEmpty()); pets.clear(); // Remove all elements print("19: " + pets); print("20: " + pets.isEmpty()); pets.addAll(Pets.arrayList(4)); print("21: " + pets); Object[] o = pets.toArray(); print("22: " + o[3]); Pet[] pa = pets.toArray(new Pet[0]); print("23: " + pa[3].id()); } } /* Output: 1: [Rat, Manx, Cymric, Mutt, Pug, Cymric, Pug] 2: [Rat, Manx, Cymric, Mutt, Pug, Cymric, Pug, Hamster] 3: true 4: Cymric 2 5: -1 6: false 7: true 8: [Rat, Manx, Mutt, Pug, Cymric, Pug] 9: [Rat, Manx, Mutt, Mouse, Pug, Cymric, Pug] subList: [Manx, Mutt, Mouse] 10: true sorted subList: [Manx, Mouse, Mutt] 11: true shuffled subList: [Mouse, Manx, Mutt] 12: true sub: [Mouse, Pug] 13: [Mouse, Pug] 14: [Rat, Mouse, Mutt, Pug, Cymric, Pug] 15: [Rat, Mutt, Cymric, Pug] 16: [Rat, Mouse, Cymric, Pug] 17: [Rat, Mouse, Mouse, Pug, Cymric, Pug] 18: false 19: [] 20: true 21: [Manx, Cymric, Rat, EgyptianMau] 22: EgyptianMau 284 Thinking in Java Bruce Eckel 23: 14 *///:~ The print lines are numbered so the output can be related to the source code. The first output line shows the original List of Pets. Unlike an array, a List allows you to add elements after it has been created, or remove elements, and it resizes itself. That’s its fundamental value: a modifiable sequence. You can see the result of adding a Hamster in output line 2—the object is appended to the end of the list. You can find out whether an object is in the list using the contains( ) method. If you want to remove an object, you can pass that object’s reference to the remove( ) method. Also, if you have a reference to an object, you can discover the index number where that object is located in the List using indexOf( ), as you can see in output line 4. When deciding whether an element is part of a List, discovering the index of an element, and removing an element from a List by reference, the equals( ) method (part of the root class Object) is used. Each Pet is defined to be a unique object, so even though there are two Cymrics in the list, if I create a new Cymric object and pass it to indexOf( ), the result will be -1 (indicating it wasn’t found), and attempts to remove( ) the object will return false. For other classes, equals( ) may be defined differently—Strings, for example, are equal if the contents of two Strings are identical. So to prevent surprises, it’s important to be aware that List behavior changes depending on equals( ) behavior. In output lines 7 and 8, removing an object that exactly matches an object in the List is shown to be successful. It’s possible to insert an element in the middle of the List, as you can see in output line 9 and the code that precedes it, but this brings up an issue: for a LinkedList, insertion and removal in the middle of a list is a cheap operation (except for, in this case, the actual random access into the middle of the list), but for an ArrayList it is an expensive operation. Does this mean you should never insert elements in the middle of an ArrayList, and switch to a LinkedList if you do? No, it just means you should be aware of the issue, and if you start doing many insertions in the middle of an ArrayList and your program starts slowing down, that you might look at your List implementation as the possible culprit (the best way to discover such a bottleneck, as you will see in the supplement at http://MindView.net/Books/BetterJava, is to use a profiler). Optimization is a tricky issue, and the best policy is to leave it alone until you discover you need to worry about it (although understanding the issues is always a good idea). The subList( ) method allows you to easily create a slice out of a larger list, and this naturally produces a true result when passed to containsAll( ) for that larger list. It’s also interesting to note that order is unimportant—you can see in output lines 11 and 12 that calling the intuitively named Collections.sort( ) and Collections.shuffle( ) on sub doesn’t affect the outcome of containsAll( ). subList( ) produces a list backed by the original list. Therefore, changes in the returned list are reflected in the original list, and vice versa. The retainAll( ) method is effectively a "set intersection" operation, in this case keeping all the elements in copy that are also in sub. Again, the resulting behavior depends on the equals( ) method. Output line 14 shows the result of removing an element using its index number, which is more straightforward than removing it by object reference since you don’t have to worry about equals( ) behavior when using indexes. The removeAll( ) method also operates based on the equals( ) method. As the name implies, it removes all the objects from the List that are in the argument List. The set( ) method is rather unfortunately named because of the potential confusion with the Set class— Holding Your Objects 285 "replace" might have been a better name here, because it replaces the element at the index (the first argument) with the second argument. Output line 17 shows that for Lists, there’s an overloaded addAll( ) method that allows you to insert the new list in the middle of the original list, instead of just appending it to the end with the addAll( ) that comes from Collection. Output lines 18-20 show the effect of the isEmpty( ) and clear( ) methods. Output lines 22 and 23 show how you can convert any Collection to an array using toArray( ). This is an overloaded method; the no-argument version returns an array of Object, but if you pass an array of the target type to the overloaded version, it will produce an array of the type specified (assuming it passes type checking). If the argument array is too small to hold all the objects in the List (as is the case here), to Array( ) will create a new array of the appropriate size. Pet objects have an id( ) method, which you can see is called on one of the objects in the resulting array. Exercise 5: (3) Modify ListFeatures.java so that it uses Integers (remember autoboxing!) instead of Pets, and explain any difference in results. Exercise 6: (2) Modify ListFeatures.java so that it uses Strings instead of Pets, and explain any difference in results. Exercise 7: (3) Create a class, then make an initialized array of objects of your class. Fill a List from your array. Create a subset of your List by using subList( ), then remove this subset from your List. Iterator In any container, you must have a way to insert elements and fetch them out again. After all, that’s the primary job of a container—to hold things. In a List, add( ) is one way to insert elements, and get( ) is one way to fetch elements. If you want to start thinking at a higher level, there’s a drawback: You need to program to the exact type of the container in order to use it. This might not seem bad at first, but what if you write code for a List, and later on you discover that it would be convenient to apply that same code to a Set? Or suppose you’d like to write, from the beginning, a piece of generalpurpose code that doesn’t know or care what type of container it’s working with, so that it can be used on different types of containers without rewriting that code? The concept of an Iterator (another design pattern) can be used to achieve this abstraction. An iterator is an object whose job is to move through a sequence and select each object in that sequence without the client programmer knowing or caring about the underlying structure of that sequence. In addition, an iterator is usually what’s called a lightweight object: one that’s cheap to create. For that reason, you’ll often find seemingly strange constraints for iterators; for example, the Java Iterator can move in only one direction. There’s not much you can do with an Iterator except: 1. Ask a Collection to hand you an Iterator using a method called iterator( ). That Iterator will be ready to return the first element in the sequence. 2. Get the next object in the sequence with next( ). 3. See if there are any more objects in the sequence with hasNext( ). 286 Thinking in Java Bruce Eckel 4. Remove the last element returned by the iterator with remove( ). To see how it works, we can again use the Pets tools from the Type Information chapter: //: holding/SimpleIteration.java import typeinfo.pets.*; import java.util.*; public class SimpleIteration { public static void main(String[] args) { List pets = Pets.arrayList(12); Iterator it = pets.iterator(); while(it.hasNext()) { Pet p = it.next(); System.out.print(p.id() + ":" + p + " "); } System.out.println(); // A simpler approach, when possible: for(Pet p : pets) System.out.print(p.id() + ":" + p + " "); System.out.println(); // An Iterator can also remove elements: it = pets.iterator(); for(int i = 0; i < 6; i++) { it.next(); it.remove(); } System.out.println(pets); } } /* Output: 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 8:Cymric 9:Rat 10:EgyptianMau 11:Hamster 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 8:Cymric 9:Rat 10:EgyptianMau 11:Hamster [Pug, Manx, Cymric, Rat, EgyptianMau, Hamster] *///:~ With an Iterator, you don’t need to worry about the number of elements in the container. That’s taken care of for you by hasNext( ) and next( ). If you’re simply moving forward through the List and not trying to modify the List object itself, you can see that the foreach syntax is more succinct. An Iterator will also remove the last element produced by next( ), which means you must call next( ) before you call remove( ).4 This idea of taking a container of objects and passing through it to perform an operation on each one is powerful and will be seen throughout this book. Now consider the creation of a display( ) method that is container-agnostic: //: holding/CrossContainerIteration.java import typeinfo.pets.*; import java.util.*; public class CrossContainerIteration { 4 remove( ) is a so-called "optional" method (there are other such methods), which means that not all Iterator implementations must implement it. This topic is covered in the Containers in Depth chapter. The standard Java library containers implement remove( ), however, so you don’t need to worry about it until that chapter. Holding Your Objects 287 public static void display(Iterator it) { while(it.hasNext()) { Pet p = it.next(); System.out.print(p.id() + ":" + p + " "); } System.out.println(); } public static void main(String[] args) { ArrayList pets = Pets.arrayList(8); LinkedList petsLL = new LinkedList(pets); HashSet petsHS = new HashSet(pets); TreeSet petsTS = new TreeSet(pets); display(pets.iterator()); display(petsLL.iterator()); display(petsHS.iterator()); display(petsTS.iterator()); } } /* Output: 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 4:Pug 6:Pug 3:Mutt 1:Manx 5:Cymric 7:Manx 2:Cymric 0:Rat 5:Cymric 2:Cymric 7:Manx 1:Manx 3:Mutt 6:Pug 4:Pug 0:Rat *///:~ Note that display( ) contains no information about the type of sequence that it is traversing, and this shows the true power of the Iterator: the ability to separate the operation of traversing a sequence from the underlying structure of that sequence. For this reason, we sometimes say that iterators unify access to containers. Exercise 8: (1) Modify Exercise l so it uses an Iterator to move through the List while calling hop( ). Exercise 9: (4) Modify innerclasses/Sequence.java so that Sequence works with an Iterator instead of a Selector. Exercise 10: (2) Change Exercise 9 in the Polymorphism chapter to use an ArrayList to hold the Rodents and an Iterator to move through the sequence of Rodents. Exercise 11: (2) Write a method that uses an Iterator to step through a Collection and print the toString( ) of each object in the container. Fill all the different types of Collections with objects and apply your method to each container. ListIterator The ListIterator is a more powerful subtype of Iterator that is produced only by List classes. While Iterator can only move forward, ListIterator is bidirectional. It can also produce the indexes of the next and previous elements relative to where the iterator is pointing in the list, and it can replace the last element that it visited using the set( ) method. You can produce a ListIterator that points to the beginning of the List by calling listIterator( ), and you can also create a ListIterator that starts out pointing to an index n in the list by calling listIterator(n). Here’s an example that demonstrates all these abilities: //: holding/ListIteration.java import typeinfo.pets.*; import java.util.*; public class ListIteration { public static void main(String[] args) { 288 Thinking in Java Bruce Eckel List pets = Pets.arrayList(8); ListIterator it = pets.listIterator(); while(it.hasNext()) System.out.print(it.next() + ", " + it.nextIndex() + ", " + it.previousIndex() + "; "); System.out.println(); // Backwards: while(it.hasPrevious()) System.out.print(it.previous().id() + " "); System.out.println(); System.out.println(pets); it = pets.listIterator(3); while(it.hasNext()) { it.next(); it.set(Pets.randomPet()); } System.out.println(pets); } } /* Output: Rat, 1, 0; Manx, 2, 1; Cymric, 3, 2; Mutt, 4, 3; Pug, 5, 4; Cymric, 6, 5; Pug, 7, 6; Manx, 8, 7; 76543210 [Rat, Manx, Cymric, Mutt, Pug, Cymric, Pug, Manx] [Rat, Manx, Cymric, Cymric, Rat, EgyptianMau, Hamster, EgyptianMau] *///:~ The Pets.randomPet( ) method is used to replace all the Pet objects in the List from location 3 onward. Exercise 12: (3) Create and populate a List. Create a second List of the same size as the first, and use ListIterators to read elements from the first List and insert them into the second in reverse order. (You may want to explore a number of different ways to solve this problem.) LinkedList The LinkedList also implements the basic List interface like ArrayList does, but it performs certain operations (insertion and removal in the middle of the List) more efficiently than does ArrayList. Conversely, it is less efficient for random-access operations. LinkedList also adds methods that allow it to be used as a stack, a Queue or a doubleended queue (deque). Some of these methods are aliases or slight variations of each other, to produce names that are more familiar within the context of a particular usage (Queue, in particular). For example, getFirst( ) and element( ) are identical—they return the head (first element) of the list without removing it, and throw NoSuchElementException if the List is empty. peek( ) is a slight variation of those two that returns null if the list is empty. removeFirst( ) and remove( ) are also identical—they remove and return the head of the list, and throw NoSuchElementException for an empty list, and poll( ) is a slight variation that returns null if this list is empty. addFirst( ) inserts an element at the beginning of the list. offer( ) is the same as add( ) and addLast( ). They all add an element to the tail (end) of a list. Holding Your Objects 289 removeLast( ) removes and returns the last element of the list. Here’s an example that shows the basic similarity and differences between these features. It doesn’t repeat the behavior that was shown in ListFeatures.java: //: holding/LinkedListFeatures.java import typeinfo.pets.*; import java.util.*; import static net.mindview.util.Print.*; public class LinkedListFeatures { public static void main(String[] args) { LinkedList pets = new LinkedList(Pets.arrayList(5)); print(pets); // Identical: print("pets.getFirst(): " + pets.getFirst()); print("pets.element(): " + pets.element()); // Only differs in empty-list behavior: print("pets.peek(): " + pets.peek()); // Identical; remove and return the first element: print("pets.remove(): " + pets.remove()); print("pets.removeFirst(): " + pets.removeFirst()); // Only differs in empty-list behavior: print("pets.poll(): " + pets.poll()); print(pets); pets.addFirst(new Rat()); print("After addFirst(): " + pets); pets.offer(Pets.randomPet()); print("After offer(): " + pets); pets.add(Pets.randomPet()); print("After add(): " + pets); pets.addLast(new Hamster()); print("After addLast(): " + pets); print("pets.removeLast(): " + pets.removeLast()); } } /* Output: [Rat, Manx, Cymric, Mutt, Pug] pets.getFirst(): Rat pets.element(): Rat pets.peek(): Rat pets.remove(): Rat pets.removeFirst(): Manx pets.poll(): Cymric [Mutt, Pug] After addFirst(): [Rat, Mutt, Pug] After offer(): [Rat, Mutt, Pug, Cymric] After add(): [Rat, Mutt, Pug, Cymric, Pug] After addLast(): [Rat, Mutt, Pug, Cymric, Pug, Hamster] pets.removeLast(): Hamster *///:~ The result of Pets.arrayList( ) is handed to the LinkedList constructor in order to populate it. If you look at the Queue interface, you’ll see the element( ), offer( ), peek( ), poll( ) and remove( ) methods that were added to LinkedList in order that it could be a Queue implementation. Full examples of Queues will be given later in this chapter. Exercise 13: (3) In the innerclasses/GreenhouseController.java example, the class Controller uses an ArrayList. Change the code to use a LinkedList instead, and use an Iterator to cycle through the set of events. 290 Thinking in Java Bruce Eckel Exercise 14: (3) Create an empty LinkedList. Using a Listlterator, add Integers to the List by always inserting them in the middle of the List. Stack A stack is sometimes referred to as a "last-in, first-out" (LIFO) container. It’s sometimes called a pushdown stack, because whatever you "push" on the stack last is the first item you can "pop" off of the stack. An often-used analogy is of cafeteria trays in a spring-loaded holder—the last ones that go in are the first ones that come out. LinkedList has methods that directly implement stack functionality, so you can also just use a LinkedList rather than making a stack class. However, a stack class can sometimes tell the story better: //: net/mindview/util/Stack.java // Making a stack from a LinkedList. package net.mindview.util; import java.util.LinkedList; public class Stack { private LinkedList storage = new LinkedList(); public void push(T v) { storage.addFirst(v); } public T peek() { return storage.getFirst(); } public T pop() { return storage.removeFirst(); } public boolean empty() { return storage.isEmpty(); } public String toString() { return storage.toString(); } } ///:~ This introduces the simplest possible example of a class definition using generics. The after the class name tells the compiler that this will be a parameterized type, and that the type parameter—the one that will be substituted with a real type when the class is used—is T. Basically, this says, "We’re defining a Stack that holds objects of type T." The Stack is implemented using a LinkedList, and the LinkedList is also told that it is holding type T. Notice that push( ) takes an object of type T, while peek( ) and pop( ) return an object of type T. The peek( ) method provides you with the top element without removing it from the top of the stack, while pop( ) removes and returns the top element. If you want only stack behavior, inheritance is inappropriate here because it would produce a class with all the rest of the LinkedList methods (you’ll see in the Containers in Depth chapter that this very mistake was made by the Java l.o designers when they created java.util.Stack). Here’s a simple demonstration of this new Stack class: //: holding/StackTest.java import net.mindview.util.*; public class StackTest { public static void main(String[] args) { Stack stack = new Stack(); for(String s : "My dog has fleas".split(" ")) stack.push(s); while(!stack.empty()) System.out.print(stack.pop() + " "); } } /* Output: fleas has dog My *///:~ Holding Your Objects 291 If you want to use this Stack class in your own code, you’ll need to fully specify the package— or change the name of the class—when you create one; otherwise, you’ll probably collide with the Stack in the java.util package. For example, if we import java.util.* into the above example, we must use package names in order to prevent collisions: //: holding/StackCollision.java import net.mindview.util.*; public class StackCollision { public static void main(String[] args) { net.mindview.util.Stack stack = new net.mindview.util.Stack(); for(String s : "My dog has fleas".split(" ")) stack.push(s); while(!stack.empty()) System.out.print(stack.pop() + " "); System.out.println(); java.util.Stack stack2 = new java.util.Stack(); for(String s : "My dog has fleas".split(" ")) stack2.push(s); while(!stack2.empty()) System.out.print(stack2.pop() + " "); } } /* Output: fleas has dog My fleas has dog My *///:~ The two Stack classes have the same interface, but there is no common Stack interface in java.util—probably because the original, poorly designed java.util.Stack class in Java 1.0 co-opted the name. Even though java.util.Stack exists, LinkedList produces a better Stack and so the net.mindview.util.Stack approach is preferable. You can also control the selection of the "preferred" Stack implementation using an explicit import: import net.mindview.util.Stack; Now any reference to Stack will select the net.mindview.util version, and to select java.util.Stack you must use full qualification. Exercise 15: (4) Stacks are often used to evaluate expressions in programming languages. Using net.mindview.util.Stack, evaluate the following expression, where’+’ means "push the following letter onto the stack," and’-’ means "pop the top of the stack and print it": "+U+n+c—+e+r+t—+a-+i-+n+t+y—+ -+r+u—+l+e+s—" Set A Set refuses to hold more than one instance of each object value. If you try to add more than one instance of an equivalent object, the Set prevents duplication. The most common use for a Set is to test for membership, so that you can easily ask whether an object is in a Set. Because of this, lookup is typically the most important operation for a Set, so you’ll usually choose a HashSet implementation, which is optimized for rapid lookup. Set has the same interface as Collection, so there isn’t any extra functionality like there is in the two different types of List. Instead, the Set is exactly a Collection—it just has different behavior. (This is the ideal use of inheritance and polymorphism: to express 292 Thinking in Java Bruce Eckel different behavior.) A Set determines membership based on the "value" of an object, a more complex topic that you will learn about in the Containers in Depth chapter. Here’s an example that uses a HashSet with Integer objects: //: holding/SetOfInteger.java import java.util.*; public class SetOfInteger { public static void main(String[] args) { Random rand = new Random(47); Set intset = new HashSet(); for(int i = 0; i < 10000; i++) intset.add(rand.nextInt(30)); System.out.println(intset); } } /* Output: [15, 8, 23, 16, 7, 22, 9, 21, 6, 1, 29, 14, 24, 4, 19, 26, 11, 18, 3, 12, 27, 17, 2, 13, 28, 20, 25, 10, 5, 0] *///:~ Ten thousand random numbers from o up to 29 are added to the Set, so you can imagine that each value has many duplications. And yet you can see that only one instance of each appears in the result. You’ll also notice that the output is in no discernible order. This is because a HashSet uses hashing for speed—hashing is covered in the Containers in Depth chapter. The order maintained by a HashSet is different from a TreeSet or a LinkedHashSet, since each implementation has a different way of storing elements. TreeSet keeps elements sorted into a red-black tree data structure, whereas HashSet uses the hashing function. LinkedHashSet also uses hashing for lookup speed, but appears to maintain elements in insertion order using a linked list. If you want the results to be sorted, one approach is to use a TreeSet instead of a HashSet: //: holding/SortedSetOfInteger.java import java.util.*; public class SortedSetOfInteger { public static void main(String[] args) { Random rand = new Random(47); SortedSet intset = new TreeSet(); for(int i = 0; i < 10000; i++) intset.add(rand.nextInt(30)); System.out.println(intset); } } /* Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29] *///:~ One of the most common operations you will perform is a test for set membership using contains( ), but there are also operations that will remind you of the Venn diagrams you may have been taught in elementary school: //: holding/SetOperations.java import java.util.*; import static net.mindview.util.Print.*; public class SetOperations { Holding Your Objects 293 public static void main(String[] args) { Set set1 = new HashSet(); Collections.addAll(set1, "A B C D E F G H I J K L".split(" ")); set1.add("M"); print("H: " + set1.contains("H")); print("N: " + set1.contains("N")); Set set2 = new HashSet(); Collections.addAll(set2, "H I J K L".split(" ")); print("set2 in set1: " + set1.containsAll(set2)); set1.remove("H"); print("set1: " + set1); print("set2 in set1: " + set1.containsAll(set2)); set1.removeAll(set2); print("set2 removed from set1: " + set1); Collections.addAll(set1, "X Y Z".split(" ")); print("‘X Y Z’ added to set1: " + set1); } } /* Output: H: true N: false set2 in set1: true set1: [D, K, C, B, L, G, I, M, A, F, J, E] set2 in set1: false set2 removed from set1: [D, C, B, G, M, A, F, E] ‘X Y Z’ added to set1: [Z, D, C, B, G, M, A, F, Y, X, E] *///:~ The method names are self-explanatory, and there are a few more that you will find in the JDK documentation. Producing a list of unique elements can be quite useful. For example, suppose you’d like to list all the words in the file SetOperations.java, above. Using the net.mindview.TextFile utility that will be introduced later in the book, you can open and read a file into a Set: //: holding/UniqueWords.java import java.util.*; import net.mindview.util.*; public class UniqueWords { public static void main(String[] args) { Set words = new TreeSet( new TextFile("SetOperations.java", "\\W+")); System.out.println(words); } } /* Output: [A, B, C, Collections, D, E, F, G, H, HashSet, I, J, K, L, M, N, Output, Print, Set, SetOperations, String, X, Y, Z, add, addAll, added, args, class, contains, containsAll, false, from, holding, import, in, java, main, mindview, net, new, print, public, remove, removeAll, removed, set1, set2, split, static, to, true, util, void] *///:~ TextFile is inherited from List. The TextFile constructor opens the file and breaks it into words according to the regular expression "\\W+", which means "one or more letters" (regular expressions are introduced in the Strings chapter). The result is handed to the TreeSet constructor, which adds the contents of the List to itself. Since it is a TreeSet, the result is sorted. In this case, the sorting is done lexicographically so that the uppercase and lowercase letters are in separate groups. If you’d like to sort it alphabetically, you can 294 Thinking in Java Bruce Eckel pass the String.CASE_INSENSITIVE_ORDER Comparator (a comparator is an object that establishes order) to the TreeSet constructor: //: holding/UniqueWordsAlphabetic.java // Producing an alphabetic listing. import java.util.*; import net.mindview.util.*; public class UniqueWordsAlphabetic { public static void main(String[] args) { Set words = new TreeSet(String.CASE_INSENSITIVE_ORDER); words.addAll( new TextFile("SetOperations.java", "\\W+")); System.out.println(words); } } /* Output: [A, add, addAll, added, args, B, C, class, Collections, contains, containsAll, D, E, F, false, from, G, H, HashSet, holding, I, import, in, J, java, K, L, M, main, mindview, N, net, new, Output, Print, public, remove, removeAll, removed, Set, set1, set2, SetOperations, split, static, String, to, true, util, void, X, Y, Z] *///:~ Comparators will be explored in detail in the Arrays chapter. Exercise 16: (5) Create a Set of the vowels. Working from UniqueWords.Java, count and display the number of vowels in each input word, and also display the total number of vowels in the input file. Map The ability to map objects to other objects can be an immensely powerful way to solve programming problems. For example, consider a program to examine the randomness of Java’s Random class. Ideally, Random would produce a perfect distribution of numbers, but to test this you need to generate many random numbers and count the ones that fall in the various ranges. A Map easily solves the problem; in this case, the key is the number produced by Random, and the value is the number of times that number appears: //: holding/Statistics.java // Simple demonstration of HashMap. import java.util.*; public class Statistics { public static void main(String[] args) { Random rand = new Random(47); Map m = new HashMap(); for(int i = 0; i < 10000; i++) { // Produce a number between 0 and 20: int r = rand.nextInt(20); Integer freq = m.get(r); m.put(r, freq == null ? 1 : freq + 1); } System.out.println(m); } } /* Output: Holding Your Objects 295 {15=497, 4=481, 19=464, 8=468, 11=531, 16=533, 18=478, 3=508, 7=471, 12=521, 17=509, 2=489, 13=506, 9=549, 6=519, 1=502, 14=477, 10=513, 5=503, 0=481} *///:~ In main( ), autoboxing converts the randomly generated int into an Integer reference that can be used with the HashMap (you can’t use primitives with containers). The get( ) method returns null if the key is not already in the container (which means that this is the first time the number has been found). Otherwise, the get( ) method produces the associated Integer value for the key, which is incremented (again, autoboxing simplifies the expression but there are actually conversions to and from Integer taking place). Here’s an example that allows you to use a String description to look up Pet objects. It also shows how you can test a Map to see if it contains a key or a value with containsKey( ) and containsValue( ): //: holding/PetMap.java import typeinfo.pets.*; import java.util.*; import static net.mindview.util.Print.*; public class PetMap { public static void main(String[] args) { Map petMap = new HashMap(); petMap.put("My Cat", new Cat("Molly")); petMap.put("My Dog", new Dog("Ginger")); petMap.put("My Hamster", new Hamster("Bosco")); print(petMap); Pet dog = petMap.get("My Dog"); print(dog); print(petMap.containsKey("My Dog")); print(petMap.containsValue(dog)); } } /* Output: {My Cat=Cat Molly, My Hamster=Hamster Bosco, My Dog=Dog Ginger} Dog Ginger true true *///:~ Maps, like arrays and Collections, can easily be expanded to multiple dimensions; you simply make a Map whose values are Maps (and the values of those Maps can be other containers, even other Maps). Thus, it’s quite easy to combine containers to quickly produce powerful data structures. For example, suppose you are keeping track of people who have multiple pets—all you need is a Map>: //: holding/MapOfList.java package holding; import typeinfo.pets.*; import java.util.*; import static net.mindview.util.Print.*; public class MapOfList { public static Map> petPeople = new HashMap>(); static { petPeople.put(new Person("Dawn"), Arrays.asList(new Cymric("Molly"),new Mutt("Spot"))); petPeople.put(new Person("Kate"), Arrays.asList(new Cat("Shackleton"), new Cat("Elsie May"), new Dog("Margrett"))); 296 Thinking in Java Bruce Eckel petPeople.put(new Person("Marilyn"), Arrays.asList( new Pug("Louie aka Louis Snorkelstein Dupree"), new Cat("Stanford aka Stinky el Negro"), new Cat("Pinkola"))); petPeople.put(new Person("Luke"), Arrays.asList(new Rat("Fuzzy"), new Rat("Fizzy"))); petPeople.put(new Person("Isaac"), Arrays.asList(new Rat("Freckly"))); } public static void main(String[] args) { print("People: " + petPeople.keySet()); print("Pets: " + petPeople.values()); for(Person person : petPeople.keySet()) { print(person + " has:"); for(Pet pet : petPeople.get(person)) print(" " + pet); } } } /* Output: People: [Person Luke, Person Marilyn, Person Isaac, Person Dawn, Person Kate] Pets: [[Rat Fuzzy, Rat Fizzy], [Pug Louie aka Louis Snorkelstein Dupree, Cat Stanford aka Stinky el Negro, Cat Pinkola], [Rat Freckly], [Cymric Molly, Mutt Spot], [Cat Shackleton, Cat Elsie May, Dog Margrett]] Person Luke has: Rat Fuzzy Rat Fizzy Person Marilyn has: Pug Louie aka Louis Snorkelstein Dupree Cat Stanford aka Stinky el Negro Cat Pinkola Person Isaac has: Rat Freckly Person Dawn has: Cymric Molly Mutt Spot Person Kate has: Cat Shackleton Cat Elsie May Dog Margrett *///:~ A Map can return a Set of its keys, a Collection of its values, or a Set of its pairs. The keySet( ) method produces a Set of all the keys in petPeople, which is used in the foreach statement to iterate through the Map. Exercise 17: (2) Take the Gerbil class in Exercise 1 and put it into a Map instead, associating each Gerbil’s name (e.g. "Fuzzy" or "Spot") as a String (the key) for each Gerbil (the value) you put in the table. Get an Iterator for the keySet( ) and use it to move through the Map, looking up the Gerbil for each key and printing out the key and telling the Gerbil to hop( ). Exercise 18: (3) Fill a HashMap with key-value pairs. Print the results to show ordering by hash code. Extract the pairs, sort by key, and place the result into a LinkedHashMap. Show that the insertion order is maintained. Exercise 19: (2) Repeat the previous exercise with a HashSet and LinkedHashSet. Holding Your Objects 297 Exercise 20: (3) Modify Exercise 16 so that you keep a count of the occurrence of each vowel. Exercise 21: (3) Using a Map, follow the form of UniqueWords.java to create a program that counts the occurrence of words in a file. Sort the results using Collections.sort( ) with a second argument of String.CASE_INSENSITIVE_ORDER (to produce an alphabetic sort), and display the result. Exercise 22: (5) Modify the previous exercise so that it uses a class containing a String and a count field to store each different word, and a Set of these objects to maintain the list of words. Exercise 23: (4) Starting with Statistics.java, create a program that runs the test repeatedly and looks to see if any one number tends to appear more than the others in the results. Exercise 24: (2) Fill a LinkedHashMap with String keys and objects of your choice. Now extract the pairs, sort them based on the keys, and reinsert them into the Map. Exercise 25: (3) Create a Map>. Use net.mindview.TextFile to open a text file and read it in a word at a time (use "\\W+" as the second argument to the TextFile constructor). Count the words as you read them in, and for each word in the file, record in the ArrayList the word count associated with that word—this is, in effect, the location in the file where that word was found. Exercise 26: (4) Take the resulting Map from the previous exercise and re-create the order of the words as they appeared in the original file. Queue A queue is typically a “first-in, first-out" (FIFO) container. That is, you put things in at one end and pull them out at the other, and the order in which you put them in will be the same order in which they come out. Queues are commonly used as a way to reliably transfer objects from one area of a program to another. Queues are especially important in concurrent programming, as you will see in the Concurrency chapter, because they safely transfer objects from one task to another. LinkedList has methods to support queue behavior and it implements the Queue interface, so a LinkedList can be used as a Queue implementation. By upcasting a LinkedList to a Queue, this example uses the Queuespecific methods in the Queue interface: //: holding/QueueDemo.java // Upcasting to a Queue from a LinkedList. import java.util.*; public class QueueDemo { public static void printQ(Queue queue) { while(queue.peek() != null) System.out.print(queue.remove() + " "); System.out.println(); } public static void main(String[] args) { Queue queue = new LinkedList(); Random rand = new Random(47); for(int i = 0; i < 10; i++) 298 Thinking in Java Bruce Eckel queue.offer(rand.nextInt(i + 10)); printQ(queue); Queue qc = new LinkedList(); for(char c : "Brontosaurus".toCharArray()) qc.offer(c); printQ(qc); } } /* Output: 8 1 1 1 5 14 3 1 0 1 Brontosaurus *///:~ offer( ) is one of the Queue-specific methods; it inserts an element at the tail of the queue if it can, or returns false. Both peek( ) and element( ) return the head of the queue without removing it, but peek( ) returns null if the queue is empty and element( ) throws NoSuchElementException. Both poll( ) and remove( ) remove and return the head of the queue, but poll( ) returns null if the queue is empty, while remove( ) throws NoSuchElementException. Autoboxing automatically converts the int result of nextInt( ) into the Integer object required by queue, and the char c into the Character object required by qc. The Queue interface narrows access to the methods of LinkedList so that only the appropriate methods are available, and you are thus less tempted to use LinkedList methods (here, you could actually cast queue back to a LinkedList, but you are at least discouraged from doing so). Notice that the Queue-specific methods provide complete and standalone functionality. That is, you can have a usable Queue without any of the methods that are in Collection, from which it is inherited. Exercise 27: (2) Write a class called Command that contains a String and has a method operation( ) that displays the String. Write a second class with a method that fills a Queue with Command objects and returns it. Pass the filled Queue to a method in a third class that consumes the objects in the Queue and calls their operation( ) methods. PriorityQueue First-in, first-out (FIFO) describes the most typical queuing discipline. A queuing discipline is what decides, given a group of elements in the queue, which one goes next. First-in, firstout says that the next element should be the one that was waiting the longest. Apriority queue says that the element that goes next is the one with the greatest need (the highest priority). For example, in an airport, a customer might be pulled out of a queue if their plane is about to leave. If you build a messaging system, some messages will be more important than others, and should be dealt with sooner, regardless of when they arrive. The PriorityQueue was added in Java SE5 to provide an automatic implementation for this behavior. When you offer( ) an object onto a PriorityQueue, that object is sorted into the queue.5 The default sorting uses the natural order of the objects in the queue, but you can modify the order by providing your own Comparator. The PriorityQueue ensures that when you call peek( ), poll( ) or remove( ), the element you get will be the one with the highest priority. 5 This actually depends on the implementation. Priority queue algorithms typically sort on insertion (maintaining a heap), but they may also perform the selection of the most important element upon removal. The choice of algorithm could be important if object priority can change while it is waiting in the queue. Holding Your Objects 299 It’s trivial to make a PriorityQueue that works with built-in types like Integer, String or Character. In the following example, the first set of values are the identical random values from the previous example, so you can see that they emerge differently from the PriorityQueue: //: holding/PriorityQueueDemo.java import java.util.*; public class PriorityQueueDemo { public static void main(String[] args) { PriorityQueue priorityQueue = new PriorityQueue(); Random rand = new Random(47); for(int i = 0; i < 10; i++) priorityQueue.offer(rand.nextInt(i + 10)); QueueDemo.printQ(priorityQueue); List ints = Arrays.asList(25, 22, 20, 18, 14, 9, 3, 1, 1, 2, 3, 9, 14, 18, 21, 23, 25); priorityQueue = new PriorityQueue(ints); QueueDemo.printQ(priorityQueue); priorityQueue = new PriorityQueue( ints.size(), Collections.reverseOrder()); priorityQueue.addAll(ints); QueueDemo.printQ(priorityQueue); String fact = "EDUCATION SHOULD ESCHEW OBFUSCATION"; List strings = Arrays.asList(fact.split("")); PriorityQueue stringPQ = new PriorityQueue(strings); QueueDemo.printQ(stringPQ); stringPQ = new PriorityQueue( strings.size(), Collections.reverseOrder()); stringPQ.addAll(strings); QueueDemo.printQ(stringPQ); Set charSet = new HashSet(); for(char c : fact.toCharArray()) charSet.add(c); // Autoboxing PriorityQueue characterPQ = new PriorityQueue(charSet); QueueDemo.printQ(characterPQ); } } /* Output: 0 1 1 1 1 1 3 5 8 14 1 1 2 3 3 9 9 14 14 18 18 20 21 22 23 25 25 25 25 23 22 21 20 18 18 14 14 9 9 3 3 2 1 1 AABCCCDDEEEFHHIILNNOOOOSSSTTUUUW WUUUTTSSSOOOONNLIIHHFEEEDDCCCBAA ABCDEFHILNOSTUW *///:~ You can see that duplicates are allowed, and the lowest values have the highest priority (in the case of String, spaces also count as values and are higher in priority than letters). To show how you can change the ordering by providing your own Comparator object, the third constructor call to PriorityQueue and the second call to PriorityQueue use the reverse-order Comparator produced by Collections.reverseOrder( ) (added in Java SE5). The last section adds a HashSet to eliminate duplicate Characters, just to make things a little more interesting. 300 Thinking in Java Bruce Eckel Integer, String and Character work with PriorityQueue because these classes already have natural ordering built in. If you want you use your own class in a PriorityQueue, you must include additional functionality to produce natural ordering, or provide your own Comparator. There’s a more sophisticated example that demonstrates this in the Containers in Depth chapter. Exercise 28: (2) Fill a PriorityQueue (using offer( )) with Double values created using java.util.Random, then remove the elements using poll( ) and display them. Exercise 29: (2) Create a simple class that inherits from Object and contains no members, and show that you cannot successfully add multiple elements of that class to a PriorityQueue. This issue will be fully explained in the Containers in Depth chapter. Collection vs. Iterator Collection is the root interface that describes what is common for all sequence containers. It might be thought of as an "incidental interface," one that appeared because of commonality between other interfaces. In addition, the java.utiLAbstractCollection class provides a default implementation for a Collection, so that you can create a new subtype of AbstractCollection without unnecessary code duplication. One argument for having an interface is that it allows you to create more generic code. By writing to an interface rather than an implementation, your code can be applied to more types of objects.6 So if I write a method that takes a Collection, that method can be applied to any type that implements Collection—and this allows a new class to choose to implement Collection in order to be used with my method. It’s interesting to note, however, that the Standard C++ Library has no common base class for its containers—all commonality between containers is achieved through iterators. In Java, it might seem sensible to follow the C++ approach, and to express commonality between containers using an iterator rather than a Collection. However, the two approaches are bound together, since implementing Collection also means providing an iterator( ) method: //: holding/InterfaceVsIterator.java import typeinfo.pets.*; import java.util.*; public class InterfaceVsIterator { public static void display(Iterator it) { while(it.hasNext()) { Pet p = it.next(); System.out.print(p.id() + ":" + p + " "); } System.out.println(); } public static void display(Collection pets) { for(Pet p : pets) System.out.print(p.id() + ":" + p + " "); System.out.println(); } public static void main(String[] args) { List petList = Pets.arrayList(8); Set petSet = new HashSet(petList); Map petMap = new LinkedHashMap(); 6 Some people advocate the automatic creation of an interface for every possible combination of methods in a class— sometimes for every single class. I believe that an interface should have more meaning than a mechanical duplication of method combinations, so I tend to wait until I see the value added by an interface before creating one. Holding Your Objects 301 String[] names = ("Ralph, Eric, Robin, Lacey, " + "Britney, Sam, Spot, Fluffy").split(", "); for(int i = 0; i < names.length; i++) petMap.put(names[i], petList.get(i)); display(petList); display(petSet); display(petList.iterator()); display(petSet.iterator()); System.out.println(petMap); System.out.println(petMap.keySet()); display(petMap.values()); display(petMap.values().iterator()); } } /* Output: 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 4:Pug 6:Pug 3:Mutt 1:Manx 5:Cymric 7:Manx 2:Cymric 0:Rat 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 4:Pug 6:Pug 3:Mutt 1:Manx 5:Cymric 7:Manx 2:Cymric 0:Rat {Ralph=Rat, Eric=Manx, Robin=Cymric, Lacey=Mutt, Britney=Pug, Sam=Cymric, Spot=Pug, Fluffy=Manx} [Ralph, Eric, Robin, Lacey, Britney, Sam, Spot, Fluffy] 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx *///:~ Both versions of display( ) work with Map objects as well as with subtypes of Collection, and both the Collection interface and the Iterator decouple the display( ) methods from knowing about the particular implementation of the underlying container. In this case the two approaches come up even. In fact, Collection pulls ahead a bit because it is Iterable, and so in the implementation of display(Collection) the foreach construct can be used, which makes the code a little cleaner. The use of Iterator becomes compelling when you implement a foreign class, one that is not a Collection, in which it would be difficult or annoying to make it implement the Collection interface. For example, if we create a Collection implementation by inheriting from a class that holds Pet objects, we must implement all the Collection methods, even if we don’t need to use them within the display( ) method. Although this can easily be accomplished by inheriting from AbstractCollection, you’re forced to implement iterator( ) anyway, along with size( ), in order to provide the methods that are not implemented by AbstractCollection, but that are used by the other methods in AbstractCollection: //: holding/CollectionSequence.java import typeinfo.pets.*; import java.util.*; public class CollectionSequence extends AbstractCollection { private Pet[] pets = Pets.createArray(8); public int size() { return pets.length; } public Iterator iterator() { return new Iterator() { private int index = 0; public boolean hasNext() { return index < pets.length; } public Pet next() { return pets[index++]; } public void remove() { // Not implemented throw new UnsupportedOperationException(); } 302 Thinking in Java Bruce Eckel }; } public static void main(String[] args) { CollectionSequence c = new CollectionSequence(); InterfaceVsIterator.display(c); InterfaceVsIterator.display(c.iterator()); } } /* Output: 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx *///:~ The remove( ) method is an "optional operation," which you will learn about in the Containers in Depth chapter. Here, it’s not necessary to implement it, and if you call it, it will throw an exception. From this example, you can see that if you implement Collection, you also implement iterator( ), and just implementing iterator( ) alone requires only slightly less effort than inheriting from AbstractCoUection. However, if your class already inherits from another class, then you cannot also inherit from AbstractCollection. In that case, to implement Collection you’d have to implement all the methods in the interface. In this case it would be much easier to inherit and add the ability to create an iterator: //: holding/NonCollectionSequence.java import typeinfo.pets.*; import java.util.*; class PetSequence { protected Pet[] pets = Pets.createArray(8); } public class NonCollectionSequence extends PetSequence { public Iterator iterator() { return new Iterator() { private int index = 0; public boolean hasNext() { return index < pets.length; } public Pet next() { return pets[index++]; } public void remove() { // Not implemented throw new UnsupportedOperationException(); } }; } public static void main(String[] args) { NonCollectionSequence nc = new NonCollectionSequence(); InterfaceVsIterator.display(nc.iterator()); } } /* Output: 0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx *///:~ Producing an Iterator is the least-coupled way of connecting a sequence to a method that consumes that sequence, and puts far fewer constraints on the sequence class than does implementing Collection. Exercise 30: (5) Modify CollectionSequence.java so that it does not inherit from AbstractCollection, but instead implements Collection. Holding Your Objects 303 Foreach and iterators So far, the foreach syntax has been primarily used with arrays, but it also works with any Collection object. You’ve actually seen a few examples of this using ArrayList, but here’s a general proof: //: holding/ForEachCollections.java // All collections work with foreach. import java.util.*; public class ForEachCollections { public static void main(String[] args) { Collection cs = new LinkedList(); Collections.addAll(cs, "Take the long way home".split(" ")); for(String s : cs) System.out.print("‘" + s + "‘ "); } } /* Output: ‘Take’ ‘the’ ‘long’ ‘way’ ‘home’ *///:~ Since cs is a Collection, this code shows that working with foreach is a characteristic of all Collection objects. The reason that this works is that Java SE5 introduced a new interface called Iterable which contains an iterator( ) method to produce an Iterator, and the Iterable interface is what foreach uses to move through a sequence. So if you create any class that implements Iterable, you can use it in a foreach statement: //: holding/IterableClass.java // Anything Iterable works with foreach. import java.util.*; public class IterableClass implements Iterable { protected String[] words = ("And that is how " + "we know the Earth to be banana-shaped.").split(" "); public Iterator iterator() { return new Iterator() { private int index = 0; public boolean hasNext() { return index < words.length; } public String next() { return words[index++]; } public void remove() { // Not implemented throw new UnsupportedOperationException(); } }; } public static void main(String[] args) { for(String s : new IterableClass()) System.out.print(s + " "); } } /* Output: And that is how we know the Earth to be banana-shaped. *///:~ 304 Thinking in Java Bruce Eckel The iterator( ) method returns an instance of an anonymous inner implementation of Iterator which delivers each word in the array. In main( ), you can see that IterableClass does indeed work in a foreach statement. In Java SE5, a number of classes have been made Iterable, primarily all Collection classes (but not Maps). For example, this code displays all the operating system environment variables: //: holding/EnvironmentVariables.java import java.util.*; public class EnvironmentVariables { public static void main(String[] args) { for(Map.Entry entry: System.getenv().entrySet()) { System.out.println(entry.getKey() + ": " + entry.getValue()); } } } /* (Execute to see output) *///:~ System.getenv( )7 returns a Map, entrySet( ) produces a Set of Map.Entry elements, and a Set is Iterable so it can be used in a foreach loop. A foreach statement works with an array or anything Iterable, but that doesn’t mean that an array is automatically an Iterable, nor is there any autoboxing that takes place: //: holding/ArrayIsNotIterable.java import java.util.*; public class ArrayIsNotIterable { static void test(Iterable ib) { for(T t : ib) System.out.print(t + " "); } public static void main(String[] args) { test(Arrays.asList(1, 2, 3)); String[] strings = { "A", "B", "C" }; // An array works in foreach, but it’s not Iterable: //! test(strings); // You must explicitly convert it to an Iterable: test(Arrays.asList(strings)); } } /* Output: 123ABC *///:~ Trying to pass an array as an Iterable argument fails. There is no automatic conversion to an Iterable; you must do it by hand. Exercise 31: (3) Modify polymorphism/shape/RandomShapeGenerator.java to make it Iterable. You’ll need to add a constructor that takes the number of elements that you want the iterator to produce before stopping. Verify that it works. 7 This was not available before Java SE5, because it was thought to be too tightly coupled to the operating system, and thus to violate "write once, run anywhere." The fact that it is included now suggests that the Java designers are becoming more pragmatic. Holding Your Objects 305 The Adapter Method idiom What if you have an existing class that is Iterable, and you’d like to add one or more new ways to use this class in a foreach statement? For example, suppose you’d like to choose whether to iterate through a list of words in either a forward or reverse direction. If you simply inherit from the class and override the iterator( ) method, you replace the existing method and you don’t get a choice. One solution is what I call the Adapter Method idiom. The "Adapter" part comes from design patterns, because you must provide a particular interface to satisfy the foreach statement. When you have one interface and you need another one, writing an adapter solves the problem. Here, I want to add the ability to produce a reverse iterator to the default forward iterator, so I can’t override. Instead, I add a method that produces an Iterable object which can then be used in the foreach statement. As you see here, this allows us to provide multiple ways to use foreach: //: holding/AdapterMethodIdiom.java // The "Adapter Method" idiom allows you to use foreach // with additional kinds of Iterables. import java.util.*; class ReversibleArrayList extends ArrayList { public ReversibleArrayList(Collection c) { super(c); } public Iterable reversed() { return new Iterable() { public Iterator iterator() { return new Iterator() { int current = size() - 1; public boolean hasNext() { return current > -1; } public T next() { return get(current--); } public void remove() { // Not implemented throw new UnsupportedOperationException(); } }; } }; } } public class AdapterMethodIdiom { public static void main(String[] args) { ReversibleArrayList ral = new ReversibleArrayList( Arrays.asList("To be or not to be".split(" "))); // Grabs the ordinary iterator via iterator(): for(String s : ral) System.out.print(s + " "); System.out.println(); // Hand it the Iterable of your choice for(String s : ral.reversed()) System.out.print(s + " "); } } /* Output: To be or not to be be to not or be To *///:~ If you simply put the ral object in the foreach statement, you get the (default) forward iterator. But if you call reversed( ) on the object, it produces different behavior. 306 Thinking in Java Bruce Eckel Using this approach, I can add two adapter methods to the IterableClass.java example: //: holding/MultiIterableClass.java // Adding several Adapter Methods. import java.util.*; public class MultiIterableClass extends IterableClass { public Iterable reversed() { return new Iterable() { public Iterator iterator() { return new Iterator() { int current = words.length - 1; public boolean hasNext() { return current > -1; } public String next() { return words[current--]; } public void remove() { // Not implemented throw new UnsupportedOperationException(); } }; } }; } public Iterable randomized() { return new Iterable() { public Iterator iterator() { List shuffled = new ArrayList(Arrays.asList(words)); Collections.shuffle(shuffled, new Random(47)); return shuffled.iterator(); } }; } public static void main(String[] args) { MultiIterableClass mic = new MultiIterableClass(); for(String s : mic.reversed()) System.out.print(s + " "); System.out.println(); for(String s : mic.randomized()) System.out.print(s + " "); System.out.println(); for(String s : mic) System.out.print(s + " "); } } /* Output: banana-shaped. be to Earth the know we how is that And is banana-shaped. Earth that how the be And we know to And that is how we know the Earth to be banana-shaped. *///:~ Notice that the second method, random( ), doesn’t create its own Iterator but simply returns the one from the shuffled List. You can see from the output that the Collections.shuffle( ) method doesn’t affect the original array, but only shuffles the references in shuffled. This is only true because the randomized( ) method wraps an ArrayList around the result of Arrays.asList( ). If the List produced by Arrays.asList( ) is shuffled directly, it will modify the underlying array, as you can see here: //: holding/ModifyingArraysAsList.java import java.util.*; public class ModifyingArraysAsList { public static void main(String[] args) { Holding Your Objects 307 Random rand = new Random(47); Integer[] ia = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; List list1 = new ArrayList(Arrays.asList(ia)); System.out.println("Before shuffling: " + list1); Collections.shuffle(list1, rand); System.out.println("After shuffling: " + list1); System.out.println("array: " + Arrays.toString(ia)); List list2 = Arrays.asList(ia); System.out.println("Before shuffling: " + list2); Collections.shuffle(list2, rand); System.out.println("After shuffling: " + list2); System.out.println("array: " + Arrays.toString(ia)); } } /* Output: Before shuffling: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] After shuffling: [4, 6, 3, 1, 8, 7, 2, 5, 10, 9] array: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Before shuffling: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] After shuffling: [9, 1, 6, 3, 7, 2, 5, 10, 4, 8] array: [9, 1, 6, 3, 7, 2, 5, 10, 4, 8] *///:~ In the first case, the output of Arrays.asList( ) is handed to the ArrayList( ) constructor, and this creates an ArrayList that references the elements of ia. Shuffling these references doesn’t modify the array. However, if you use the result of Arrays.asList(ia) directly, shuffling modifies the order of ia. It’s important to be aware that Arrays.asList( ) produces a List object that uses the underlying array as its physical implementation. If you do anything to that List that modifies it, and you don’t want the original array modified, you should make a copy into another container. Exercise 32: (2) Following the example of MultilterableClass, add reversed( ) and randomized( ) methods to NonCollectionSequence.java, as well as making NonCollectionSequence implement Iterable, and show that all the approaches work in foreach statements. Summary Java provides a number of ways to hold objects: 1. An array associates numerical indexes to objects. It holds objects of a known type so that you don’t have to cast the result when you’re looking up an object. It can be multidimensional, and it can hold primitives. However, its size cannot be changed once you create it. 2. A Collection holds single elements, and a Map holds associated pairs. With Java generics, you specify the type of object to be held in the containers, so you can’t put the wrong type into a container and you don’t have to cast elements when you fetch them out of a container. Both Collections and Maps automatically resize themselves as you add more elements. A container won’t hold primitives, but autoboxing takes care of translating primitives back and forth to the wrapper types held in the container. 3. Like an array, a List also associates numerical indexes to objects— thus, arrays and Lists are ordered containers. 308 Thinking in Java Bruce Eckel 4. Use an ArrayList if you’re doing a lot of random accesses, but a LinkedList if you will be doing a lot of insertions and removals in the middle of the list. 5. The behavior of Queues and stacks is provided via the LinkedList. 6. A Map is a way to associate not integral values, but objects with other objects. HashMaps are designed for rapid access, whereas a TreeMap keeps its keys in sorted order, and thus is not as fast as a HashMap. A LinkedHashMap keeps its elements in insertion order, but provides rapid access with hashing. 7. A Set only accepts one of each type of object. HashSets provide maximally fast lookups, whereas TreeSets keep the elements in sorted order. LinkedHashSets keep elements in insertion order. 8. There’s no need to use the legacy classes Vector, Hashtable, and Stack in new code. It’s helpful to look at a simplified diagram of the Java containers (without the abstract classes or legacy components). This only includes the interfaces and classes that you will encounter on a regular basis. Simple Container Taxonomy You’ll see that there are really only four basic container components—Map, List, Set, and Queue—and only two or three implementations of each one (the java.util.concurrent implementations of Queue are not included in this diagram). The containers that you will use most often have heavy black lines around them. The dotted boxes represent interfaces, and the solid boxes are regular (concrete) classes. The dotted lines with hollow arrows indicate that a particular class is implementing an interface. The solid arrows show that a class can produce objects of the class the arrow is pointing to. For example, any Collection can produce an Iterator, and a List can produce a ListIterator (as well as an ordinary Iterator, since List is inherited from Collection). Here’s an example that shows the difference in methods between the various classes. The actual code is from the Generics chapter; I’m just calling it here to produce the output. The output also shows the interfaces that are implemented in each class or interface: //: holding/ContainerMethods.java import net.mindview.util.*; Holding Your Objects 309 public class ContainerMethods { public static void main(String[] args) { ContainerMethodDifferences.main(args); } } /* Output: (Sample) Collection: [add, addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray] Interfaces in Collection: [Iterable] Set extends Collection, adds: [] Interfaces in Set: [Collection] HashSet extends Set, adds: [] Interfaces in HashSet: [Set, Cloneable, Serializable] LinkedHashSet extends HashSet, adds: [] Interfaces in LinkedHashSet: [Set, Cloneable, Serializable] TreeSet extends Set, adds: [pollLast, navigableHeadSet, descendingIterator, lower, headSet, ceiling, pollFirst, subSet, navigableTailSet, comparator, first, floor, last, navigableSubSet, higher, tailSet] Interfaces in TreeSet: [NavigableSet, Cloneable, Serializable] List extends Collection, adds: [listIterator, indexOf, get, subList, set, lastIndexOf] Interfaces in List: [Collection] ArrayList extends List, adds: [ensureCapacity, trimToSize] Interfaces in ArrayList: [List, RandomAccess, Cloneable, Serializable] LinkedList extends List, adds: [pollLast, offer, descendingIterator, addFirst, peekLast, removeFirst, peekFirst, removeLast, getLast, pollFirst, pop, poll, addLast, removeFirstOccurrence, getFirst, element, peek, offerLast, push, offerFirst, removeLastOccurrence] Interfaces in LinkedList: [List, Deque, Cloneable, Serializable] Queue extends Collection, adds: [offer, element, peek, poll] Interfaces in Queue: [Collection] PriorityQueue extends Queue, adds: [comparator] Interfaces in PriorityQueue: [Serializable] Map: [clear, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, values] HashMap extends Map, adds: [] Interfaces in HashMap: [Map, Cloneable, Serializable] LinkedHashMap extends HashMap, adds: [] Interfaces in LinkedHashMap: [Map] SortedMap extends Map, adds: [subMap, comparator, firstKey, lastKey, headMap, tailMap] Interfaces in SortedMap: [Map] TreeMap extends Map, adds: [descendingEntrySet, subMap, pollLastEntry, lastKey, floorEntry, lastEntry, lowerKey, navigableHeadMap, navigableTailMap, descendingKeySet, tailMap, ceilingEntry, higherKey, pollFirstEntry, comparator, firstKey, floorKey, higherEntry, firstEntry, navigableSubMap, headMap, lowerEntry, ceilingKey] Interfaces in TreeMap: [NavigableMap, Cloneable, Serializable] *///:~ You can see that all Sets except TreeSet have exactly the same interface as Collection. List and Collection differ significantly, although List requires methods that are in Collection. On the other hand, the methods in the Queue interface stand alone; the Collection methods are not required to create a functioning Queue implementation. Finally, the only intersection between Map and Collection is the fact that a Map can produce Collections using the entrySet( ) and values( ) methods. Notice the tagging interface java.util.RandomAccess, which is attached to ArrayList but not to LinkedList. This provides information for algorithms that might want to dynamically change their behavior depending on the use of a particular List. 310 Thinking in Java Bruce Eckel It’s true that this organization is somewhat odd, as object-oriented hierarchies go. However, as you learn more about the containers in java.util (in particular, in the Containers in Depth chapter), you’ll see that there are more issues than just a slightly odd inheritance structure. Container libraries have always been difficult design problems—solving these problems involves satisfying a set of forces that often oppose each other. So you should be prepared for some compromises here and there. Despite these issues, the Java containers are fundamental tools that you can use on a day-to-day basis to make your programs simpler, more powerful, and more effective. It might take you a little while to get comfortable with some aspects of the library, but I think you’ll find yourself rapidly acquiring and using the classes in this library. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Holding Your Objects 311 Error Handling with Exceptions The basic philosophy of Java is that "badly formed code will not be run." The ideal time to catch an error is at compile time, before you even try to run the program. However, not all errors can be detected at compile time. The rest of the problems must be handled at run time through some formality that allows the originator of the error to pass appropriate information to a recipient who will know how to handle the difficulty properly. Improved error recovery is one of the most powerful ways that you can increase the robustness of your code. Error recovery is a fundamental concern for every program you write, but it’s especially important in Java, where one of the primary goals is to create program components for others to use. To create a robust system, each component must be robust. By providing a consistent error-reporting model using exceptions, Java allows components to reliably communicate problems to client code. The goals for exception handling in Java are to simplify the creation of large, reliable programs using less code than currently possible, and to do so with more confidence that your application doesn’t have an unhandled error. Exceptions are not terribly difficult to learn, and are one of those features that provide immediate and significant benefits to your project. Because exception handling is the only official way that Java reports errors, and it is enforced by the Java compiler, there are only so many examples that can be written in this book without learning about exception handling. This chapter introduces you to the code that you need to write to properly handle exceptions, and shows how you can generate your own exceptions if one of your methods gets into trouble. Concepts C and other earlier languages often had multiple error-handling schemes, and these were generally established by convention and not as part of the programming language. Typically, you returned a special value or set a flag, and the recipient was supposed to look at the value or the flag and determine that something was amiss. However, as the years passed, it was discovered that programmers who use a library tend to think of themselves as invincible—as in "Yes, errors might happen to others, but not in my code." So, not too surprisingly, they wouldn’t check for the error conditions (and sometimes the error conditions were too silly to check for1). If you were thorough enough to check for an error every time you called a method, your code could turn into an unreadable nightmare. Because programmers could still coax systems out of these languages, they were resistant to admitting the truth: that this approach to handling errors was a major limitation to creating large, robust, maintainable programs. The solution is to take the casual nature out of error handling and to enforce formality. This actually has a long history, because implementations of exception handling go back to operating systems in the 1960s, and even to BASIC’S "on error goto." But C++ exception 1 The C programmer can look up the return value of printf( ) for an example of this. handling was based on Ada, and Java’s is based primarily on C++ (although it looks more like Object Pascal). The word "exception" is meant in the sense of "I take exception to that." At the point where the problem occurs, you might not know what to do with it, but you do know that you can’t just continue on merrily; you must stop, and somebody, somewhere, must figure out what to do. But you don’t have enough information in the current context to fix the problem. So you hand the problem out to a higher context where someone is qualified to make the proper decision. The other rather significant benefit of exceptions is that they tend to reduce the complexity of error-handling code. Without exceptions, you must check for a particular error and deal with it at multiple places in your program. With exceptions, you no longer need to check for errors at the point of the method call, since the exception will guarantee that someone catches it. You only need to handle the problem in one place, in the so-called exception handler. This saves you code, and it separates the code that describes what you want to do during normal execution from the code that is executed when things go awry. In general, reading, writing, and debugging code becomes much clearer with exceptions than when using the old way of error handling. Basic exceptions An exceptional condition is a problem that prevents the continuation of the current method or scope. It’s important to distinguish an exceptional condition from a normal problem, in which you have enough information in the current context to somehow cope with the difficulty. With an exceptional condition, you cannot continue processing because you don’t have the information necessary to deal with the problem in the current context. All you can do is jump out of the current context and relegate that problem to a higher context. This is what happens when you throw an exception. Division is a simple example. If you’re about to divide by zero, it’s worth checking for that condition. But what does it mean that the denominator is zero? Maybe you know, in the context of the problem you’re trying to solve in that particular method, how to deal with a zero denominator. But if it’s an unexpected value, you can’t deal with it and so must throw an exception rather than continuing along that execution path. When you throw an exception, several things happen. First, the exception object is created in the same way that any Java object is created: on the heap, with new. Then the current path of execution (the one you couldn’t continue) is stopped and the reference for the exception object is ejected from the current context. At this point the exception-handling mechanism takes over and begins to look for an appropriate place to continue executing the program. This appropriate place is the exception handler, whose job is to recover from the problem so the program can either try another tack or just continue. As a simple example of throwing an exception, consider an object reference called t. It’s possible that you might be passed a reference that hasn’t been initialized, so you might want to check before trying to call a method using that object reference. You can send information about the error into a larger context by creating an object representing your information and "throwing" it out of your current context. This is called throwing an exception. Here’s what it looks like: if(t == null) throw new NullPointerException(); This throws the exception, which allows you—in the current context—to abdicate responsibility for thinking about the issue further. It’s just magically handled somewhere else. Precisely where will be shown shortly. 314 Thinking in Java Bruce Eckel Exceptions allow you to think of everything that you do as a transaction, and the exceptions guard those transactions: "...the fundamental premise of transactions is that we needed exception handling in distributed computations. Transactions are the computer equivalent of contract law. If anything goes wrong, we’ll just blow away the whole computation."2 You can also think about exceptions as a built-in undo system, because (with some care) you can have various recovery points in your program. If a part of the program fails, the exception will "undo" back to a known stable point in the program. One of the most important aspects of exceptions is that if something bad happens, they don’t allow a program to continue along its ordinary path. This has been a real problem in languages like C and C++; especially C, which had no way to force a program to stop going down a path if a problem occurred, so it was possible to ignore problems for a long time and get into a completely inappropriate state. Exceptions allow you to (if nothing else) force the program to stop and tell you what went wrong, or (ideally) force the program to deal with the problem and return to a stable state. Exception arguments As with any object in Java, you always create exceptions on the heap using new, which allocates storage and calls a constructor. There are two constructors in all standard exceptions: The first is the default constructor, and the second takes a string argument so that you can place pertinent information in the exception: throw new NullPointerException("t = null"); This string can later be extracted using various methods, as you’ll see. The keyword throw produces a number of interesting results. After creating an exception object with new, you give the resulting reference to throw. The object is, in effect, "returned" from the method, even though that object type isn’t normally what the method is designed to return. A simplistic way to think about exception handling is as a different kind of return mechanism, although you get into trouble if you take that analogy too far. You can also exit from ordinary scopes by throwing an exception. In either case, an exception object is returned, and the method or scope exits. Any similarity to an ordinary return from a method ends here, because where you return is someplace completely different from where you return for a normal method call. (You end up in an appropriate exception handler that might be far away—many levels on the call stack— from where the exception was thrown.) In addition, you can throw any type of Throwable, which is the exception root class. Typically, you’ll throw a different class of exception for each different type of error. The information about the error is represented both inside the exception object and implicitly in the name of the exception class, so someone in the bigger context can figure out what to do with your exception. (Often, the only information is the type of exception, and nothing meaningful is stored within the exception object.) Catching an exception To see how an exception is caught, you must first understand the concept of a guarded region. This is a section of code that might produce exceptions and is followed by the code to handle those exceptions. 2 Jim Gray, Turing Award winner for his team’s contributions on transactions, in an interview on www.acmqueue.org. Error Handling with Exceptions 315 The try block If you’re inside a method and you throw an exception (or another method that you call within this method throws an exception), that method will exit in the process of throwing. If you don’t want a throw to exit the method, you can set up a special block within that method to capture the exception. This is called the try block because you "try" your various method calls there. The try block is an ordinary scope preceded by the keyword try: try { // Code that might generate exceptions } If you were checking for errors carefully in a programming language that didn’t support exception handling, you’d have to surround every method call with setup and error-testing code, even if you call the same method several times. With exception handling, you put everything in a try block and capture all the exceptions in one place. This means your code is much easier to write and read because the goal of the code is not confused with the error checking. Exception handlers Of course, the thrown exception must end up someplace. This "place" is the exception handler, and there’s one for every exception type you want to catch. Exception handlers immediately follow the try block and are denoted by the keyword catch: try { // Code that might generate exceptions } catch(Type1 id1)|{ // Handle exceptions of Type1 } catch(Type2 id2) { // Handle exceptions of Type2 } catch(Type3 id3) { // Handle exceptions of Type3 } // etc... Each catch clause (exception handler) is like a little method that takes one and only one argument of a particular type. The identifier (id1, id2, and so on) can be used inside the handler, just like a method argument. Sometimes you never use the identifier because the type of the exception gives you enough information to deal with the exception, but the identifier must still be there. The handlers must appear directly after the try block. If an exception is thrown, the exception-handling mechanism goes hunting for the first handler with an argument that matches the type of the exception. Then it enters that catch clause, and the exception is considered handled. The search for handlers stops once the catch clause is finished. Only the matching catch clause executes; it’s not like a switch statement in which you need a break after each case to prevent the remaining ones from executing. Note that within the try block, a number of different method calls might generate the same exception, but you need only one handler. 316 Thinking in Java Bruce Eckel Termination vs. resumption There are two basic models in exception-handling theory. Java supportst termination,3 in which you assume that the error is so critical that there’s no way to get back to where the exception occurred. Whoever threw the exception decided that there was no way to salvage the situation, and they don’t want to come back. The alternative is called resumption. It means that the exception handler is expected to do something to rectify the situation, and then the faulting method is retried, presuming success the second time. If you want resumption, it means you still hope to continue execution after the exception is handled. If you want resumption-like behavior in Java, don’t throw an exception when you encounter an error. Instead, call a method that fixes the problem. Alternatively, place your try block inside a while loop that keeps reentering the try block until the result is satisfactory. Historically, programmers using operating systems that supported resumptive exception handling eventually ended up using termination-like code and skipping resumption. So although resumption sounds attractive at first, it isn’t quite so useful in practice. The dominant reason is probably the coupling that results: A resumptive handler would need to be aware of where the exception is thrown, and contain non-generic code specific to the throwing location. This makes the code difficult to write and maintain, especially for large systems where the exception can be generated from many points. Creating your own exceptions You’re not stuck using the existing Java exceptions. The Java exception hierarchy can’t foresee all the errors you might want to report, so you can create your own to denote a special problem that your library might encounter. To create your own exception class, you must inherit from an existing exception class, preferably one that is close in meaning to your new exception (although this is often not possible). The most trivial way to create a new type of exception is just to let the compiler create the default constructor for you, so it requires almost no code at all: //: exceptions/InheritingExceptions.java // Creating your own exceptions. class SimpleException extends Exception {} public class InheritingExceptions { public void f() throws SimpleException { System.out.println("Throw SimpleException from f()"); throw new SimpleException(); } public static void main(String[] args) { InheritingExceptions sed = new InheritingExceptions(); try { sed.f(); } catch(SimpleException e) { System.out.println("Caught it!"); } } } /* Output: Throw SimpleException from f() 3 As do most languages, including C++, C#, Python, D, etc. Error Handling with Exceptions 317 Caught it! *///:~ The compiler creates a default constructor, which automatically (and invisibly) calls the baseclass default constructor. Of course, in this case you don’t get a SimpleException(String) constructor, but in practice that isn’t used much. As you’ll see, the most important thing about an exception is the class name, so most of the time an exception like the one shown here is satisfactory. Here, the result is printed to the console, where it is automatically captured and tested with this book’s output-display system. However, you may want to send error output to the standard error stream by writing to System.err. This is usually a better place to send error information than System.out, which may be redirected. If you send output to System.err, it will not be redirected along with System.out so the user is more likely to notice it. You can also create an exception class that has a constructor with a String argument: //: exceptions/FullConstructors.java class MyException extends Exception { public MyException() {} public MyException(String msg) { super(msg); } } public class FullConstructors { public static void f() throws MyException { System.out.println("Throwing MyException from f()"); throw new MyException(); } public static void g() throws MyException { System.out.println("Throwing MyException from g()"); throw new MyException("Originated in g()"); } public static void main(String[] args) { try { f(); } catch(MyException e) { e.printStackTrace(System.out); } try { g(); } catch(MyException e) { e.printStackTrace(System.out); } } } /* Output: Throwing MyException from f() MyException at FullConstructors.f(FullConstructors.java:11) at FullConstructors.main(FullConstructors.java:19) Throwing MyException from g() MyException: Originated in g() at FullConstructors.g(FullConstructors.java:15) at FullConstructors.main(FullConstructors.java:24) *///:~ The added code is small: two constructors that define the way MyException is created. In the second constructor, the base-class constructor with a String argument is explicitly invoked by using the super keyword. In the handlers, one of the Throwable (from which Exception is inherited) methods is called: printStackTrace( ). As you can see from the output, this produces information 318 Thinking in Java Bruce Eckel about the sequence of methods that were called to get to the point where the exception happened. Here, the information is sent to System.out, and automatically captured and displayed in the output. However, if you call the default version: e.printStackTrace(); the information goes to the standard error stream. Exercise 1: (2) Create a class with a main( ) that throws an object of class Exception inside a try block. Give the constructor for Exception a String argument. Catch the exception inside a catch clause and print the String argument. Add a finally clause and print a message to prove you were there. Exercise 2: (1) Define an object reference and initialize it to null. Try to call a method through this reference. Now wrap the code in a try-catch clause to catch the exception. Exercise 3: (1) Write code to generate and catch an ArraylndexOutOfBoundsException. Exercise 4: (2) Create your own exception class using the extends keyword. Write a constructor for this class that takes a String argument and stores it inside the object with a String reference. Write a method that displays the stored String. Create a try-catch clause to exercise your new exception. Exercise 5: (3) Create your own resumption-like behavior using a while loop that repeats until an exception is no longer thrown. Exceptions and logging You may also want to log the output using the java.util.logging facility. Although full details of logging are introduced in the supplement at http://MindView.net/Books/BetterJava, basic logging is straightforward enough to be used here. //: exceptions/LoggingExceptions.java // An exception that reports through a Logger. import java.util.logging.*; import java.io.*; class LoggingException extends Exception { private static Logger logger = Logger.getLogger("LoggingException"); public LoggingException() { StringWriter trace = new StringWriter(); printStackTrace(new PrintWriter(trace)); logger.severe(trace.toString()); } } public class LoggingExceptions { public static void main(String[] args) { try { throw new LoggingException(); } catch(LoggingException e) { System.err.println("Caught " + e); } try { throw new LoggingException(); Error Handling with Exceptions 319 } catch(LoggingException e) { System.err.println("Caught " + e); } } } /* Output: (85% match) Aug 30, 2005 4:02:31 PM LoggingException SEVERE: LoggingException at LoggingExceptions.main(LoggingExceptions.java:19) Caught LoggingException Aug 30, 2005 4:02:31 PM LoggingException SEVERE: LoggingException at LoggingExceptions.main(LoggingExceptions.java:24) Caught LoggingException *///:~ The static Logger.getLogger( ) method creates a Logger object associated with the String argument (usually the name of the package and class that the errors are about) which sends its output to System.err. The easiest way to write to a Logger is just to call the method associated with the level of logging message; here, severe( ) is used. To produce the String for the logging message, we’d like to have the stack trace where the exception is thrown, but printStackTrace( ) doesn’t produce a String by default. To get a String, we need to use the overloaded printStackTrace( ) that takes a java.io.PrintWriter object as an argument (all of this will be fully explained in the I/O chapter). If we hand the Print Writer constructor a java.io.StringWriter object, the output can be extracted as a String by calling toString( ). Although the approach used by LoggingException is very convenient because it builds all the logging infrastructure into the exception itself, and thus it works automatically without client programmer intervention, it’s more common that you will be catching and logging someone else’s exception, so you must generate the log message in the exception handler: //: exceptions/LoggingExceptions2.java // Logging caught exceptions. import java.util.logging.*; import java.io.*; public class LoggingExceptions2 { private static Logger logger = Logger.getLogger("LoggingExceptions2"); static void logException(Exception e) { StringWriter trace = new StringWriter(); e.printStackTrace(new PrintWriter(trace)); logger.severe(trace.toString()); } public static void main(String[] args) { try { throw new NullPointerException(); } catch(NullPointerException e) { logException(e); } } } /* Output: (90% match) Aug 30, 2005 4:07:54 PM LoggingExceptions2 logException SEVERE: java.lang.NullPointerException at LoggingExceptions2.main(LoggingExceptions2.java:16) *///:~ The process of creating your own exceptions can be taken further. You can add extra constructors and members: 320 Thinking in Java Bruce Eckel //: exceptions/ExtraFeatures.java // Further embellishment of exception classes. import static net.mindview.util.Print.*; class MyException2 extends Exception { private int x; public MyException2() {} public MyException2(String msg) { super(msg); } public MyException2(String msg, int x) { super(msg); this.x = x; } public int val() { return x; } public String getMessage() { return "Detail Message: "+ x + " "+ super.getMessage(); } } public class ExtraFeatures { public static void f() throws MyException2 { print("Throwing MyException2 from f()"); throw new MyException2(); } public static void g() throws MyException2 { print("Throwing MyException2 from g()"); throw new MyException2("Originated in g()"); } public static void h() throws MyException2 { print("Throwing MyException2 from h()"); throw new MyException2("Originated in h()", 47); } public static void main(String[] args) { try { f(); } catch(MyException2 e) { e.printStackTrace(System.out); } try { g(); } catch(MyException2 e) { e.printStackTrace(System.out); } try { h(); } catch(MyException2 e) { e.printStackTrace(System.out); System.out.println("e.val() = " + e.val()); } } } /* Output: Throwing MyException2 from f() MyException2: Detail Message: 0 null at ExtraFeatures.f(ExtraFeatures.java:22) at ExtraFeatures.main(ExtraFeatures.java:34) Throwing MyException2 from g() MyException2: Detail Message: 0 Originated in g() at ExtraFeatures.g(ExtraFeatures.java:26) at ExtraFeatures.main(ExtraFeatures.java:39) Throwing MyException2 from h() MyException2: Detail Message: 47 Originated in h() at ExtraFeatures.h(ExtraFeatures.java:30) at ExtraFeatures.main(ExtraFeatures.java:44) e.val() = 47 Error Handling with Exceptions 321 *///:~ A field x has been added, along with a method that reads that value and an additional constructor that sets it. In addition, Throwable.getMessage( ) has been overridden to produce a more interesting detail message. getMessage( ) is something like toString( ) for exception classes. Since an exception is just another kind of object, you can continue this process of embellishing the power of your exception classes. Keep in mind, however, that all this dressing-up might be lost on the client programmers using your packages, since they might simply look for the exception to be thrown and nothing more. (That’s the way most of the Java library exceptions are used.) Exercise 6: (1) Create two exception classes, each of which performs its own logging automatically. Demonstrate that these work. Exercise 7: (1) Modify Exercise 3 so that the catch clause logs the results. The exception specification In Java, you’re encouraged to inform the client programmer, who calls your method, of the exceptions that might be thrown from your method. This is civilized, because the caller can then know exactly what code to write to catch all potential exceptions. Of course, if the source code is available, the client programmer could hunt through and look for throw statements, but a library might not come with sources. To prevent this from being a problem, Java provides syntax (and forces you to use that syntax) to allow you to politely tell the client programmer what exceptions this method throws, so the client programmer can handle them. This is the exception specification and it’s part of the method declaration, appearing after the argument list. The exception specification uses an additional keyword, throws, followed by a list of all the potential exception types. So your method definition might look like this: void f() throws TooBig, TooSmall, DivZero { //... However, if you say void f() { //... it means that no exceptions are thrown from the method {except for the exceptions inherited from RuntimeException, which can be thrown anywhere without exception specifications—these will be described later). You can’t lie about an exception specification. If the code within your method causes exceptions, but your method doesn’t handle them, the compiler will detect this and tell you that you must either handle the exception or indicate with an exception specification that it may be thrown from your method. By enforcing exception specifications from top to bottom, Java guarantees that a certain level of exception correctness can be ensured at compile time. There is one place you can lie: You can claim to throw an exception that you really don’t. The compiler takes your word for it, and forces the users of your method to treat it as if it really does throw that exception. This has the beneficial effect of being a placeholder for that exception, so you can actually start throwing the exception later without requiring changes to existing code. It’s also important for creating abstract base classes and interfaces whose derived classes or implementations may need to throw exceptions. 322 Thinking in Java Bruce Eckel Exceptions that are checked and enforced at compile time are called checked exceptions. Exercise 8: (1) Write a class with a method that throws an exception of the type created in Exercise 4. Try compiling it without an exception specification to see what the compiler says. Add the appropriate exception specification. Try out your class and its exception inside a try-catch clause. Catching any exception It is possible to create a handler that catches any type of exception. You do this by catching the base-class exception type Exception (there are other types of base exceptions, but Exception is the base that’s pertinent to virtually all programming activities): catch(Exception e) { System.out.println("Caught an exception"); } This will catch any exception, so if you use it you’ll want to put it at the end of your list of handlers to avoid preempting any exception handlers that might otherwise follow it. Since the Exception class is the base of all the exception classes that are important to the programmer, you don’t get much specific information about the exception, but you can call the methods that come from its base type Throwable: String getMessage( ) String getLocalizedMessage( ) Gets the detail message, or a message adjusted for this particular locale. String toString( ) Returns a short description of the Throwable, including the detail message if there is one. void printStackTrace( ) voidprintStackTrace(PrintStream) voidprintStackTrace(java.io.PrintWriter) Prints the Throwable and the Throwable’s call stack trace. The call stack shows the sequence of method calls that brought you to the point at which the exception was thrown. The first version prints to standard error, the second and third print to a stream of your choice (in the I/O chapter, you’ll understand why there are two types of streams). Throwable fillInStackTrace( ) Records information within this Throwable object about the current state of the stack frames. Useful when an application is rethrowing an error or exception (more about this shortly). In addition, you get some other methods from Throwable’s base type Object (everybody’s base type). The one that might come in handy for exceptions is getClass( ), which returns an object representing the class of this object. You can in turn query this Class object for its name with getName( ), which includes package information, or getSimpleName( ), which produces the class name alone. Here’s an example that shows the use of the basic Exception methods: //: exceptions/ExceptionMethods.java // Demonstrating the Exception Methods. import static net.mindview.util.Print.*; Error Handling with Exceptions 323 public class ExceptionMethods { public static void main(String[] args) { try { throw new Exception("My Exception"); } catch(Exception e) { print("Caught Exception"); print("getMessage():" + e.getMessage()); print("getLocalizedMessage():" + e.getLocalizedMessage()); print("toString():" + e); print("printStackTrace():"); e.printStackTrace(System.out); } } } /* Output: Caught Exception getMessage():My Exception getLocalizedMessage():My Exception toString():java.lang.Exception: My Exception printStackTrace(): java.lang.Exception: My Exception at ExceptionMethods.main(ExceptionMethods.java:8) *///:~ You can see that the methods provide successively more information—each is effectively a superset of the previous one. Exercise 9: (2) Create three new types of exceptions. Write a class with a method that throws all three. In main( ), call the method but only use a single catch clause that will catch all three types of exceptions. The stack trace The information provided by printStackTrace( ) can also be accessed directly using getStackTrace( ). This method returns an array of stack trace elements, each representing one stack frame. Element zero is the top of the stack, and is the last method invocation in the sequence (the point this Throwable was created and thrown). The last element of the array and the bottom of the stack is the first method invocation in the sequence. This program provides a simple demonstration: //: exceptions/WhoCalled.java // Programmatic access to stack trace information. public class WhoCalled { static void f() { // Generate an exception to fill in the stack trace try { throw new Exception(); } catch (Exception e) { for(StackTraceElement ste : e.getStackTrace()) System.out.println(ste.getMethodName()); } } static void g() { f(); } static void h() { g(); } public static void main(String[] args) { f(); System.out.println("--------------------------------"); g(); System.out.println("--------------------------------"); 324 Thinking in Java Bruce Eckel h(); } } /* Output: f main -------------------------------f g main -------------------------------f g h main *///:~ Here, we just print the method name, but you can also print the entire StackTraceElement, which contains additional information. Rethrowing an exception Sometimes you’ll want to rethrow the exception that you just caught, particularly when you use Exception to catch any exception. Since you already have the reference to the current exception, you can simply rethrow that reference: catch(Exception e) { System.out.println("An exception was thrown"); throw e; } Rethrowing an exception causes it to go to the exception handlers in the nexthigher context. Any further catch clauses for the same try block are still ignored. In addition, everything about the exception object is preserved, so the handler at the higher context that catches the specific exception type can extract all the information from that object. If you simply rethrow the current exception, the information that you print about that exception in printStackTrace( ) will pertain to the exception’s origin, not the place where you rethrow it. If you want to install new stack trace information, you can do so by calling fillInStackTrace( ), which returns a Throwable object that it creates by stuffing the current stack information into the old exception object. Here’s what it looks like: //: exceptions/Rethrowing.java // Demonstrating fillInStackTrace() public class Rethrowing { public static void f() throws Exception { System.out.println("originating the exception in f()"); throw new Exception("thrown from f()"); } public static void g() throws Exception { try { f(); } catch(Exception e) { System.out.println("Inside g(),e.printStackTrace()"); e.printStackTrace(System.out); throw e; } } public static void h() throws Exception { Error Handling with Exceptions 325 try { f(); } catch(Exception e) { System.out.println("Inside h(),e.printStackTrace()"); e.printStackTrace(System.out); throw (Exception)e.fillInStackTrace(); } } public static void main(String[] args) { try { g(); } catch(Exception e) { System.out.println("main: printStackTrace()"); e.printStackTrace(System.out); } try { h(); } catch(Exception e) { System.out.println("main: printStackTrace()"); e.printStackTrace(System.out); } } } /* Output: originating the exception in f() Inside g(),e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:7) at Rethrowing.g(Rethrowing.java:11) at Rethrowing.main(Rethrowing.java:29) main: printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:7) at Rethrowing.g(Rethrowing.java:11) at Rethrowing.main(Rethrowing.java:29) originating the exception in f() Inside h(),e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:7) at Rethrowing.h(Rethrowing.java:20) at Rethrowing.main(Rethrowing.java:35) main: printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.h(Rethrowing.java:24) at Rethrowing.main(Rethrowing.java:35) *///:~ The line where fillInStackTrace( ) is called becomes the new point of origin of the exception. It’s also possible to rethrow a different exception from the one you caught. If you do this, you get a similar effect as when you use fillInStackTrace( )— the information about the original site of the exception is lost, and what you’re left with is the information pertaining to the new throw: //: exceptions/RethrowNew.java // Rethrow a different object from the one that was caught. class OneException extends Exception { public OneException(String s) { super(s); } } class TwoException extends Exception { 326 Thinking in Java Bruce Eckel public TwoException(String s) { super(s); } } public class RethrowNew { public static void f() throws OneException { System.out.println("originating the exception in f()"); throw new OneException("thrown from f()"); } public static void main(String[] args) { try { try { f(); } catch(OneException e) { System.out.println( "Caught in inner try, e.printStackTrace()"); e.printStackTrace(System.out); throw new TwoException("from inner try"); } } catch(TwoException e) { System.out.println( "Caught in outer try, e.printStackTrace()"); e.printStackTrace(System.out); } } } /* Output: originating the exception in f() Caught in inner try, e.printStackTrace() OneException: thrown from f() at RethrowNew.f(RethrowNew.java:15) at RethrowNew.main(RethrowNew.java:20) Caught in outer try, e.printStackTrace() TwoException: from inner try at RethrowNew.main(RethrowNew.java:25) *///:~ The final exception knows only that it came from the inner try block and not from f( ). You never have to worry about cleaning up the previous exception, or any exceptions for that matter. They’re all heap-based objects created with new, so the garbage collector automatically cleans them all up. Exception chaining Often you want to catch one exception and throw another, but still keep the information about the originating exception—this is called exception chaining. Prior to JDK 1.4, programmers had to write their own code to preserve the original exception information, but now all Throwable subclasses have the option to take a cause object in their constructor. The cause is intended to be the originating exception, and by passing it in you maintain the stack trace back to its origin, even though you’re creating and throwing a new exception. It’s interesting to note that the only Throwable subclasses that provide the cause argument in the constructor are the three fundamental exception classes Error (used by the JVM to report system errors), Exception, and RuntimeException. If you want to chain any other exception types, you do it through the initCause( ) method rather than the constructor. Here’s an example that allows you to dynamically add fields to a DynamicFields object at run time: //: exceptions/DynamicFields.java Error Handling with Exceptions 327 // A Class that dynamically adds fields to itself. // Demonstrates exception chaining. import static net.mindview.util.Print.*; class DynamicFieldsException extends Exception {} public class DynamicFields { private Object[][] fields; public DynamicFields(int initialSize) { fields = new Object[initialSize][2]; for(int i = 0; i < initialSize; i++) fields[i] = new Object[] { null, null }; } public String toString() { StringBuilder result = new StringBuilder(); for(Object[] obj : fields) { result.append(obj[0]); result.append(": "); result.append(obj[1]); result.append("\n"); } return result.toString(); } private int hasField(String id) { for(int i = 0; i < fields.length; i++) if(id.equals(fields[i][0])) return i; return -1; } private int getFieldNumber(String id) throws NoSuchFieldException { int fieldNum = hasField(id); if(fieldNum == -1) throw new NoSuchFieldException(); return fieldNum; } private int makeField(String id) { for(int i = 0; i < fields.length; i++) if(fields[i][0] == null) { fields[i][0] = id; return i; } // No empty fields. Add one: Object[][] tmp = new Object[fields.length + 1][2]; for(int i = 0; i < fields.length; i++) tmp[i] = fields[i]; for(int i = fields.length; i < tmp.length; i++) tmp[i] = new Object[] { null, null }; fields = tmp; // Recursive call with expanded fields: return makeField(id); } public Object getField(String id) throws NoSuchFieldException { return fields[getFieldNumber(id)][1]; } public Object setField(String id, Object value) throws DynamicFieldsException { if(value == null) { // Most exceptions don’t have a "cause" constructor. // In these cases you must use initCause(), // available in all Throwable subclasses. DynamicFieldsException dfe = 328 Thinking in Java Bruce Eckel new DynamicFieldsException(); dfe.initCause(new NullPointerException()); throw dfe; } int fieldNumber = hasField(id); if(fieldNumber == -1) fieldNumber = makeField(id); Object result = null; try { result = getField(id); // Get old value } catch(NoSuchFieldException e) { // Use constructor that takes "cause": throw new RuntimeException(e); } fields[fieldNumber][1] = value; return result; } public static void main(String[] args) { DynamicFields df = new DynamicFields(3); print(df); try { df.setField("d", "A value for d"); df.setField("number", 47); df.setField("number2", 48); print(df); df.setField("d", "A new value for d"); df.setField("number3", 11); print("df: " + df); print("df.getField(\"d\") : " + df.getField("d")); Object field = df.setField("d", null); // Exception } catch(NoSuchFieldException e) { e.printStackTrace(System.out); } catch(DynamicFieldsException e) { e.printStackTrace(System.out); } } } /* Output: null: null null: null null: null d: A value for d number: 47 number2: 48 df: d: A new value for d number: 47 number2: 48 number3: 11 df.getField("d") : A new value for d DynamicFieldsException at DynamicFields.setField(DynamicFields.java:64) at DynamicFields.main(DynamicFields.java:94) Caused by: java.lang.NullPointerException at DynamicFields.setField(DynamicFields.java:66) ... 1 more *///:~ Each DynamicFields object contains an array of Object-Object pairs. The first object is the field identifier (a String), and the second is the field value, which can be any type except an unwrapped primitive. When you create the object, you make an educated guess about how many fields you need. When you call setField( ), it either finds the existing field by that Error Handling with Exceptions 329 name or creates a new one, and puts in your value. If it runs out of space, it adds new space by creating an array of length one longer and copying the old elements in. If you try to put in a null value, then it throws a DynamicFieldsException by creating one and using initCause( ) to insert a NullPointerException as the cause. As a return value, setField( ) also fetches out the old value at that field location using getField( ), which could throw a NoSuchFieldException. If the client programmer calls getField( ), then they are responsible for handling NoSuchFieldException, but if this exception is thrown inside setField( ), it’s a programming error, so the NoSuchFieldException is converted to a RuntimeException using the constructor that takes a cause argument. You’ll notice that toString( ) uses a StringBuilder to create its result. You’ll learn more about StringBuilder in the Strings chapter, but in general you’ll want to use it whenever you’re writing a toString( ) that involves looping, as is the case here. Exercise 10: (2) Create a class with two methods, f( ) and g( ). In g( ), throw an exception of a new type that you define. In f( ), call g( ), catch its exception and, in the catch clause, throw a different exception (of a second type that you define). Test your code in main( ). Exercise 11: (1) Repeat the previous exercise, but inside the catch clause, wrap g( )’s exception in a RuntimeException. Standard Java exceptions The Java class Throwable describes anything that can be thrown as an exception. There are two general types of Throwable objects ("types of = "inherited from"). Error represents compile-time and system errors that you don’t worry about catching (except in very special cases). Exception is the basic type that can be thrown from any of the standard Java library class methods and from your methods and runtime accidents. So the Java programmer’s base type of interest is usually Exception. The best way to get an overview of the exceptions is to browse the JDK documentation. It’s worth doing this once just to get a feel for the various exceptions, but you’ll soon see that there isn’t anything special between one exception and the next except for the name. Also, the number of exceptions in Java keeps expanding; basically, it’s pointless to print them in a book. Any new library you get from a third-party vendor will probably have its own exceptions as well. The important thing to understand is the concept and what you should do with the exceptions. The basic idea is that the name of the exception represents the problem that occurred, and the exception name is intended to be relatively selfexplanatory. The exceptions are not all defined in java.lang; some are created to support other libraries such as util, net, and io, which you can see from their full class names or what they are inherited from. For example, all I/O exceptions are inherited from java.io.IOException. Special case: RuntimeException The first example in this chapter was if(t == null) throw new NullPointerException(); 330 Thinking in Java Bruce Eckel It can be a bit horrifying to think that you must check for null on every reference that is passed into a method (since you can’t know if the caller has passed you a valid reference). Fortunately, you don’t—this is part of the standard runtime checking that Java performs for you, and if any call is made to a null reference, Java will automatically throw a NullPointerException. So the above bit of code is always superfluous, although you may want to perform other checks in order to guard against the appearance of a NullPointerException. There’s a whole group of exception types that are in this category. They’re always thrown automatically by Java and you don’t need to include them in your exception specifications. Conveniently enough, they’re all grouped together by putting them under a single base class called RuntimeException, which is a perfect example of inheritance: It establishes a family of types that have some characteristics and behaviors in common. Also, you never need to write an exception specification saying that a method might throw a RuntimeException (or any type inherited from RuntimeException), because they are unchecked exceptions. Because they indicate bugs, you don’t usually catch a RuntimeException—it’s dealt with automatically. If you were forced to check for RuntimeExceptions, your code could get too messy. Even though you don’t typically catch RuntimeExceptions, in your own packages you might choose to throw some of the RuntimeExceptions. What happens when you don’t catch such exceptions? Since the compiler doesn’t enforce exception specifications for these, it’s quite plausible that a RuntimeException could percolate all the way out to your main( ) method without being caught. To see what happens in this case, try the following example: //: exceptions/NeverCaught.java // Ignoring RuntimeExceptions. // {ThrowsException} public class NeverCaught { static void f() { throw new RuntimeException("From f()"); } static void g() { f(); } public static void main(String[] args) { g(); } } ///:~ You can already see that a RuntimeException (or anything inherited from it) is a special case, since the compiler doesn’t require an exception specification for these types. The output is reported to System.err: Exception in thread "main" Java.lang.RuntimeException: From f() at NeverCaught.f(NeverCaught.Java:7) at NeverCaught.g(NeverCaught.Java:10) at NeverCaught.main(NeverCaught.Java:13) So the answer is: If a RuntimeException gets all the way out to main( ) without being caught, printStackTrace( ) is called for that exception as the program exits. Keep in mind that only exceptions of type RuntimeException (and subclasses) can be ignored in your coding, since the compiler carefully enforces the handling of all checked exceptions. The reasoning is that a RuntimeException represents a programming error, which is: Error Handling with Exceptions 331 1. An error you cannot anticipate. For example, a null reference that is outside of your control. 2. An error that you, as a programmer, should have checked for in your code (such as ArraylndexOutOfBoundsException where you should have paid attention to the size of the array). An exception that happens from point #1 often becomes an issue for point #2. You can see what a tremendous benefit it is to have exceptions in this case, since they help in the debugging process. It’s interesting to notice that you cannot classify Java exception handling as a single-purpose tool. Yes, it is designed to handle those pesky runtime errors that will occur because of forces outside your code’s control, but it’s also essential for certain types of programming bugs that the compiler cannot detect. Exercise 12: (3) Modify innerclasses/Sequence.java so that it throws an appropriate exception if you try to put in too many elements. Performing cleanup with finally There’s often some piece of code that you want to execute whether or not an exception is thrown within a try block. This usually pertains to some operation other than memory recovery (since that’s taken care of by the garbage collector). To achieve this effect, you use a finally clause4 at the end of all the exception handlers. The full picture of an exceptionhandling section is thus: try { // The guarded region: Dangerous activities // that might throw A, B, or C } catch(A a1) { // Handler for situation A } catch(B b1) { // Handler for situation B } catch(C c1) { // Handler for situation C } finally { // Activities that happen every time } To demonstrate that the finally clause always runs, try this program: //: exceptions/FinallyWorks.java // The finally clause is always executed. class ThreeException extends Exception {} public class FinallyWorks { static int count = 0; public static void main(String[] args) { while(true) { try { 4 C++ exception handling does not have the finally clause because it relies on destructors to accomplish this sort of cleanup. 332 Thinking in Java Bruce Eckel // Post-increment is zero first time: if(count++ == 0) throw new ThreeException(); System.out.println("No exception"); } catch(ThreeException e) { System.out.println("ThreeException"); } finally { System.out.println("In finally clause"); if(count == 2) break; // out of "while" } } } } /* Output: ThreeException In finally clause No exception In finally clause *///:~ From the output, you can see that the finally clause is executed whether or not an exception is thrown. This program also gives a hint for how you can deal with the fact that exceptions in Java do not allow you to resume back to where the exception was thrown, as discussed earlier. If you place your try block in a loop, you can establish a condition that must be met before you continue the program. You can also add a static counter or some other device to allow the loop to try several different approaches before giving up. This way you can build a greater level of robustness into your programs. What’s finally for? In a language without garbage collection and without automatic destructor calls,5 finally is important because it allows the programmer to guarantee the release of memory regardless of what happens in the try block. But Java has garbage collection, so releasing memory is virtually never a problem. Also, it has no destructors to call. So when do you need to use finally in Java? The finally clause is necessary when you need to set something other than memory back to its original state. This is some kind of cleanup like an open file or network connection, something you’ve drawn on the screen, or even a switch in the outside world, as modeled in the following example: //: exceptions/Switch.java import static net.mindview.util.Print.*; public class Switch { private boolean state = false; public boolean read() { return state; } public void on() { state = true; print(this); } public void off() { state = false; print(this); } public String toString() { return state ? "on" : "off"; } } ///:~ //: exceptions/OnOffException1.java public class OnOffException1 extends Exception {} ///:~ 5 A destructor is a function that’s always called when an object becomes unused. You always know exactly where and when the destructor gets called. C++ has automatic destructor calls, and C# (which is much more like Java) has a way that automatic destruction can occur. Error Handling with Exceptions 333 //: exceptions/OnOffException2.java public class OnOffException2 extends Exception {} ///:~ //: exceptions/OnOffSwitch.java // Why use finally? public class OnOffSwitch { private static Switch sw = new Switch(); public static void f() throws OnOffException1,OnOffException2 {} public static void main(String[] args) { try { sw.on(); // Code that can throw exceptions... f(); sw.off(); } catch(OnOffException1 e) { System.out.println("OnOffException1"); sw.off(); } catch(OnOffException2 e) { System.out.println("OnOffException2"); sw.off(); } } } /* Output: on off *///:~ The goal here is to make sure that the switch is off when main( ) is completed, so sw.off( ) is placed at the end of the try block and at the end of each exception handler. But it’s possible that an exception might be thrown that isn’t caught here, so sw.off( ) would be missed. However, with finally you can place the cleanup code from a try block in just one place: //: exceptions/WithFinally.java // Finally Guarantees cleanup. public class WithFinally { static Switch sw = new Switch(); public static void main(String[] args) { try { sw.on(); // Code that can throw exceptions... OnOffSwitch.f(); } catch(OnOffException1 e) { System.out.println("OnOffException1"); } catch(OnOffException2 e) { System.out.println("OnOffException2"); } finally { sw.off(); } } } /* Output: on off *///:~ Here the sw.off( ) has been moved to just one place, where it’s guaranteed to run no matter what happens. 334 Thinking in Java Bruce Eckel Even in cases in which the exception is not caught in the current set of catch clauses, finally will be executed before the exception-handling mechanism continues its search for a handler at the next higher level: //: exceptions/AlwaysFinally.java // Finally is always executed. import static net.mindview.util.Print.*; class FourException extends Exception {} public class AlwaysFinally { public static void main(String[] args) { print("Entering first try block"); try { print("Entering second try block"); try { throw new FourException(); } finally { print("finally in 2nd try block"); } } catch(FourException e) { System.out.println( "Caught FourException in 1st try block"); } finally { System.out.println("finally in 1st try block"); } } } /* Output: Entering first try block Entering second try block finally in 2nd try block Caught FourException in 1st try block finally in 1st try block *///:~ The finally statement will also be executed in situations in which break and continue statements are involved. Note that, along with the labeled break and labeled continue, finally eliminates the need for a goto statement in Java. Exercise 13: (2) Modify Exercise 9 by adding a finally clause. Verify that your finally clause is executed, even if a NullPointerException is thrown. Exercise 14: (2) Show that OnOffSwitch.java can fail by throwing a RuntimeException inside the try block. Exercise 15: (2) Show that WithFinally.java doesn’t fail by throwing a RuntimeException inside the try block. Using finally during return Because a finally clause is always executed, it’s possible to return from multiple points within a method and still guarantee that important cleanup will be performed: //: exceptions/MultipleReturns.java import static net.mindview.util.Print.*; public class MultipleReturns { public static void f(int i) { print("Initialization that requires cleanup"); Error Handling with Exceptions 335 try { print("Point 1"); if(i == 1) return; print("Point 2"); if(i == 2) return; print("Point 3"); if(i == 3) return; print("End"); return; } finally { print("Performing cleanup"); } } public static void main(String[] args) { for(int i = 1; i <= 4; i++) f(i); } } /* Output: Initialization that requires cleanup Point 1 Performing cleanup Initialization that requires cleanup Point 1 Point 2 Performing cleanup Initialization that requires cleanup Point 1 Point 2 Point 3 Performing cleanup Initialization that requires cleanup Point 1 Point 2 Point 3 End Performing cleanup *///:~ You can see from the output that it doesn’t matter where you return from inside the finally class. Exercise 16: (2) Modify reusing/CADSystem.java to demonstrate that returning from the middle of a try-finally will still perform proper cleanup. Exercise 17: (3) Modify polymorphism/Frog.java so that it uses try-finally to guarantee proper cleanup, and show that this works even if you return from the middle of the try-finally. Pitfall: the lost exception Unfortunately, there’s a flaw in Java’s exception implementation. Although exceptions are an indication of a crisis in your program and should never be ignored, it’s possible for an exception to simply be lost. This happens with a particular configuration using a finally clause: //: exceptions/LostMessage.java // How an exception can be lost. class VeryImportantException extends Exception { public String toString() { 336 Thinking in Java Bruce Eckel return "A very important exception!"; } } class HoHumException extends Exception { public String toString() { return "A trivial exception"; } } public class LostMessage { void f() throws VeryImportantException { throw new VeryImportantException(); } void dispose() throws HoHumException { throw new HoHumException(); } public static void main(String[] args) { try { LostMessage lm = new LostMessage(); try { lm.f(); } finally { lm.dispose(); } } catch(Exception e) { System.out.println(e); } } } /* Output: A trivial exception *///:~ You can see from the output that there’s no evidence of the VerylmportantException, which is simply replaced by the HoHumException in the finally clause. This is a rather serious pitfall, since it means that an exception can be completely lost, and in a far more subtle and difficult-to-detect fashion than the preceding example. In contrast, C++ treats the situation in which a second exception is thrown before the first one is handled as a dire programming error. Perhaps a future version of Java will repair this problem (on the other hand, you will typically wrap any method that throws an exception, such as dispose( ) in the example above, inside a try-catch clause). An even simpler way to lose an exception is just to return from inside a finally clause: //: exceptions/ExceptionSilencer.java public class ExceptionSilencer { public static void main(String[] args) { try { throw new RuntimeException(); } finally { // Using ‘return’ inside the finally block // will silence any thrown exception. return; } } } ///:~ If you run this program you’ll see that it produces no output, even though an exception is thrown. Error Handling with Exceptions 337 Exercise 18: (3) Add a second level of exception loss to LostMessage.java so that the HoHumException is itself replaced by a third exception. Exercise 19: (2) Repair the problem in LostMessage.java by guarding the call in the finally clause. Exception restrictions When you override a method, you can throw only the exceptions that have been specified in the base-class version of the method. This is a useful restriction, since it means that code that works with the base class will automatically work with any object derived from the base class (a fundamental OOP concept, of course), including exceptions. This example demonstrates the kinds of restrictions imposed (at compile time) for exceptions: //: exceptions/StormyInning.java // Overridden methods may throw only the exceptions // specified in their base-class versions, or exceptions // derived from the base-class exceptions. class BaseballException extends Exception {} class Foul extends BaseballException {} class Strike extends BaseballException {} abstract class Inning { public Inning() throws BaseballException {} public void event() throws BaseballException { // Doesn’t actually have to throw anything } public abstract void atBat() throws Strike, Foul; public void walk() {} // Throws no checked exceptions } class StormException extends Exception {} class RainedOut extends StormException {} class PopFoul extends Foul {} interface Storm { public void event() throws RainedOut; public void rainHard() throws RainedOut; } public class StormyInning extends Inning implements Storm { // OK to add new exceptions for constructors, but you // must deal with the base constructor exceptions: public StormyInning() throws RainedOut, BaseballException {} public StormyInning(String s) throws Foul, BaseballException {} // Regular methods must conform to base class: //! void walk() throws PopFoul {} //Compile error // Interface CANNOT add exceptions to existing // methods from the base class: //! public void event() throws RainedOut {} // If the method doesn’t already exist in the // base class, the exception is OK: public void rainHard() throws RainedOut {} // You can choose to not throw any exceptions, 338 Thinking in Java Bruce Eckel // even if the base version does: public void event() {} // Overridden methods can throw inherited exceptions: public void atBat() throws PopFoul {} public static void main(String[] args) { try { StormyInning si = new StormyInning(); si.atBat(); } catch(PopFoul e) { System.out.println("Pop foul"); } catch(RainedOut e) { System.out.println("Rained out"); } catch(BaseballException e) { System.out.println("Generic baseball exception"); } // Strike not thrown in derived version. try { // What happens if you upcast? Inning i = new StormyInning(); i.atBat(); // You must catch the exceptions from the // base-class version of the method: } catch(Strike e) { System.out.println("Strike"); } catch(Foul e) { System.out.println("Foul"); } catch(RainedOut e) { System.out.println("Rained out"); } catch(BaseballException e) { System.out.println("Generic baseball exception"); } } } ///:~ In Inning, you can see that both the constructor and the event( ) method say that they will throw an exception, but they never do. This is legal because it allows you to force the user to catch any exceptions that might be added in overridden versions of event( ). The same idea holds for abstract methods, as seen in atBat( ). The interface Storm is interesting because it contains one method (event( )) that is defined in Inning, and one method that isn’t. Both methods throw a new type of exception, RainedOut. When Stormylnning extends Inning and implements Storm, you’ll see that the event( ) method in Storm cannot change the exception interface of event( ) in Inning. Again, this makes sense because otherwise you’d never know if you were catching the correct thing when working with the base class. Of course, if a method described in an interface is not in the base class, such as rainHard( ), then there’s no problem if it throws exceptions. The restriction on exceptions does not apply to constructors. You can see in Stormylnning that a constructor can throw anything it wants, regardless of what the base-class constructor throws. However, since a base-class constructor must always be called one way or another (here, the default constructor is called automatically), the derived-class constructor must declare any base-class constructor exceptions in its exception specification. A derived-class constructor cannot catch exceptions thrown by its base-class constructor. The reason StormyInning.walk( ) will not compile is that it throws an exception, but Inning.walk( ) does not. If this were allowed, then you could write code that called Inning.walk( ) and that didn’t have to handle any exceptions, but then when you substituted an object of a class derived from Inning, exceptions would be thrown so your Error Handling with Exceptions 339 code would break. By forcing the derived-class methods to conform to the exception specifications of the base-class methods, substitutability of objects is maintained. The overridden event( ) method shows that a derived-class version of a method may choose not to throw any exceptions, even if the base-class version does. Again, this is fine since it doesn’t break code that is written assuming the base-class version throws exceptions. Similar logic applies to atBat( ), which throws PopFoul, an exception that is derived from Foul thrown by the base-class version of atBat( ). This way, if you write code that works with Inning and calls atBat( ), you must catch the Foul exception. Since PopFoul is derived from Foul, the exception handler will also catch PopFoul. The last point of interest is in main( ). Here, you can see that if you’re dealing with exactly a StormyInning object, the compiler forces you to catch only the exceptions that are specific to that class, but if you upcast to the base type, then the compiler (correctly) forces you to catch the exceptions for the base type. All these constraints produce much more robust exceptionhandling code.6 Although exception specifications are enforced by the compiler during inheritance, the exception specifications are not part of the type of a method, which comprises only the method name and argument types. Therefore, you cannot overload methods based on exception specifications. In addition, just because an exception specification exists in a baseclass version of a method doesn’t mean that it must exist in the derived-class version of the method. This is quite different from inheritance rules, where a method in the base class must also exist in the derived class. Put another way, the "exception specification interface" for a particular method may narrow during inheritance and overriding, but it may not widen—this is precisely the opposite of the rule for the class interface during inheritance. Exercise 20: (3) Modify StormyInning.java by adding an UmpireArgument exception type and methods that throw this exception. Test the modified hierarchy. Constructors It’s important that you always ask, "If an exception occurs, will everything be properly cleaned up?" Most of the time you’re fairly safe, but with constructors there’s a problem. The constructor puts the object into a safe starting state, but it might perform some operation— such as opening a filethat doesn’t get cleaned up until the user is finished with the object and calls a special cleanup method. If you throw an exception from inside a constructor, these cleanup behaviors might not occur properly. This means that you must be especially diligent while you write your constructor. You might think that finally is the solution. But it’s not quite that simple, because finally performs the cleanup code every time. If a constructor fails partway through its execution, it might not have successfully created some part of the object that will be cleaned up in the finally clause. In the following example, a class called InputFile is created that opens a file and allows you to read it one line at a time. It uses the classes FileReader and BufferedReader from the Java standard I/O library that will be discussed in the I/O chapter. These classes are simple enough that you probably won’t have any trouble understanding their basic use: //: exceptions/InputFile.java // Paying attention to exceptions in constructors. import java.io.*; 6 ISO C++ added similar constraints that require derived-method exceptions to be the same as, or derived from, the exceptions thrown by the base-class method. This is one case in which C++ is actually able to check exception specifications at compile time. 340 Thinking in Java Bruce Eckel public class InputFile { private BufferedReader in; public InputFile(String fname) throws Exception { try { in = new BufferedReader(new FileReader(fname)); // Other code that might throw exceptions } catch(FileNotFoundException e) { System.out.println("Could not open " + fname); // Wasn’t open, so don’t close it throw e; } catch(Exception e) { // All other exceptions must close it try { in.close(); } catch(IOException e2) { System.out.println("in.close() unsuccessful"); } throw e; // Rethrow } finally { // Don’t close it here!!! } } public String getLine() { String s; try { s = in.readLine(); } catch(IOException e) { throw new RuntimeException("readLine() failed"); } return s; } public void dispose() { try { in.close(); System.out.println("dispose() successful"); } catch(IOException e2) { throw new RuntimeException("in.close() failed"); } } } ///:~ The constructor for InputFile takes a String argument, which is the name of the file you want to open. Inside a try block, it creates a FileReader using the file name. A FileReader isn’t particularly useful until you use it to create a BufferedReader. One of the benefits of InputFile is that it combines these two actions. If the FileReader constructor is unsuccessful, it throws a FileNotFoundException. This is the one case in which you don’t want to close the file, because it wasn’t successfully opened. Any other catch clauses must close the file because it was opened by the time those catch clauses are entered. (Of course, this gets trickier if more than one method can throw a FileNotFoundException. In that case, you’ll usually have to break things into several try blocks.) The close( ) method might throw an exception so it is tried and caught even though it’s within the block of another catch clause—it’s just another pair of curly braces to the Java compiler. After performing local operations, the exception is rethrown, which is appropriate because this constructor failed, and you don’t want the calling method to assume that the object has been properly created and is valid. In this example, the finally clause is definitely not the place to close( ) the file, since that would close it every time the constructor completed. We want the file to be open for the useful lifetime of the InputFile object. Error Handling with Exceptions 341 The getLine( ) method returns a String containing the next line in the file. It calls readLine( ), which can throw an exception, but that exception is caught so that getLine( ) doesn’t throw any exceptions. One of the design issues with exceptions is whether to handle an exception completely at this level, to handle it partially and pass the same exception (or a different one) on, or whether to simply pass it on. Passing it on, when appropriate, can certainly simplify coding. In this situation, the getLine( ) method converts the exception to a RuntimeException to indicate a programming error. The dispose( ) method must be called by the user when the InputFile object is no longer needed. This will release the system resources (such as file handles) that are used by the BufferedReader and/or FileReader objects. You don’t want to do this until you’re finished with the InputFile object. You might think of putting such functionality into a finalize( ) method, but as mentioned in the Initialization & Cleanup chapter, you can’t always be sure that finalize( ) will be called (even if you can be sure that it will be called, you don’t know when). This is one of the downsides to Java: All cleanupother than memory cleanup—doesn’t happen automatically, so you must inform the client programmers that they are responsible. The safest way to use a class which might throw an exception during construction and which requires cleanup is to use nested try blocks: //: exceptions/Cleanup.java // Guaranteeing proper cleanup of a resource. public class Cleanup { public static void main(String[] args) { try { InputFile in = new InputFile("Cleanup.java"); try { String s; int i = 1; while((s = in.getLine()) != null) ; // Perform line-by-line processing here... } catch(Exception e) { System.out.println("Caught Exception in main"); e.printStackTrace(System.out); } finally { in.dispose(); } } catch(Exception e) { System.out.println("InputFile construction failed"); } } } /* Output: dispose() successful *///:~ Look carefully at the logic here: The construction of the InputFile object is effectively in its own try block. If that construction fails, the outer catch clause is entered and dispose( ) is not called. However, if construction succeeds then you want to make sure the object is cleaned up, so immediately after construction you create a new try block. The finally that performs cleanup is associated with the inner try block; this way, the finally clause is not executed if construction fails, and it is always executed if construction succeeds. This general cleanup idiom should still be used if the constructor throws no exceptions. The basic rule is: Right after you create an object that requires cleanup, begin a try-finally: //: exceptions/CleanupIdiom.java // Each disposable object must be followed by a try-finally 342 Thinking in Java Bruce Eckel class NeedsCleanup { // Construction can’t fail private static long counter = 1; private final long id = counter++; public void dispose() { System.out.println("NeedsCleanup " + id + " disposed"); } } class ConstructionException extends Exception {} class NeedsCleanup2 extends NeedsCleanup { // Construction can fail: public NeedsCleanup2() throws ConstructionException {} } public class CleanupIdiom { public static void main(String[] args) { // Section 1: NeedsCleanup nc1 = new NeedsCleanup(); try { // ... } finally { nc1.dispose(); } // Section 2: // If construction cannot fail you can group objects: NeedsCleanup nc2 = new NeedsCleanup(); NeedsCleanup nc3 = new NeedsCleanup(); try { // ... } finally { nc3.dispose(); // Reverse order of construction nc2.dispose(); } // Section 3: // If construction can fail you must guard each one: try { NeedsCleanup2 nc4 = new NeedsCleanup2(); try { NeedsCleanup2 nc5 = new NeedsCleanup2(); try { // ... } finally { nc5.dispose(); } } catch(ConstructionException e) { // nc5 constructor System.out.println(e); } finally { nc4.dispose(); } } catch(ConstructionException e) { // nc4 constructor System.out.println(e); } } } /* Output: NeedsCleanup 1 disposed NeedsCleanup 3 disposed NeedsCleanup 2 disposed NeedsCleanup 5 disposed NeedsCleanup 4 disposed *///:~ Error Handling with Exceptions 343 In main( ), section 1 is fairly straightforward: You follow a disposable object with a tryfinally. If the object construction cannot fail, no catch is necessary. In section 2, you can see that objects with constructors that cannot fail can be grouped together for both construction and cleanup. Section 3 shows how to deal with objects whose constructors can fail and which need cleanup. To properly handle this situation, things get messy, because you must surround each construction with its own try-catch, and each object construction must be followed by a try-finally to guarantee cleanup. The messiness of exception handling in this case is a strong argument for creating constructors that cannot fail, although this is not always possible. Note that if dispose( ) can throw an exception you might need additional try blocks. Basically, you must think carefully about all the possibilities and guard for each one. Exercise 21: (2) Demonstrate that a derived-class constructor cannot catch exceptions thrown by its base-class constructor. Exercise 22: (2) Create a class called FailingConstructor with a constructor that might fail partway through the construction process and throw an exception. In main( ), write code that properly guards against this failure. Exercise 23: (4) Add a class with a dispose( ) method to the previous exercise. Modify FailingConstructor so that the constructor creates one of these disposable objects as a member object, after which the constructor might throw an exception, after which it creates a second disposable member object. Write code to properly guard against failure, and in main( ) verify that all possible failure situations are covered. Exercise 24: (3) Add a dispose( ) method to the FailingConstructor class and write code to properly use this class. Exception matching When an exception is thrown, the exception-handling system looks through the "nearest" handlers in the order they are written. When it finds a match, the exception is considered handled, and no further searching occurs. Matching an exception doesn’t require a perfect match between the exception and its handler. A derived-class object will match a handler for the base class, as shown in this example: //: exceptions/Human.java // Catching exception hierarchies. class Annoyance extends Exception {} class Sneeze extends Annoyance {} public class Human { public static void main(String[] args) { // Catch the exact type: try { throw new Sneeze(); } catch(Sneeze s) { System.out.println("Caught Sneeze"); } catch(Annoyance a) { System.out.println("Caught Annoyance"); 344 Thinking in Java Bruce Eckel } // Catch the base type: try { throw new Sneeze(); } catch(Annoyance a) { System.out.println("Caught Annoyance"); } } } /* Output: Caught Sneeze Caught Annoyance *///:~ The Sneeze exception will be caught by the first catch clause that it matches, which is the first one, of course. However, if you remove the first catch clause, leaving only the catch clause for Annoyance, the code still works because it’s catching the base class of Sneeze. Put another way, catch(Annoyance a) will catch an Annoyance or any class derived from it. This is useful because if you decide to add more derived exceptions to a method, then the client programmer’s code will not need changing as long as the client catches the baseclass exceptions. If you try to "mask" the derived-class exceptions by putting the base-class catch clause first, like this: try { throw new Sneeze(); } catch(Annoyance a) { // ... } catch(Sneeze s) { // ... } the compiler will give you an error message, since it sees that the Sneeze catch clause can never be reached. Exercise 25: (2) Create a three-level hierarchy of exceptions. Now create a base-class A with a method that throws an exception at the base of your hierarchy. Inherit B from A and override the method so it throws an exception at level two of your hierarchy. Repeat by inheriting class C from B. In main( ), create a C and upcast it to A, then call the method. Alternative approaches An exception-handling system is a trapdoor that allows your program to abandon execution of the normal sequence of statements. The trapdoor is used when an "exceptional condition" occurs, such that normal execution is no longer possible or desirable. Exceptions represent conditions that the current method is unable to handle. The reason exception-handling systems were developed is because the approach of dealing with each possible error condition produced by each function call was too onerous, and programmers simply weren’t doing it. As a result, they were ignoring the errors. It’s worth observing that the issue of programmer convenience in handling errors was a prime motivation for exceptions in the first place. One of the important guidelines in exception handling is "Don’t catch an exception unless you know what to do with it." In fact, one of the important goals of exception handling is to move the error-handling code away from the point where the errors occur. This allows you to focus on what you want to accomplish in one section of your code, and how you’re going to deal with problems in a distinct separate section of your code. As a result, your mainline code is not cluttered with error-handling logic, and it’s much easier to understand and maintain. Error Handling with Exceptions 345 Exception handling also tends to reduce the amount of error-handling code, by allowing one handler to deal with many error sites. Checked exceptions complicate this scenario a bit, because they force you to add catch clauses in places where you may not be ready to handle an error. This results in the "harmful if swallowed" problem: try { // ... to do something useful } catch(ObligatoryException e) {} // Gulp! Programmers (myself included, in the 1st edition of this book) would just do the simplest thing, and "swallow" the exception—often unintentionally, but once you do it, the compiler has been satisfied, so unless you remember to revisit and correct the code, the exception will be lost. The exception happens, but it vanishes completely when swallowed. Because the compiler forces you to write code right away to handle the exception, this seems like the easiest solution even though it’s probably the worst thing you can do. Horrified upon realizing that I had done this, in the 2nd edition I "fixed" the problem by printing the stack trace inside the handler (as is still seen— appropriately—in a number of examples in this chapter). While this is useful to trace the behavior of exceptions, it still indicates that you don’t really know what to do with the exception at that point in your code. In this section you’ll learn about some of the issues and complications arising from checked exceptions, and options that you have when dealing with them. This topic seems simple. But it is not only complicated, it is also an issue of some volatility. There are people who are staunchly rooted on either side of the fence and who feel that the correct answer (theirs) is blatantly obvious. I believe the reason for one of these positions is the distinct benefit seen in going from a poorly typed language like pre-ANSI C to a strong, statically typed language (that is, checked at compile time) like C++ or Java. When you make that transition (as I did), the benefits are so dramatic that it can seem like static type checking is always the best answer to most problems. My hope is to relate a little bit of my own evolution that has brought the absolute value of static type checking into question; clearly, it’s very helpful much of the time, but there’s a fuzzy line we cross when it begins to get in the way and become a hindrance (one of my favorite quotes is "All models are wrong. Some are useful."). History Exception handling originated in systems like PL/1 and Mesa, and later appeared in CLU, Smalltalk, Modula-3, Ada, Eiffel, C++, Python, Java, and the post-Java languages Ruby and C#. The Java design is similar to C++, except in places where the Java designers felt that the C++ approach caused problems. To provide programmers with a framework that they were more likely to use for error handling and recovery, exception handling was added to C++ rather late in the standardization process, promoted by Bjarne Stroustrup, the language’s original author. The model for C++ exceptions came primarily from CLU. However, other languages existed at that time that also supported exception handling: Ada, Smalltalk (both of these had exceptions but no exception specifications) and Modula-3 (which included both exceptions and specifications). In their seminal paper7 on the subject, Liskov and Snyder observe that a major defect of languages like C, which report errors in a transient fashion, is that: 7 Barbara Liskov and Alan Snyder, Exception Handling in CLU, IEEE Transactions on Software Engineering, Vol. SE-5, No. 6, November 1979. This paper is not available on the Internet, only in print form, so you’ll have to contact a library to get a copy. 346 Thinking in Java Bruce Eckel "...every invocation must be followed by a conditional test to determine what the outcome was. This requirement leads to programs that are difficult to read, and probably inefficient as well, thus discouraging programmers from signaling and handling exceptions." Thus one of the original motivations of exception handling was to prevent this requirement, but with checked exceptions in Java we commonly see exactly this kind of code. They go on to say: "...requiring that the text of a handler be attached to the invocation that raises the exception would lead to unreadable programs in which expressions were broken up with handlers." Following the CLU approach when designing C++ exceptions, Stroustrup stated that the goal was to reduce the amount of code required to recover from errors. I believe that he was observing that programmers were typically not writing error-handling code in C because the amount and placement of such code was daunting and distracting. As a result, they were used to doing it the C way, ignoring errors in code and using debuggers to track down problems. To use exceptions, these C programmers had to be convinced to write "additional" code that they weren’t normally writing. Thus, to draw them into a better way of handling errors, the amount of code they would need to "add" must not be onerous. I think it’s important to keep this goal in mind when looking at the effects of checked exceptions in Java. C++ brought an additional idea over from CLU: the exception specification, to programmatically state in the method signature the exceptions that could result from calling that method. The exception specification really has two purposes. It can say, "I’m originating this exception in my code; you handle it." But it can also mean, "I’m ignoring this exception that can occur as a result of my code; you handle it." We’ve been focusing on the "you handle it" part when looking at the mechanics and syntax of exceptions, but here I’m particularly interested in the fact that we often ignore exceptions and that’s what the exception specification can state. In C++ the exception specification is not part of the type information of a function. The only compile-time checking is to ensure that exception specifications are used consistently; for example, if a function or method throws exceptions, then the overloaded or derived versions must also throw those exceptions. Unlike Java, however, no compile-time checking occurs to determine whether or not the function or method will actually throw that exception, or whether the exception specification is complete (that is, whether it accurately describes all exceptions that maybe thrown). That validation does happen, but only at run time. If an exception is thrown that violates the exception specification, the C++ program will call the standard library function unexpected( ). It is interesting to note that, because of the use of templates, exception specifications are not used at all in the Standard C++ Library. In Java, there are restrictions on the way that Java generics can be used with exception specifications. Perspectives First, it’s worth noting that Java effectively invented the checked exception (clearly inspired by C++ exception specifications and the fact that C++ programmers typically don’t bother with them). However, it was an experiment which no subsequent language has chosen to duplicate. Secondly, checked exceptions appear to be an "obvious good thing" when seen in introductory examples and in small programs. It has been suggested that the subtle difficulties begin to appear when programs start to get large. Of course, largeness usually doesn’t happen overnight; it creeps. Languages that may not be suited for large-scale projects Error Handling with Exceptions 347 are used for small projects. These projects grow, and at some point we realize that things have gone from "manageable" to "difficult." This is what I’m suggesting may be the case with too much type checking; in particular, with checked exceptions. The scale of the program seems to be a significant issue. This is a problem because most discussions tend to use small programs as demonstrations. One of the C# designers observed that: "Examination of small programs leads to the conclusion that requiring exception specifications could both enhance developer productivity and enhance code quality, but experience with large software projects suggests a different result—decreased productivity and little or no increase in code quality."8 In reference to uncaught exceptions, the CLU creators stated: "We felt it was unrealistic to require the programmer to provide handlers in situations where no meaningful action can be taken."9 When explaining why a function declaration with no specification means that it can throw any exception, rather than no exceptions, Stroustrup states: "However, that would require exception specifications for essentially every function, would be a significant cause for recompilation, and would inhibit cooperation with software written in other languages. This would encourage programmers to subvert the exception-handling mechanisms and to write spurious code to suppress exceptions. It would provide a false sense of security to people who failed to notice the exception."10 We see this very behavior—subverting the exceptions—happening with checked exceptions in Java. Martin Fowler (author of UML Distilled, Refactoring, and Analysis Patterns) wrote the following to me: "...on the whole I think that exceptions are good, but Java checked exceptions are more trouble than they are worth." I now think that Java’s important step was to unify the error-reporting model, so that all errors are reported using exceptions. This wasn’t happening with C++, because for backward compatibility with C the old model of just ignoring errors was still available. But if you have consistent reporting with exceptions, then exceptions can be used if desired, and if not, they will propagate out to the highest level (the console or other container program). When Java modified the C++ model so that exceptions were the only way to report errors, the extra enforcement of checked exceptions may have become less necessary. In the past, I have been a strong believer that both checked exceptions and static type checking were essential to robust program development. However, both anecdotal and direct experience11 with languages that are more dynamic than static has led me to think that the great benefits actually come from: 8 http://discuss.develop.com/archives/wa.exe?A2=indoonA&L=DOTNET&P=R32820 9 Exception Handling in CLU, Liskov & Snyder. 10 Bjarne Stroustrup, The C++ Programming Language, 3rd Edition (Addison-Wesley, 1997), P- 376. 11 Indirectly with Smalltalk via conversations with many experienced programmers in that language; directly with Python (www.Python.org). 348 Thinking in Java Bruce Eckel 1. A unified error-reporting model via exceptions, regardless of whether the programmer is forced by the compiler to handle them. 2. Type checking, regardless of when it takes place. That is, as long as proper use of a type is enforced, it often doesn’t matter if it happens at compile time or run time. On top of this, there are very significant productivity benefits to reducing the compile-time constraints upon the programmer. Indeed, reflection and generics are required to compensate for the overconstraining nature of static typing, as you shall see in a number of examples throughout the book. I’ve already been told by some that what I say here constitutes blasphemy, and by uttering these words my reputation will be destroyed, civilizations will fall, and a higher percentage of programming projects will fail. The belief that the compiler can save your project by pointing out errors at compile time runs strong, but it’s even more important to realize the limitation of what the compiler is able to do; in the supplement you will find at http://MindView.net/Books/BetterJava, I emphasize the value of an automated build process and unit testing, which give you far more leverage than you get by trying to turn everything into a syntax error. It’s worth keeping in mind that: "A good programming language is one that helps programmers write good programs. No programming language will prevent its users from writing bad programs."12 In any event, the likelihood of checked exceptions ever being removed from Java seems dim. It would be too radical of a language change, and proponents within Sun appear to be quite strong. Sun has a history and policy of absolute backwards compatibility—to give you a sense of this, virtually all Sun software runs on all Sun hardware, no matter how old. However, if you find that some checked exceptions are getting in your way, or especially if you find yourself being forced to catch exceptions, but you don’t know what to do with them, there are some alternatives. Passing exceptions to the console In simple programs, like many of those in this book, the easiest way to preserve the exceptions without writing a lot of code is to pass them out of main( ) to the console. For example, if you want to open a file for reading (something you’ll learn about in detail in the I/O chapter), you must open and close a FilelnputStream, which throws exceptions. For a simple program, you can do this (you’ll see this approach used in numerous places throughout this book): //: exceptions/MainException.java import java.io.*; public class MainException { // Pass all exceptions to the console: public static void main(String[] args) throws Exception { // Open the file: FileInputStream file = new FileInputStream("MainException.java"); // Use the file ... // Close the file: file.close(); } } ///:~ 12 Kees Koster, designer of the CDL language, as quoted by Bertrand Meyer, designer of the Eiffel language, www.elj.com/elj/vi/ni/bm/right/. Error Handling with Exceptions 349 Note that main( ) is also a method that may have an exception specification, and here the type of exception is Exception, the root class of all checked exceptions. By passing it out to the console, you are relieved from writing try-catch clauses within the body of main( ). (Unfortunately, file I/O is significantly more complex than it would appear to be from this example, so don’t get too excited until after you’ve read the I/O chapter). Exercise 26: (1) Change the file name string in MainException.java to name a file that doesn’t exist. Run the program and note the result. Converting checked to unchecked exceptions Throwing an exception from main( ) is convenient when you’re writing simple programs for your own consumption, but is not generally useful. The real problem is when you are writing an ordinary method body, and you call another method and realize, "I have no idea what to do with this exception here, but I don’t want to swallow it or print some banal message." With chained exceptions, a new and simple solution prevents itself. You simply "wrap" a checked exception inside a RuntimeException by passing it to the RuntimeException constructor, like this: try{ // ... to do something useful } catch(IDontKnowWhatToDoWithThisCheckedException e) { throw new RuntimeException(e); } This seems to be an ideal solution if you want to "turn off the checked exception—you don’t swallow it, and you don’t have to put it in your method’s exception specification, but because of exception chaining you don’t lose any information from the original exception. This technique provides the option to ignore the exception and let it bubble up the call stack without being required to write try-catch clauses and/or exception specifications. However, you may still catch and handle the specific exception by using getCause( ), as seen here: //: exceptions/TurnOffChecking.java // "Turning off" Checked exceptions. import java.io.*; import static net.mindview.util.Print.*; class WrapCheckedException { void throwRuntimeException(int type) { try { switch(type) { case 0: throw new FileNotFoundException(); case 1: throw new IOException(); case 2: throw new RuntimeException("Where am I?"); default: return; } } catch(Exception e) { // Adapt to unchecked: throw new RuntimeException(e); } } } class SomeOtherException extends Exception {} public class TurnOffChecking { public static void main(String[] args) { WrapCheckedException wce = new WrapCheckedException(); // You can call throwRuntimeException() without a try 350 Thinking in Java Bruce Eckel // block, and let RuntimeExceptions leave the method: wce.throwRuntimeException(3); // Or you can choose to catch exceptions: for(int i = 0; i < 4; i++) try { if(i < 3) wce.throwRuntimeException(i); else throw new SomeOtherException(); } catch(SomeOtherException e) { print("SomeOtherException: " + e); } catch(RuntimeException re) { try { throw re.getCause(); } catch(FileNotFoundException e) { print("FileNotFoundException: " + e); } catch(IOException e) { print("IOException: " + e); } catch(Throwable e) { print("Throwable: " + e); } } } } /* Output: FileNotFoundException: java.io.FileNotFoundException IOException: java.io.IOException Throwable: java.lang.RuntimeException: Where am I? SomeOtherException: SomeOtherException *///:~ WrapCheckedException.throwRuntimeException( ) contains code that generates different types of exceptions. These are caught and wrapped inside RuntimeException objects, so they become the "cause" of those exceptions. In TurnOffChecking, you can see that it’s possible to call throwRuntimeException( ) with no try block because the method does not throw any checked exceptions. However, when you’re ready to catch exceptions, you still have the ability to catch any exception you want by putting your code inside a try block. You start by catching all the exceptions you explicitly know might emerge from the code in your try block—in this case, SomeOtherException is caught first. Lastly, you catch RuntimeException and throw the result of getCause( ) (the wrapped exception). This extracts the originating exceptions, which can then be handled in their own catch clauses. The technique of wrapping a checked exception in a RuntimeException will be used when appropriate throughout the rest of this book. Another solution is to create your own subclass of RuntimeException. This way, it doesn’t need to be caught, but someone can catch it if they want to. Exercise 27: (1) Modify Exercise 3 to convert the exception to a RuntimeException. Exercise 28: (1) Modify Exercise 4 so that the custom exception class inherits from RuntimeException, and show that the compiler allows you to leave out the try block. Exercise 29: (1) Modify all the exception types in Stormylnning.java so that they extend RuntimeException, and show that no exception specifications or try blocks are necessary. Remove the ‘//!’ comments and show how the methods can be compiled without specifications. Error Handling with Exceptions 351 Exercise 30: (2) Modify Human.java so that the exceptions inherit from RuntimeException. Modify main( ) so that the technique in TurnOffChecking.java is used to handle the different types of exceptions. Exception guidelines Use exceptions to: 1. Handle problems at the appropriate level. (Avoid catching exceptions unless you know what to do with them.) 2. Fix the problem and call the method that caused the exception again. 3. Patch things up and continue without retrying the method. 4. Calculate some alternative result instead of what the method was supposed to produce. 5. Do whatever you can in the current context and rethrow the same exception to a higher context. 6. Do whatever you can in the current context and throw a different exception to a higher context. 7. Terminate the program. 8. Simplify. (If your exception scheme makes things more complicated, then it is painful and annoying to use.) 9. Make your library and program safer. (This is a short-term investment for debugging, and a long-term investment for application robustness.) Summary Exceptions are integral to programming with Java; you can accomplish only so much without knowing how to work with them. For that reason, exceptions are introduced at this point in the book—there are many libraries (like I/O, mentioned earlier) that you can’t use without handling exceptions. One of the advantages of exception handling is that it allows you to concentrate on the problem you’re trying to solve in one place, and then deal with the errors from that code in another place. And although exceptions are generally explained as tools that allow you to report and recover from errors at run time, I have come to wonder how often the "recovery" aspect is implemented, or even possible. My perception is that it is less than 10 percent of the time, and even then it probably amounts to unwinding the stack to a known stable state rather than actually performing any kind of resumptive behavior. Whether or not this is true, I have come to believe that the "reporting" function is where the essential value of exceptions lie. The fact that Java effectively insists that all errors be reported in the form of exceptions is what gives it a great advantage over languages like C++, which allow you to report errors in a number of different ways, or not at all. A consistent errorreporting system means that you no longer have to ask the question "Are errors slipping through the cracks?" with each piece of code you write (as long as you don’t "swallow" the exceptions, that is!). 352 Thinking in Java Bruce Eckel As you will see in future chapters, by laying this question to rest—even if you do so by throwing a RuntimeException—your design and implementation efforts can be focused on more interesting and challenging issues. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Error Handling with Exceptions 353 Strings String manipulation is arguably one of the most common activities in computer programming. This is especially true in Web systems, where Java is heavily used. In this chapter, we’ll look more deeply at what is certainly the most commonly used class in the language, String, along with some of its associated classes and utilities. Immutable Strings Objects of the String class are immutable. If you examine the JDK documentation for the String class, you’ll see that every method in the class that appears to modify a String actually creates and returns a brand new String object containing the modification. The original String is left untouched. Consider the following code: //: strings/Immutable.java import static net.mindview.util.Print.*; public class Immutable { public static String upcase(String s) { return s.toUpperCase(); } public static void main(String[] args) { String q = "howdy"; print(q); // howdy String qq = upcase(q); print(qq); // HOWDY print(q); // howdy } } /* Output: howdy HOWDY howdy *///:~ When q is passed in to upcase( ) it’s actually a copy of the reference to q. The object this reference is connected to stays in a single physical location. The references are copied as they are passed around. Looking at the definition for upcase( ), you can see that the reference that’s passed in has the name s, and it exists for only as long as the body of upcase( ) is being executed. When upcase( ) completes, the local reference s vanishes. upcase( ) returns the result, which is the original string with all the characters set to uppercase. Of course, it actually returns a reference to the result. But it turns out that the reference that it returns is for a new object, and the original q is left alone. This behavior is usually what you want. Suppose you say: String s = "asdf"; String x = Immutable.upcase(s); Do you really want the upcase( ) method to change the argument? To the reader of the code, an argument usually looks like a piece of information provided to the method, not something to be modified. This is an important guarantee, since it makes code easier to write and understand. Overloading ‘+’ vs. StringBuilder Since String objects are immutable, you can alias to a particular String as many times as you want. Because a String is read-only, there’s no possibility that one reference will change something that will affect the other references. Immutability can have efficiency issues. A case in point is the operator ‘+’ that has been overloaded for String objects. Overloading means that an operation has been given an extra meaning when used with a particular class. (The ‘+’ and ‘+=‘ for String are the only operators that are overloaded in Java, and Java does not allow the programmer to overload any others.)1 The’+’ operator allows you to concatenate Strings: //: strings/Concatenation.java public class Concatenation { public static void main(String[] args) { String mango = "mango"; String s = "abc" + mango + "def" + 47; System.out.println(s); } } /* Output: abcmangodef47 *///:~ You could imagine how this might work. The String "abc" could have a method append( ) that creates a new String object containing "abc" concatenated with the contents of mango. The new String object would then create another new String that added "def," and so on. This would certainly work, but it requires the creation of a lot of String objects just to put together this new String, and then you have a bunch of intermediate String objects that need to be garbage collected. I suspect that the Java designers tried this approach first (which is a lesson in software design—you don’t really know anything about a system until you try it out in code and get something working). I also suspect that they discovered it delivered unacceptable performance. To see what really happens, you can decompile the above code using the javap tool that comes as part of the JDK. Here’s the command line: javap -c Concatenation The -c flag will produce the JVM bytecodes. After we strip out the parts we’re not interested in and do a bit of editing, here are the relevant bytecodes: public static void main(java.lang.String[]); Code: Stack=2, Locals=3, Args_size=1 0: ldc #2; //String mango 1 C++ allows the programmer to overload operators at will. Because this can often be a complicated process (see Chapter 10 of Thinking in C++, 2nd Edition, Prentice Hall, 2000), the Java designers deemed it a "bad" feature that shouldn’t be included in Java. It wasn’t so bad that they didn’t end up doing it themselves, and ironically enough, operator overloading would be much easier to use in Java than in C++. This can be seen in Python (see www.Python.org) and C#, which have garbage collection and straightforward operator overloading. 356 Thinking in Java Bruce Eckel 2: astore_1 3: new #3; //class StringBuilder 6: dup 7: invokespecial #4; //StringBuilder."":() 10: ldc #5; // String abc 12 invokevirtual #6; //StringBuilder.append:(String) 15 aload_1 16 invokevirtual #6; //StringBuilder.append:(String) 19 ldc #7; //String def 21 invokevirtual #6; //StringBuilder.append:(String) 24 bipush 47 26 invokevirtual #8; //StringBuilder.append:(I) 29 invokevirtual #9; //StringBuilder.toString:() 32 astore_2 33 getstatic #10; //Field System.out:PrintStream; 36 aload_2 37 invokevirtual #11; // PrintStream.println:(String) 40 return If you’ve had experience with assembly language, this may look familiar to you—statements like dup and invokevirtual are the Java Virtual Machine (JVM) equivalent of assembly language. If you’ve never seen assembly language, don’t worry about it—the important part to notice is the introduction by the compiler of the java.lang.StringBuilder class. There was no mention of StringBuilder in the source code, but the compiler decided to use it anyway, because it is much more efficient. In this case, the compiler creates a StringBuilder object to build the String s, and calls append( ) four times, one for each of the pieces. Finally, it calls toString( ) to produce the result, which it stores (with astore_2) as s. Before you assume that you should just use Strings everywhere and that the compiler will make everything efficient, let’s look a little more closely at what the compiler is doing. Here’s an example that produces a String result in two ways: using Strings, and by hand-coding with StringBuilder: //: strings/WhitherStringBuilder.java public class WhitherStringBuilder { public String implicit(String[] fields) { String result = ""; for(int i = 0; i < fields.length; i++) result += fields[i]; return result; } public String explicit(String[] fields) { StringBuilder result = new StringBuilder(); for(int i = 0; i < fields.length; i++) result.append(fields[i]); return result.toString(); } } ///:~ Now if you run javap -c WitherStringBuilder, you can see the (simplified) code for the two different methods. First, implicit( ): public java.lang.String implicit(java.lang.String[]); Code: 0: ldc #2; //String 2: astore_2 3: iconst_0 4: istore_3 Strings 357 5: iload_3 6: aload_1 7: arraylength 8: if_icmpge 38 11: new #3; //class StringBuilder 14: dup 15: invokespecial #4; // StringBuilder.””:() 18: aload_2 19: invokevirtual #5; // StringBuilder.append:() 22: aload_1 23 iload_3 24 aaload 25: invokevirtual #5; // StringBuilder.append:() 28: invokevirtual #6; // StringBuiIder.toString:() 31: astore_2 32: iinc 3, 1 35: goto 5 38: aload_2 39 areturn Notice 8: and 35:, which together form a loop. 8: does an "integer compare greater than or equal to" of the operands on the stack and jumps to 38: when the loop is done. 35: is a goto back to the beginning of the loop, at 5:. The important thing to note is that the StringBuilder construction happens inside this loop, which means you’re going to get a new StringBuilder object every time you pass through the loop. Here are the bytecodes for explicit( ): public java.lang.String explicit(java.lang.String[]); Code: 0: new #3; //class StringBuilder 3: dup 4: invokespecial #4; // StringBuilder.””:() 7: astore_2 8: iconst_0 9: istore_3 10: iload_3 11: aload_1 12: arraylength 13: if_icmpge 30 16: aload_2 17: aload_1 18: iload_3 19: aaload 20 invokevirtual #5; // StringBuilder.append:() 23 pop 24: iinc 3,1 27: goto 10 30: aload_2 31: invokevirtual #6; // StringBuiIder.toString:() 34: areturn Not only is the loop code shorter and simpler, the method only creates a single StringBuilder object. Creating an explicit StringBuilder also allows you to preallocate its size if you have extra information about how big it might need to be, so that it doesn’t need to constantly reallocate the buffer. 358 Thinking in Java Bruce Eckel Thus, when you create a toString( ) method, if the operations are simple ones that the compiler can figure out on its own, you can generally rely on the compiler to build the result in a reasonable fashion. But if looping is involved, you should explicitly use a StringBuilder in your toString( ), like this: //: strings/UsingStringBuilder.java import java.util.*; public class UsingStringBuilder { public static Random rand = new Random(47); public String toString() { StringBuilder result = new StringBuilder("["); for(int i = 0; i < 25; i++) { result.append(rand.nextInt(100)); result.append(", "); } result.delete(result.length()-2, result.length()); result.append("]"); return result.toString(); } public static void main(String[] args) { UsingStringBuilder usb = new UsingStringBuilder(); System.out.println(usb); } } /* Output: [58, 55, 93, 61, 61, 29, 68, 0, 22, 7, 88, 28, 51, 89, 9, 78, 98, 61, 20, 58, 16, 40, 11, 22, 4] *///:~ Notice that each piece of the result is added with an append( ) statement. If you try to take shortcuts and do something like append(a + ": " + c), the compiler will jump in and start making more StringBuilder objects again. If you are in doubt about which approach to use, you can always run javap to double-check. Although StringBuilder has a full complement of methods, including insert( ), replace( ), substring( ) and even reverse( ), the ones you will generally use are append( ) and toString( ). Note the use of delete( ) to remove the last comma and space before adding the closing square bracket. StringBuilder was introduced in Java SE5. Prior to this, Java used StringBuffer, which ensured thread safety (see the Concurrency chapter) and so was significantly more expensive. Thus, string operations in Java SE5/6 should be faster. Exercise 1: (2) Analyze SprinklerSystem.toString( ) in reusing/SprinklerSystem.java to discover whether writing the toString( ) with an explicit StringBuilder will save any StringBuilder creations. Unintended recursion Because (like every other class) the Java standard containers are ultimately inherited from Object, they contain a toString( ) method. This has been overridden so that they can produce a String representation of themselves, including the objects they hold. ArrayList.toString( ), for example, steps through the elements of the Array List and calls toString( ) for each one: //: strings/ArrayListDisplay.java import generics.coffee.*; Strings 359 import java.util.*; public class ArrayListDisplay { public static void main(String[] args) { ArrayList coffees = new ArrayList(); for(Coffee c : new CoffeeGenerator(10)) coffees.add(c); System.out.println(coffees); } } /* Output: [Americano 0, Latte 1, Americano 2, Mocha 3, Mocha 4, Breve 5, Americano 6, Latte 7, Cappuccino 8, Cappuccino 9] *///:~ Suppose you’d like your toString( ) to print the address of your class. It seems to make sense to simply refer to this: //: strings/InfiniteRecursion.java // Accidental recursion. // {RunByHand} import java.util.*; public class InfiniteRecursion { public String toString() { return " InfiniteRecursion address: " + this + "\n"; } public static void main(String[] args) { List v = new ArrayList(); for(int i = 0; i < 10; i++) v.add(new InfiniteRecursion()); System.out.println(v); } } ///:~ If you create an InfiniteRecursion object and then print it, you’ll get a very long sequence of exceptions. This is also true if you place the InfiniteRecursion objects in an ArrayList and print that ArrayList as shown here. What’s happening is automatic type conversion for Strings. When you say: "InfiniteRecursion address: " + this The compiler sees a String followed by a’+’ and something that’s not a String, so it tries to convert this to a String. It does this conversion by calling toString( ), which produces a recursive call. If you really do want to print the address of the object, the solution is to call the ObjecttoString( ) method, which does just that. So instead of saying this, you’d say super.toString( ). Exercise 2: (1) Repair InfiniteRecursion.java. 360 Thinking in Java Bruce Eckel Operations on Strings Here are some of the basic methods available for String objects. Methods that are overloaded are summarized in a single row: Method Constructor length( ) charAt( ) getChars( ), getBytes( ) toCharArray( ) equals( ), equalsIgnoreCase( ) compareTo( ) contains( ) contentEquals( ) equalsIgnoreCase( ) regionMatches( ) startsWith( ) endsWith( ) indexOf( ), lastIndexOf( ) Strings Arguments, Overloading Use Overloaded: default, String, StringBuilder, StringBuffer, char arrays, byte arrays. Creating String objects. Number of characters in the String. int Index The char at a location in the String. The beginning and end from which to copy, the array to copy into, an index into the destination array. Copy chars or bytes into an external array. Produces a char[] containing the characters in the String. A String to compare with. An equality check on the contents of the two Strings. A String to compare with. Result is negative, zero, or positive depending on the lexicographical ordering of the String and the argument. Uppercase and lowercase are not equal! A CharSequence to search Result is true if the for. argument is contained in the String. A CharSequence or StringBuffer to compare to. Result is true if there’s an exact match with the argument. A String to compare with. Result is true if the contents are equal, ignoring case. Offset into this String, the other String and its offset and length to compare. Overload adds "ignore case." boolean result indicates whether the region matches. String that it might start with. Overload adds offset into argument. boolean result indicates whether the String starts with the argument. String that might be a suffix boolean result indicates of this String. whether the argument is a suffix. Overloaded: char, char and Returns -1 if the argument is starting index, String, not found within this String; otherwise, returns 361 Method substring( ) (also subSequence( )) concat( ) replace() toLowerCase( ) toUpperCase( ) trim( ) valueOf( ) intern( ) Arguments, Overloading String and starting index. Overloaded: starting index; starting index + ending index. The String to concatenate. The old character to search for, the new character to replace it with. Can also replace a CharSequence with a CharSequence. Overloaded: Object, char[], char[] and offset and count, boolean, char, int, long, float, double. Use the index where the argument starts. lastIndexOf( ) searches backward from end. Returns a new String object containing the specified character set. Returns a new String object containing the original String’s characters followed by the characters in the argument. Returns a new String object with the replacements made. Uses the old String if no match is found. Returns a new String object with the case of all letters changed. Uses the old String if no changes need to be made. Returns a new String object with the whitespace removed from each end. Uses the old String if no changes need to be made. Returns a String containing a character representation of the argument. Produces one and only one String reference per unique character sequence. You can see that every String method carefully returns a new String object when it’s necessary to change the contents. Also notice that if the contents don’t need changing, the method will just return a reference to the original String. This saves storage and overhead. The String methods involving regular expressions will be explained later in this chapter. Formatting output One of the long-awaited features that has finally appeared in Java SE5 is output formatting in the style of C’s printf( ) statement. Not only does this allow for simplified output code, but it also gives Java developers powerful control over output formatting and alignment.2 2 Mark Welsh assisted in the creation of this section, and the "Scanning input" section. 362 Thinking in Java Bruce Eckel printf() C’s printf( ) doesn’t assemble strings the way Java does, but takes a single format string and inserts values into it, formatting as it goes. Instead of using the overloaded ‘+’ operator (which C doesn’t overload) to concatenate quoted text and variables, printf( ) uses special placeholders to show where the data should go. The arguments that are inserted into the format string follow in a comma-separated list. For example: printf("Row 1: [%d %f]\n", x, y); At run time, the value of x is inserted into %d and the value of y is inserted into %f. These placeholders are called/ormaf specifiers and, in addition to telling where to insert the value, they also tell what kind of variable is to be inserted and how to format it. For instance, the ‘%d’ above says that x is an integer and the ‘%f says y is a floating point value (a float or double). System.out.format() Java SE5 introduced the format( ) method, available to PrintStream or PrintWriter objects (which you’ll learn more about in the I/O chapter), which includes System.out. The format( ) method is modeled after C’s printf( ). There’s even a convenience printf( ) method that you can use if you’re feeling nostalgic, which just calls format( ). Here’s a simple example: //: strings/SimpleFormat.java public class SimpleFormat { public static void main(String[] args) { int x = 5; double y = 5.332542; // The old way: System.out.println("Row 1: [" + x + " " + y + "]"); // The new way: System.out.format("Row 1: [%d %f]\n", x, y); // or System.out.printf("Row 1: [%d %f]\n", x, y); } } /* Output: Row 1: [5 5.332542] Row 1: [5 5.332542] Row 1: [5 5.332542] *///:~ You can see that format( ) and printf( ) are equivalent. In both cases, there’s only a single format string, followed by one argument for each format specifier. The Formatter class All of Java’s new formatting functionality is handled by the Formatter class in the java.util package. You can think of Formatter as a translator that converts your format string and data into the desired result. When you create a Formatter object, you tell it where you want this result to go by passing that information to the constructor: //: strings/Turtle.java Strings 363 import java.io.*; import java.util.*; public class Turtle { private String name; private Formatter f; public Turtle(String name, Formatter f) { this.name = name; this.f = f; } public void move(int x, int y) { f.format("%s The Turtle is at (%d,%d)\n", name, x, y); } public static void main(String[] args) { PrintStream outAlias = System.out; Turtle tommy = new Turtle("Tommy", new Formatter(System.out)); Turtle terry = new Turtle("Terry", new Formatter(outAlias)); tommy.move(0,0); terry.move(4,8); tommy.move(3,4); terry.move(2,5); tommy.move(3,3); terry.move(3,3); } } /* Output: Tommy The Turtle is at (0,0) Terry The Turtle is at (4,8) Tommy The Turtle is at (3,4) Terry The Turtle is at (2,5) Tommy The Turtle is at (3,3) Terry The Turtle is at (3,3) *///:~ All the tommy output goes to System.out and all the terry output goes to an alias of System.out. The constructor is overloaded to take a range of output locations, but the most useful are PrintStreams (as above), OutputStreams, and Files. You’ll learn more about these in the I/O chapter. Exercise 3: (1) Modify Turtle.java so that it sends all output to System.err. The previous example uses a new format specifier, ‘%s’. This indicates a String argument and is an example of the simplest kind of format specifier-one that has only a conversion type. Format specifiers To control spacing and alignment when data is inserted, you need more elaborate format specifiers. Here’s the general syntax: %[argument_index$][flags][width][.precision]conversion Often, you’ll need to control the minimum size of a field. This can be accomplished by specifying a width. The Formatter guarantees that a field is at least a certain number of characters wide by padding it with spaces if necessary. By default, the data is right justified, but this can be overridden by including a ‘-’ in the flags section. 364 Thinking in Java Bruce Eckel The opposite of width is precision, which is used to specify a maximum. Unlike the width, which is applicable to all of the data conversion types and behaves the same with each, precision has a different meaning for different types. For Strings, the precision specifies the maximum number of characters from the String to print. For floating point numbers, precision specifies the number of decimal places to display (the default is 6), rounding if there are too many or adding trailing zeroes if there are too few. Since integers have no fractional part, precision isn’t applicable to them and you’ll get an exception if you use precision with an integer conversion type. This example uses format specifiers to print a shopping receipt: //: strings/Receipt.java import java.util.*; public class Receipt { private double total = 0; private Formatter f = new Formatter(System.out); public void printTitle() { f.format("%-15s %5s %10s\n", "Item", "Qty", "Price"); f.format("%-15s %5s %10s\n", "----", "---", "-----"); } public void print(String name, int qty, double price) { f.format("%-15.15s %5d %10.2f\n", name, qty, price); total += price; } public void printTotal() { f.format("%-15s %5s %10.2f\n", "Tax", "", total*0.06); f.format("%-15s %5s %10s\n", "", "", "-----"); f.format("%-15s %5s %10.2f\n", "Total", "", total * 1.06); } public static void main(String[] args) { Receipt receipt = new Receipt(); receipt.printTitle(); receipt.print("Jack’s Magic Beans", 4, 4.25); receipt.print("Princess Peas", 3, 5.1); receipt.print("Three Bears Porridge", 1, 14.29); receipt.printTotal(); } } /* Output: Item Qty Price ---- --- ----- Jack’s Magic Be 4 4.25 Princess Peas 3 5.10 Three Bears Por 1 14.29 Tax 1.42 ----- Total 25.06 *///:~ As you can see, the Formatter provides powerful control over spacing and alignment with fairly concise notation. Here, the format strings are simply copied in order to produce the appropriate spacing. Exercise 4: (3) Modify Receipt.java so that the widths are all controlled by a single set of constant values. The goal is to allow you to easily change a width by changing a single value in one place. Strings 365 Formatter conversions These are the conversions you’ll come across most frequently: Conversion Characters d Integral (as decimal) c Unicode character b Boolean value s String f Floating point (as decimal) e Floating point (in scientific notation) x Integral (as hex) h Hash code (as hex) % Literal "%" Here’s an example that shows these conversions in action: //: strings/Conversion.java import java.math.*; import java.util.*; public class Conversion { public static void main(String[] args) { Formatter f = new Formatter(System.out); char u = ‘a’; System.out.println("u = ‘a’"); f.format("s: %s\n", u); // f.format("d: %d\n", u); f.format("c: %c\n", u); f.format("b: %b\n", u); // f.format("f: %f\n", u); // f.format("e: %e\n", u); // f.format("x: %x\n", u); f.format("h: %h\n", u); int v = 121; System.out.println("v = 121"); f.format("d: %d\n", v); f.format("c: %c\n", v); f.format("b: %b\n", v); f.format("s: %s\n", v); // f.format("f: %f\n", v); // f.format("e: %e\n", v); f.format("x: %x\n", v); f.format("h: %h\n", v); BigInteger w = new BigInteger("50000000000000"); System.out.println( "w = new BigInteger(\"50000000000000\")"); 366 Thinking in Java Bruce Eckel f.format("d: %d\n", w); // f.format("c: %c\n", w); f.format("b: %b\n", w); f.format("s: %s\n", w); // f.format("f: %f\n", w); // f.format("e: %e\n", w); f.format("x: %x\n", w); f.format("h: %h\n", w); double x = 179.543; System.out.println("x = 179.543"); // f.format("d: %d\n", x); // f.format("c: %c\n", x); f.format("b: %b\n", x); f.format("s: %s\n", x); f.format("f: %f\n", x); f.format("e: %e\n", x); // f.format("x: %x\n", x); f.format("h: %h\n", x); Conversion y = new Conversion(); System.out.println("y = new Conversion()"); // f.format("d: %d\n", y); // f.format("c: %c\n", y); f.format("b: %b\n", y); f.format("s: %s\n", y); // f.format("f: %f\n", y); // f.format("e: %e\n", y); // f.format("x: %x\n", y); f.format("h: %h\n", y); boolean z = false; System.out.println("z = false"); // f.format("d: %d\n", z); // f.format("c: %c\n", z); f.format("b: %b\n", z); f.format("s: %s\n", z); // f.format("f: %f\n", z); // f.format("e: %e\n", z); // f.format("x: %x\n", z); f.format("h: %h\n", z); } } /* Output: (Sample) u = ‘a’ s: a c: a b: true h: 61 v = 121 d: 121 c: y b: true s: 121 x: 79 h: 79 w = new BigInteger("50000000000000") d: 50000000000000 b: true s: 50000000000000 x: 2d79883d2000 h: 8842a1a7 x = 179.543 b: true Strings 367 s: 179.543 f: 179.543000 e: 1.795430e+02 h: 1ef462c y = new Conversion() b: true s: Conversion@9cab16 h: 9cab16 z = false b: false s: false h: 4d5 *///:~ The commented lines show conversions that are invalid for that particular variable type; executing them will trigger an exception. Notice that the ‘b’ conversion works for each variable above. Although it’s valid for any argument type, it might not behave as you’d expect. For boolean primitives or Boolean objects, the result will be true or false, accordingly. However, for any other argument, as long as the argument type is not null the result is always true. Even the numeric value of zero, which is synonymous with false in many languages (including C), will produce true, so be careful when using this conversion with non-boolean types. There are more obscure conversion types and other format specifier options. You can read about these in the JDK documentation for the Formatter class. Exercise 5: (5) For each of the basic conversion types in the above table, write the most complex formatting expression possible. That is, use all the possible format specifiers available for that conversion type. String.format() Java SE5 also took a cue from C’s sprintf( ), which is used to create Strings. String.format( ) is a static method which takes all the same arguments as Formatter’s format( ) but returns a String. It can come in handy when you only need to call format( ) once: //: strings/DatabaseException.java public class DatabaseException extends Exception { public DatabaseException(int transactionID, int queryID, String message) { super(String.format("(t%d, q%d) %s", transactionID, queryID, message)); } public static void main(String[] args) { try { throw new DatabaseException(3, 7, "Write failed"); } catch(Exception e) { System.out.println(e); } } } /* Output: DatabaseException: (t3, q7) Write failed *///:~ 368 Thinking in Java Bruce Eckel Under the hood, all String.format( ) does is instantiate a Formatter and pass your arguments to it, but using this convenience method can often be clearer and easier than doing it by hand. A hex dump tool As a second example, often you want to look at the bytes inside a binary file using hex format. Here’s a small utility that displays a binary array of bytes in a readable hex format, using String.format( ): //: net/mindview/util/Hex.java package net.mindview.util; import java.io.*; public class Hex { public static String format(byte[] data) { StringBuilder result = new StringBuilder(); int n = 0; for(byte b : data) { if(n % 16 == 0) result.append(String.format("%05X: ", n)); result.append(String.format("%02X ", b)); n++; if(n % 16 == 0) result.append("\n"); } result.append("\n"); return result.toString(); } public static void main(String[] args) throws Exception { if(args.length == 0) // Test by displaying this class file: System.out.println( format(BinaryFile.read("Hex.class"))); else System.out.println( format(BinaryFile.read(new File(args[0])))); } } /* Output: (Sample) 00000: CA FE BA BE 00 00 00 31 00 52 0A 00 05 00 22 07 00010: 00 23 0A 00 02 00 22 08 00 24 07 00 25 0A 00 26 00020: 00 27 0A 00 28 00 29 0A 00 02 00 2A 08 00 2B 0A 00030: 00 2C 00 2D 08 00 2E 0A 00 02 00 2F 09 00 30 00 00040: 31 08 00 32 0A 00 33 00 34 0A 00 15 00 35 0A 00 00050: 36 00 37 07 00 38 0A 00 12 00 39 0A 00 33 00 3A ... *///:~ To open and read the binary file, this uses another utility that will be introduced in the I/O chapter: net.mindview.util.BinaryFile. The read( ) method returns the entire file as a byte array. Exercise 6: (2) Create a class that contains int, long, float and double fields. Create a toString( ) method for this class that uses String.format( ), and demonstrate that your class works correctly. Strings 369 Regular expressions Regular expressions have long been integral to standard Unix utilities like sed and awk, and languages like Python and Perl (some would argue that they are the predominant reason for Perl’s success). String manipulation tools were previously delegated to the String, StringBuffer, and StringTokenizer classes in Java, which had relatively simple facilities compared to regular expressions. Regular expressions are powerful and flexible text-processing tools. They allow you to specify, programmatically, complex patterns of text that can be discovered in an input string. Once you discover these patterns, you can then react to them any way you want. Although the syntax of regular expressions can be intimidating at first, they provide a compact and dynamic language that can be employed to solve all sorts of string processing, matching and selection, editing, and verification problems in a completely general way. Basics A regular expression is a way to describe strings in general terms, so that you can say, "If a string has these things in it, then it matches what I’m looking for." For example, to say that a number might or might not be preceded by a minus sign, you put in the minus sign followed by a question mark, like this: -? To describe an integer, you say that it’s one or more digits. In regular expressions, a digit is described by saying ‘\d’. If you have any experience with regular expressions in other languages, you’ll immediately notice a difference in the way backslashes are handled. In other languages, ‘\\’ means "I want to insert a plain old (literal) backslash in the regular expression. Don’t give it any special meaning." In Java, ‘ \ \ ‘ means "I’m inserting a regular expression backslash, so that the following character has special meaning." For example, if you want to indicate a digit, your regular expression string will be ‘\\d’. If you want to insert a literal backslash, you say ‘\\\\’- However, things like newlines and tabs just use a single backslash: ‘\n\t’. To indicate "one or more of the preceding expression," you use a ‘+’. So to say, "possibly a minus sign, followed by one or more digits," you write: -?\\d+ The simplest way to use regular expressions is to use the functionality built into the String class. For example, we can see whether a String matches the regular expression above: //: strings/IntegerMatch.java public class IntegerMatch { public static void main(String[] args) { System.out.println("-1234".matches("-?\\d+")); System.out.println("5678".matches("-?\\d+")); System.out.println("+911".matches("-?\\d+")); System.out.println("+911".matches("(-|\\+)?\\d+")); } } /* Output: true true false true *///:~ 370 Thinking in Java Bruce Eckel The first two expressions match, but the third one starts with a ‘+’, which is a legitimate sign but means the number doesn’t match the regular expression. So we need a way to say, "may start with a + or a -." In regular expressions, parentheses have the effect of grouping an expression, and the vertical bar ‘|’ means OR. So (-I\\+)? means that this part of the string may be either a ‘-’ or a ‘+’ or nothing (because of the ‘?’). Because the ‘+’ character has special meaning in regular expressions, it must be escaped with a ‘\\’ in order to appear as an ordinary character in the expression. A useful regular expression tool that’s built into String is split( ), which means, "Split this string around matches of the given regular expression." //: strings/Splitting.java import java.util.*; public class Splitting { public static String knights = "Then, when you have found the shrubbery, you must " + "cut down the mightiest tree in the forest... " + "with... a herring!"; public static void split(String regex) { System.out.println( Arrays.toString(knights.split(regex))); } public static void main(String[] args) { split(" "); // Doesn’t have to contain regex chars split("\\W+"); // Non-word characters split("n\\W+"); // ‘n’ followed by non-word characters } } /* Output: [Then,, when, you, have, found, the, shrubbery,, you, must, cut, down, the, mightiest, tree, in, the, forest..., with..., a, herring!] [Then, when, you, have, found, the, shrubbery, you, must, cut, down, the, mightiest, tree, in, the, forest, with, a, herring] [The, whe, you have found the shrubbery, you must cut dow, the mightiest tree i, the forest... with... a herring!] *///:~ First, note that you may use ordinary characters as regular expressions—a regular expression doesn’t have to contain special characters, as you can see in the first call to split( ), which just splits on whitespace. The second and third calls to split( ) use ‘\W’, which means a non-word character (the lowercase version, ‘\w’, means a word character)—you can see that the punctuation has been removed in the second case. The third call to split( ) says, "the letter n followed by one or more non-word characters." You can see that the split patterns do not appear in the result. An overloaded version of String. split( ) allows you to limit the number of splits that occur. The final regular expression tool built into String is replacement. You can either replace the first occurrence, or all of them: //: strings/Replacing.java import static net.mindview.util.Print.*; public class Replacing { static String s = Splitting.knights; public static void main(String[] args) { Strings 371 print(s.replaceFirst("f\\w+", "located")); print(s.replaceAll("shrubbery|tree|herring","banana")); } } /* Output: Then, when you have located the shrubbery, you must cut down the mightiest tree in the forest... with... a herring! Then, when you have found the banana, you must cut down the mightiest banana in the forest... with... a banana! *///:~ The first expression matches the letter f followed by one or more word characters (note that the w is lowercase this time). It only replaces the first match that it finds, so the word "found" is replaced by the word "located." The second expression matches any of the three words separated by the OR vertical bars, and it replaces all matches that it finds. You’ll see that the non-String regular expressions have more powerful replacement tools— for example, you can call methods to perform replacements. Non-String regular expressions are also significantly more efficient if you need to use the regular expression more than once. Exercise 7: (5) Using the documentation for java.util.regex.Pattern as a resource, write and test a regular expression that checks a sentence to see that it begins with a capital letter and ends with a period. Exercise 8: (2) Split the string Splitting.knights on the words "the" or “you." Exercise 9: (4) Using the documentation for java.util.regex.Pattern as a resource, replace all the vowels in Splitting.knights with underscores. Creating regular expressions You can begin learning regular expressions with a subset of the possible constructs. A complete list of constructs for building regular expressions can be found in the JDK documentation for the Pattern class for package java.util.regex. B \xhh \uhhhh \t \n \r \f \e Characters The specific character B Character with hex value oxhh The Unicode character with hex representation 0xhhhh Tab Newline Carriage return Form feed Escape 372 Thinking in Java Bruce Eckel The power of regular expressions begins to appear when you are defining character classes. Here are some typical ways to create character classes, and some predefined classes: . [abc] [^abc] [a-zA-Z] [abc[hij]] [a-z&&[hij]] \s \S \d \D \w \W Character Classes Any character Any of the characters a, b, or c (same as a|b|c) Any character except a, b, and c (negation) Any character a through z or A through Z (range) Any of a,b,c,h,I,j (same as a|b|c|h|i|j) (union) Either h, i, or j (intersection) A whitespace character (space, tab, newline, form feed, carriage return) A non-whitespace character ([^\s]) A numeric digit [0-9] A non-digit [^o-9] A word character [a-zA-Z_0-9] A non-word character [^\w] What’s shown here is only a sample; you’ll want to bookmark the JDK documentation page for java.util.regex.Pattern so you can easily access all the possible regular expression patterns. Logical Operators XY X followed by Y X|Y X or Y (X) A capturing group. You can refer to the ith captured group later in the expression with \i. Boundary Matchers ^ Beginning of a line $ End of a line \b Word boundary \B Non-word boundary \G End of the previous match As an example, each of the following successfully matches the character sequence "Rudolph": //: strings/Rudolph.java public class Rudolph { public static void main(String[] args) { for(String pattern : new String[]{ "Rudolph", "[rR]udolph", "[rR][aeiou][a-z]ol.*", "R.*" }) System.out.println("Rudolph".matches(pattern)); Strings 373 } } /* Output: true true true true *///:~ Of course, your goal should not be to create the most obfuscated regular expression, but rather the simplest one necessary to do the job. You’ll find that, once you start writing regular expressions, you’ll often use your code as a reference when writing new regular expressions. Quantifiers A quantifier describes the way that a pattern absorbs input text: • Greedy: Quantifiers are greedy unless otherwise altered. A greedy expression finds as many possible matches for the pattern as possible. A typical cause of problems is to assume that your pattern will only match the first possible group of characters, when it’s actually greedy and will keep going until it’s matched the largest possible string. • Reluctant: Specified with a question mark, this quantifier matches the minimum number of characters necessary to satisfy the pattern. Also called lazy, minimal matching, non-greedy, or ungreedy. • Possessive: Currently this is only available in Java (not in other languages) and is more advanced, so you probably won’t use it right away. As a regular expression is applied to a string, it generates many states so that it can backtrack if the match fails. Possessive quantifiers do not keep those intermediate states, and thus prevent backtracking. They can be used to prevent a regular expression from running away and also to make it execute more efficiently. Greedy X? X* x+ X{n} X{n,} X{n,m} Reluctant X?? X*? x+? X{n}? X{n,}? X{n,m}? Possessive Matches X?+ X, one or none x*+ X, zero or more X++ X, one or more X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but not more than m times Keep in mind that the expression ‘X’ will often need to be surrounded in parentheses for it to work the way you desire. For example: abc+ might seem like it would match the sequence ‘abc’ one or more times, and if you apply it to the input string ‘abcabcabc’, you will in fact get three matches. However, the expression actually says, "Match ‘ab’ followed by one or more occurrences of ‘c’." To match the entire string ‘abc’ one or more times, you must say: 374 Thinking in Java Bruce Eckel (abc)+ You can easily be fooled when using regular expressions; it’s an orthogonal language, on top of Java. CharSequence The interface called CharSequence establishes a generalized definition of a character sequence abstracted from the CharBuffer, String, StringBuffer, or StringBuilder classes: interface CharSequence { charAt(int i); length(); subSequence(int start,| int end); toString(); } The aforementioned classes implement this interface. Many regular expression operations take CharSequence arguments. Pattern and Matcher In general, you’ll compile regular expression objects rather than using the fairly limited String utilities. To do this, you import java.util.regex, then compile a regular expression by using the static Pattern.compile( ) method. This produces a Pattern object based on its String argument. You use the Pattern by calling the matcher( ) method, passing the string that you want to search. The matcher( ) method produces a Matcher object, which has a set of operations to choose from (you can see all of these in the JDK documentation for java.util.regex.Matcher). For example, the replaceAll( ) method replaces all the matches with its argument. As a first example, the following class can be used to test regular expressions against an input string. The first command-line argument is the input string to match against, followed by one or more regular expressions to be applied to the input. Under Unix/Linux, the regular expressions must be quoted on the command line. This program can be useful in testing regular expressions as you construct them to see that they produce your intended matching behavior. //: strings/TestRegularExpression.java // Allows you to easily try out regular expressions. // {Args: abcabcabcdefabc "abc+" "(abc)+" "(abc){2,}" } import java.util.regex.*; import static net.mindview.util.Print.*; public class TestRegularExpression { public static void main(String[] args) { if(args.length < 2) { print("Usage:\njava TestRegularExpression " + "characterSequence regularExpression+"); System.exit(0); } print("Input: \"" + args[0] + "\""); for(String arg : args) { print("Regular expression: \"" + arg + "\""); Pattern p = Pattern.compile(arg); Matcher m = p.matcher(args[0]); while(m.find()) { Strings 375 print("Match \"" + m.group() + "\" at positions " + m.start() + "-" + (m.end() - 1)); } } } } /* Output: Input: "abcabcabcdefabc" Regular expression: "abcabcabcdefabc" Match "abcabcabcdefabc" at positions 0-14 Regular expression: "abc+" Match "abc" at positions 0-2 Match "abc" at positions 3-5 Match "abc" at positions 6-8 Match "abc" at positions 12-14 Regular expression: "(abc)+" Match "abcabcabc" at positions 0-8 Match "abc" at positions 12-14 Regular expression: "(abc){2,}" Match "abcabcabc" at positions 0-8 *///:~ A Pattern object represents the compiled version of a regular expression. As seen in the preceding example, you can use the matcher( ) method and the input string to produce a Matcher object from the compiled Pattern object. Pattern also has a static method: static boolean matches(String regex, CharSequence input) to check whether regex matches the entire input CharSequence, and a split( ) method that produces an array of String that has been broken around matches of the regex. A Matcher object is generated by calling Pattern.matcher( ) with the input string as an argument. The Matcher object is then used to access the results, using methods to evaluate the success or failure of different types of matches: boolean matches() boolean lookingAt() boolean find() boolean find(int start) The matches ( ) method is successful if the pattern matches the entire input string, while lookingAt( ) is successful if the input string, starting at the beginning, is a match to the pattern. Exercise 10: (2) For the phrase "Java now has regular expressions" evaluate whether the following expressions will find a match: ^Java \Breg.* n.w\s+h(a|i)s s? s* s+ s{4} S{1}. s{0,3} 376 Thinking in Java Bruce Eckel Exercise 11: (2) Apply the regular expression (?i)((^[aeiou])|(\s+[aeiou]))\w+?[aeiou]\b to "Arline ate eight apples and one orange while Anita hadn’t any" find() Matcher.find( ) can be used to discover multiple pattern matches in the CharSequence to which it is applied. For example: //: strings/Finding.java import java.util.regex.*; import static net.mindview.util.Print.*; public class Finding { public static void main(String[] args) { Matcher m = Pattern.compile("\\w+") .matcher("Evening is full of the linnet’s wings"); while(m.find()) printnb(m.group() + " "); print(); int i = 0; while(m.find(i)) { printnb(m.group() + " "); i++; } } } /* Output: Evening is full of the linnet s wings Evening vening ening ning ing ng g is is s full full ull ll l of of f the the he e linnet linnet innet nnet net et t s s wings wings ings ngs gs s *///:~ The pattern ‘\\w+’ splits the input into words. find( ) is like an iterator, moving forward through the input string. However, the second version of find( ) can be given an integer argument that tells it the character position for the beginning of the search—this version resets the search position to the value of the argument, as you can see from the output. Groups Groups are regular expressions set off by parentheses that can be called up later with their group number. Group o indicates the whole expression match, group l is the first parenthesized group, etc. Thus in A(B(C))D there are three groups: Group 0 is ABCD, group 1 is BC, and group 2 is C. The Matcher object has methods to give you information about groups: public int groupCount( ) returns the number of groups in this matcher’s pattern. Group o is not included in this count. Strings 377 public String group( ) returns group 0 (the entire match) from the previous match operation (find( ), for example). public String group(int i) returns the given group number during the previous match operation. If the match was successful, but the group specified failed to match any part of the input string, then null is returned. public int start(int group) returns the start index of the group found in the previous match operation. public int end(int group) returns the index of the last character, plus one, of the group found in the previous match operation. Here’s an example: //: strings/Groups.java import java.util.regex.*; import static net.mindview.util.Print.*; public class Groups { static public final String POEM = "Twas brillig, and the slithy toves\n" + "Did gyre and gimble in the wabe.\n" + "All mimsy were the borogoves,\n" + "And the mome raths outgrabe.\n\n" + "Beware the Jabberwock, my son,\n" + "The jaws that bite, the claws that catch.\n" + "Beware the Jubjub bird, and shun\n" + "The frumious Bandersnatch."; public static void main(String[] args) { Matcher m = Pattern.compile("(?m)(\\S+)\\s+((\\S+)\\s+(\\S+))$") .matcher(POEM); while(m.find()) { for(int j = 0; j <= m.groupCount(); j++) printnb("[" + m.group(j) + "]"); print(); } } } /* Output: [the slithy toves][the][slithy toves][slithy][toves] [in the wabe.][in][the wabe.][the][wabe.] [were the borogoves,][were][the borogoves,][the][borogoves,] [mome raths outgrabe.][mome][raths outgrabe.][raths][outgrabe.] [Jabberwock, my son,][Jabberwock,][my son,][my][son,] [claws that catch.][claws][that catch.][that][catch.] [bird, and shun][bird,][and shun][and][shun] [The frumious Bandersnatch.][The][frumious Bandersnatch.][frumious][Bandersnatch.] *///:~ The poem is the first part of Lewis Carroll’s "Jabberwocky," from Through the Looking Glass. You can see that the regular expression pattern has a number of parenthesized groups, consisting of any number of non-whitespace characters (‘\S+’) followed by any number of whitespace characters (‘\s+’). The goal is to capture the last three words on each line; the end of a line is delimited by ‘$’. However, the normal behavior is to match ‘$’ with the end of the entire input sequence, so you must explicitly tell the regular expression to pay attention to newlines within the input. This is accomplished with the ‘(?m)’ pattern flag at the beginning of the sequence (pattern flags will be shown shortly). 378 Thinking in Java Bruce Eckel Exercise 12: (5) Modify Groups.java to count all of the unique words that do not start with a capital letter. start() and end() Following a successful matching operation, start( ) returns the start index of the previous match, and end( ) returns the index of the last character matched, plus one. Invoking either start( ) or end( ) following an unsuccessful matching operation (or before attempting a matching operation) produces an IllegalStateException. The following program also demonstrates matches( ) and lookingAt( ):3 //: strings/StartEnd.java import java.util.regex.*; import static net.mindview.util.Print.*; public class StartEnd { public static String input = "As long as there is injustice, whenever a\n" + "Targathian baby cries out, wherever a distress\n" + "signal sounds among the stars ... We’ll be there.\n" + "This fine ship, and this fine crew ...\n" + "Never give up! Never surrender!"; private static class Display { private boolean regexPrinted = false; private String regex; Display(String regex) { this.regex = regex; } void display(String message) { if(!regexPrinted) { print(regex); regexPrinted = true; } print(message); } } static void examine(String s, String regex) { Display d = new Display(regex); Pattern p = Pattern.compile(regex); Matcher m = p.matcher(s); while(m.find()) d.display("find() ‘" + m.group() + "‘ start = "+ m.start() + " end = " + m.end()); if(m.lookingAt()) // No reset() necessary d.display("lookingAt() start = " + m.start() + " end = " + m.end()); if(m.matches()) // No reset() necessary d.display("matches() start = " + m.start() + " end = " + m.end()); } public static void main(String[] args) { for(String in : input.split("\n")) { print("input : " + in); for(String regex : new String[]{"\\w*ere\\w*", "\\w*ever", "T\\w+", "Never.*?!"}) examine(in, regex); } } } /* Output: input : As long as there is injustice, whenever a 3 Quote from one of Commander Taggart’s speeches on Galaxy Quest. Strings 379 \w*ere\w* find() ‘there’ start = 11 end = 16 \w*ever find() ‘whenever’ start = 31 end = 39 input : Targathian baby cries out, wherever a distress \w*ere\w* find() ‘wherever’ start = 27 end = 35 \w*ever find() ‘wherever’ start = 27 end = 35 T\w+ find() ‘Targathian’ start = 0 end = 10 lookingAt() start = 0 end = 10 input : signal sounds among the stars ... We’ll be there. \w*ere\w* find() ‘there’ start = 43 end = 48 input : This fine ship, and this fine crew ... T\w+ find() ‘This’ start = 0 end = 4 lookingAt() start = 0 end = 4 input : Never give up! Never surrender! \w*ever find() ‘Never’ start = 0 end = 5 find() ‘Never’ start = 15 end = 20 lookingAt() start = 0 end = 5 Never.*?! find() ‘Never give up!’ start = 0 end = 14 find() ‘Never surrender!’ start = 15 end = 31 lookingAt() start = 0 end = 14 matches() start = 0 end = 31 *///:~ Notice that find( ) will locate the regular expression anywhere in the input, but lookingAt( ) and matches( ) only succeed if the regular expression starts matching at the very beginning of the input. While matches( ) only succeeds if the entire input matches the regular expression, lookingAt( )4 succeeds if only the first part of the input matches. Exercise 13: (2) Modify StartEnd.java so that it uses Groups.POEM as input, but still produces positive outputs for find( ), lookingAt( ) and matches( ). Pattern flags An alternative compile( ) method accepts flags that affect matching behavior: Pattern Pattern.compile(String regex, int flag) where flag is drawn from among the following Pattern class constants: 4 I have no idea how they came up with this method name, or what it’s supposed to refer to. But it’s reassuring to know that whoever comes up with nonintuitive method names is still employed at Sun. And that their apparent policy of not reviewing code designs is still in place. Sorry for the sarcasm, but this kind of thing gets tiresome after a few years. 380 Thinking in Java Bruce Eckel Compile Flag Effect Pattern.CANON_EQ Pattern.CASE INSENSITIVE (?i) Pattern.COMMENTS (?x) Pattern.DOTALL (?s) Pattern.MULTILINE (?m) Pattern.UNICODE CASE (?u) Pattern.UNIX LINES (?d) Two characters will be considered to match if, and only if, their full canonical decompositions match. The expression ‘\u003F’, for example, will match the string ‘?’ when this flag is specified. By default, matching does not take canonical equivalence into account. By default, case-insensitive matching assumes that only characters in the USASCII character set are being matched. This flag allows your pattern to match without regard to case (upper or lower). Unicode-aware case-insensitive matching can be enabled by specifying the UNICODE_CASE flag in conjunction with this flag. In this mode, whitespace is ignored, and embedded comments starting with # are ignored until the end of a line. Unix lines mode can also be enabled via the embedded flag expression. In dotall mode, the expression’.’ matches any character, including a line terminator. By default, the ‘.’ expression does not match line terminators. In multiline mode, the expressions ‘^’ and ‘$’ match the beginning and ending of a line, respectively.’^’ also matches the beginning of the input string, and ‘$’ also matches the end of the input string. By default, these expressions only match at the beginning and the end of the entire input string. Case-insensitive matching, when enabled by the CASE_INSENSITIVE flag, is done in a manner consistent with the Unicode Standard. By default, caseinsensitive matching assumes that only characters in the US-ASCII character set are being matched. In this mode, only the ‘\n’ line terminator is recognized in the behavior of ‘.’, ‘^’, and ‘$’. Particularly useful among these flags are Pattern.CASE_INSENSITIVE, Pattern.MULTILINE, and Pattern.COMMENTS (which is helpful for clarity and/or documentation). Note that the behavior of most of the flags can also be obtained by inserting the parenthesized characters, shown beneath the flags in the table, into your regular expression preceding the place where you want the mode to take effect. Strings 381 You can combine the effect of these and other flags through an "OR" (‘|’) operation: //: strings/ReFlags.java import java.util.regex.*; public class ReFlags { public static void main(String[] args) { Pattern p = Pattern.compile("^java", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE); Matcher m = p.matcher( "java has regex\nJava has regex\n" + "JAVA has pretty good regular expressions\n" + "Regular expressions are in Java"); while(m.find()) System.out.println(m.group()); } } /* Output: java Java JAVA *///:~ This creates a pattern that will match lines starting with "Java," "Java," "JAVA," etc., and attempt a match for each line within a multiline set (matches starting at the beginning of the character sequence and following each line terminator within the character sequence). Note that the group( ) method only produces the matched portion. split() split( ) divides an input string into an array of String objects, delimited by the regular expression. String[] split(CharSequence input) String[] split(CharSequence input, int limit) This is a handy way to break input text on a common boundary: //: strings/SplitDemo.java import java.util.regex.*; import java.util.*; import static net.mindview.util.Print.*; public class SplitDemo { public static void main(String[] args) { String input = "This!!unusual use!!of exclamation!!points"; print(Arrays.toString( Pattern.compile("!!").split(input))); // Only do the first three: print(Arrays.toString( Pattern.compile("!!").split(input, 3))); } } /* Output: [This, unusual use, of exclamation, points] [This, unusual use, of exclamation!!points] *///:~ The second form of split( ) limits the number of splits that occur. 382 Thinking in Java Bruce Eckel Exercise 14: (1) Rewrite SplitDemo using String.split( ). Replace operations Regular expressions are especially useful to replace text. Here are the available methods: replaceFirst(String replacement) replaces the first matching part of the input string with replacement. replaceAll(String replacement) replaces every matching part of the input string with replacement. appendReplacement(StringBuffer sbuf, String replacement) performs step-by-step replacements into sbuf, rather than replacing only the first one or all of them, as in replaceFirst( ) and replaceAll( ), respectively. This is a very important method, because it allows you to call methods and perform other processing in order to produce replacement (replaceFirst( ) and replaceAll( ) are only able to put in fixed strings). With this method, you can programmatically pick apart the groups and create powerful replacements. appendTail(StringBuffer sbuf, String replacement) is invoked after one or more invocations of the appendReplacement( ) method in order to copy the remainder of the input string. Here’s an example that shows the use of all the replace operations. The block of commented text at the beginning is extracted and processed with regular expressions for use as input in the rest of the example: //: strings/TheReplacements.java import java.util.regex.*; import net.mindview.util.*; import static net.mindview.util.Print.*; /*! Here’s a block of text to use as input to the regular expression matcher. Note that we’ll first extract the block of text by looking for the special delimiters, then process the extracted block. !*/ public class TheReplacements { public static void main(String[] args) throws Exception { String s = TextFile.read("TheReplacements.java"); // Match the specially commented block of text above: Matcher mInput = Pattern.compile("/\\*!(.*)!\\*/", Pattern.DOTALL) .matcher(s); if(mInput.find()) s = mInput.group(1); // Captured by parentheses // Replace two or more spaces with a single space: s = s.replaceAll(" {2,}", " "); // Replace one or more spaces at the beginning of each // line with no spaces. Must enable MULTILINE mode: s = s.replaceAll("(?m)^ +", ""); print(s); s = s.replaceFirst("[aeiou]", "(VOWEL1)"); StringBuffer sbuf = new StringBuffer(); Pattern p = Pattern.compile("[aeiou]"); Matcher m = p.matcher(s); // Process the find information as you Strings 383 // perform the replacements: while(m.find()) m.appendReplacement(sbuf, m.group().toUpperCase()); // Put in the remainder of the text: m.appendTail(sbuf); print(sbuf); } } /* Output: Here’s a block of text to use as input to the regular expression matcher. Note that we’ll first extract the block of text by looking for the special delimiters, then process the extracted block. H(VOWEL1)rE’s A blOck Of tExt tO UsE As InpUt tO thE rEgUlAr ExprEssIOn mAtchEr. NOtE thAt wE’ll fIrst ExtrAct thE blOck Of tExt by lOOkIng fOr thE spEcIAl dElImItErs, thEn prOcEss thE ExtrActEd blOck. *///:~ The file is opened and read using the TextFile class in the net.mindview.util library (the code for this will be shown in the I/O chapter). The static read( ) method reads the entire file and returns it as a String. mInput is created to match all the text (notice the grouping parentheses) between ‘/*!’ and ‘!*/’. Then, more than two spaces are reduced to a single space, and any space at the beginning of each line is removed (in order to do this on all lines and not just the beginning of the input, multiline mode must be enabled). These two replacements are performed with the equivalent (but more convenient, in this case) replaceAll( ) that’s part of String. Note that since each replacement is only used once in the program, there’s no extra cost to doing it this way rather than precompiling it as a Pattern. replaceFirst( ) only performs the first replacement that it finds. In addition, the replacement strings in replaceFirst( ) and replaceAll( ) are just literals, so if you want to perform some processing on each replacement, they don’t help. In that case, you need to use appendReplacement( ), which allows you to write any amount of code in the process of performing the replacement. In the preceding example, a group( ) is selected and processed—in this situation, setting the vowel found by the regular expression to uppercase— as the resulting sbuf is being built. Normally, you step through and perform all the replacements and then call appendTail( ), but if you want to simulate replaceFirst( ) (or "replace n"), you just do the replacement one time and then call appendTail( ) to put the rest into sbuf. appendReplacement( ) also allows you to refer to captured groups directly in the replacement string by saying "$g", where ‘g’ is the group number. However, this is for simpler processing and wouldn’t give you the desired results in the preceding program. reset() An existing Matcher object can be applied to a new character sequence using the reset( ) methods: //: strings/Resetting.java import java.util.regex.*; public class Resetting { public static void main(String[] args) throws Exception { Matcher m = Pattern.compile("[frb][aiu][gx]") .matcher("fix the rug with bags"); while(m.find()) 384 Thinking in Java Bruce Eckel System.out.print(m.group() + " "); System.out.println(); m.reset("fix the rig with rags"); while(m.find()) System.out.print(m.group() + " "); } } /* Output: fix rug bag fix rig rag *///:~ reset( ) without any arguments sets the Matcher to the beginning of the current sequence. Regular expressions and Java I/O Most of the examples so far have shown regular expressions applied to static strings. The following example shows one way to apply regular expressions to search for matches in a file. Inspired by Unix’s grep, JGrep.java takes two arguments: a file name and the regular expression that you want to match. The output shows each line where a match occurs and the match position(s) within the line. //: strings/JGrep.java // A very simple version of the "grep" program. // {Args: JGrep.java "\\b[Ssct]\\w+"} import java.util.regex.*; import net.mindview.util.*; public class JGrep { public static void main(String[] args) throws Exception { if(args.length < 2) { System.out.println("Usage: java JGrep file regex"); System.exit(0); } Pattern p = Pattern.compile(args[1]); // Iterate through the lines of the input file: int index = 0; Matcher m = p.matcher(""); for(String line : new TextFile(args[0])) { m.reset(line); while(m.find()) System.out.println(index++ + ": " + m.group() + ": " + m.start()); } } } /* Output: (Sample) 0: strings: 4 1: simple: 10 2: the: 28 3: Ssct: 26 4: class: 7 5: static: 9 6: String: 26 7: throws: 41 8: System: 6 9: System: 6 10: compile: 24 11: through: 15 12: the: 23 13: the: 36 14: String: 8 15: System: 8 Strings 385 16: start: 31 *///:~ The file is opened as a net.mindview.util.TextFile object (which will be shown in the I/O chapter), which reads the lines of the file into an ArrayList. This means that the foreach syntax can iterate through the lines in the TextFile object. Although it’s possible to create a new Matcher object within the for loop, it is slightly more optimal to create an empty Matcher object outside the loop and use the reset( ) method to assign each line of the input to the Matcher. The result is scanned with find( ). The test arguments open the JGrep.java file to read as input, and search for words starting with [Ssct]. You can learn much more about regular expressions in Mastering Regular Expressions, 2nd Edition, by Jeffrey E. F. Friedl (O’Reilly, 2002). There are also numerous introductions to regular expressions on the Internet, and you can often find helpful information in the documentation for languages like Perl and Python. Exercise 15: (5) Modify JGrep.java to accept flags as arguments (e.g., Pattern.CASE_INSENSITIVE, Pattern.MULTILINE). Exercise 16: (5) Modify JGrep.java to accept a directory name or a file name as argument (if a directory is provided, search should include all files in the directory). Hint: You can generate a list of file names with: File[] files = new File(".").listFiles(); Exercise 17: (8) Write a program that reads a Java source-code file (you provide the file name on the command line) and displays all the comments. Exercise 18: (8) Write a program that reads a Java source-code file (you provide the file name on the command line) and displays all the string literals in the code. Exercise 19: (8) Building on the previous two exercises, write a program that examines Java source code and produces all the class names used in a particular program. Scanning input Until now it has been relatively painful to read data from a human-readable file or from standard input. The usual solution is to read in a line of text, tokenize it, and then use the various parse methods of Integer, Double, etc., to parse the data: //: strings/SimpleRead.java import java.io.*; public class SimpleRead { public static BufferedReader input = new BufferedReader( new StringReader("Sir Robin of Camelot\n22 1.61803")); public static void main(String[] args) { try { System.out.println("What is your name?"); String name = input.readLine(); System.out.println(name); System.out.println( "How old are you? What is your favorite double?"); 386 Thinking in Java Bruce Eckel System.out.println("(input: )"); String numbers = input.readLine(); System.out.println(numbers); String[] numArray = numbers.split(" "); int age = Integer.parseInt(numArray[0]); double favorite = Double.parseDouble(numArray[1]); System.out.format("Hi %s.\n", name); System.out.format("In 5 years you will be %d.\n", age + 5); System.out.format("My favorite double is %f.", favorite / 2); } catch(IOException e) { System.err.println("I/O exception"); } } } /* Output: What is your name? Sir Robin of Camelot How old are you? What is your favorite double? (input: ) 22 1.61803 Hi Sir Robin of Camelot. In 5 years you will be 27. My favorite double is 0.809015. *///:~ The input field uses classes from java.io, which will not officially be introduced until the I/O chapter. A StringReader turns a String into a readable stream, and this object is used to create a BufferedReader because BufferedReader has a readLine( ) method. The result is that the input object can be read a line at a time, just as if it were standard input from the console. readLine( ) is used to get the String for each line of input. It’s fairly straightforward when you want to get one input for each line of data, but if two input values are on a single line, things get messy—the line must be split so we can parse each input separately. Here, the splitting takes place when creating numArray, but note that the split( ) method was introduced in J2SE1.4, so before that you had to do something else. The Scanner class, added in Java SE5, relieves much of the burden of scanning input: //: strings/BetterRead.java import java.util.*; public class BetterRead { public static void main(String[] args) { Scanner stdin = new Scanner(SimpleRead.input); System.out.println("What is your name?"); String name = stdin.nextLine(); System.out.println(name); System.out.println( "How old are you? What is your favorite double?"); System.out.println("(input: )"); int age = stdin.nextInt(); double favorite = stdin.nextDouble(); System.out.println(age); System.out.println(favorite); System.out.format("Hi %s.\n", name); System.out.format("In 5 years you will be %d.\n", age + 5); System.out.format("My favorite double is %f.", favorite / 2); Strings 387 } } /* Output: What is your name? Sir Robin of Camelot How old are you? What is your favorite double? (input: ) 22 1.61803 Hi Sir Robin of Camelot. In 5 years you will be 27. My favorite double is 0.809015. *///:~ The Scanner constructor can take just about any kind of input object, including a File object (which will also be covered in the I/O chapter), an InputStream, a String, or in this case a Readable, which is an interface introduced in Java SE5 to describe "something that has a read( ) method." The BufferedReader from the previous example falls into this category. With Scanner, the input, tokenizing, and parsing are all ensconced in various different kinds of "next" methods. A plain next( ) returns the next String token, and there are "next" methods for all the primitive types (except char) as well as for BigDecimal and Biglnteger. All of the "next" methods block, meaning they will return only after a complete data token is available for input. There are also corresponding "hasNext" methods that return true if the next input token is of the correct type. An interesting difference between the two previous examples above is the lack of a try block for IOExceptions in BetterRead.java. One of the assumptions made by the Scanner is that an IOException signals the end of input, and so these are swallowed by the Scanner. However, the most recent exception is available through the ioException( ) method, so you are able to examine it if necessary. Exercise 20: (2) Create a class that contains int, long, float and double and String fields. Create a constructor for this class that takes a single String argument, and scans that string into the various fields. Add a toString( ) method and demonstrate that your class works correctly. Scanner delimiters By default, a Scanner splits input tokens along whitespace, but you can also specify your own delimiter pattern in the form of a regular expression: //: strings/ScannerDelimiter.java import java.util.*; public class ScannerDelimiter { public static void main(String[] args) { Scanner scanner = new Scanner("12, 42, 78, 99, 42"); scanner.useDelimiter("\\s*,\\s*"); while(scanner.hasNextInt()) System.out.println(scanner.nextInt()); } } /* Output: 12 42 78 99 42 *///:~ 388 Thinking in Java Bruce Eckel This example uses commas (surrounded by arbitrary amounts of whitespace) as the delimiter when reading from the given String. This same technique can be used to read from commadelimited files. In addition to useDelimiter( ) for setting the delimiter pattern, there is also delimiter( ), which returns the current Pattern being used as a delimiter. Scanning with regular expressions In addition to scanning for predefined primitive types, you can also scan for your own userdefined patterns, which is helpful when scanning more complex data. This example scans threat data from a log like your firewall might produce: //: strings/ThreatAnalyzer.java import java.util.regex.*; import java.util.*; public class ThreatAnalyzer { static String threatData = "58.27.82.161@02/10/2005\n" + "204.45.234.40@02/11/2005\n" + "58.27.82.161@02/11/2005\n" + "58.27.82.161@02/12/2005\n" + "58.27.82.161@02/12/2005\n" + "[Next log section with different data format]"; public static void main(String[] args) { Scanner scanner = new Scanner(threatData); String pattern = "(\\d+[.]\\d+[.]\\d+[.]\\d+)@" + "(\\d{2}/\\d{2}/\\d{4})"; while(scanner.hasNext(pattern)) { scanner.next(pattern); MatchResult match = scanner.match(); String ip = match.group(1); String date = match.group(2); System.out.format("Threat on %s from %s\n", date,ip); } } } /* Output: Threat on 02/10/2005 from 58.27.82.161 Threat on 02/11/2005 from 204.45.234.40 Threat on 02/11/2005 from 58.27.82.161 Threat on 02/12/2005 from 58.27.82.161 Threat on 02/12/2005 from 58.27.82.161 *///:~ When you use next( ) with a specific pattern, that pattern is matched against the next input token. The result is made available by the match( ) method, and as you can see above, it works just like the regular expression matching you saw earlier. There’s one caveat when scanning with regular expressions. The pattern is matched against the next input token only, so if your pattern contains a delimiter it will never be matched. StringTokenizer Before regular expressions (in J2SE1.4) or the Scanner class (in Java SE5), the way to split a string into parts was to "tokenize" it with StringTokenizer. But now it’s much easier and more succinct to do the same thing with regular expressions or the Scanner class. Here’s a simple comparison of StringTokenizer to the other two techniques: //: strings/ReplacingStringTokenizer.java Strings 389 import java.util.*; public class ReplacingStringTokenizer { public static void main(String[] args) { String input = "But I’m not dead yet! I feel happy!"; StringTokenizer stoke = new StringTokenizer(input); while(stoke.hasMoreElements()) System.out.print(stoke.nextToken() + " "); System.out.println(); System.out.println(Arrays.toString(input.split(" "))); Scanner scanner = new Scanner(input); while(scanner.hasNext()) System.out.print(scanner.next() + " "); } } /* Output: But I’m not dead yet! I feel happy! [But, I’m, not, dead, yet!, I, feel, happy!] But I’m not dead yet! I feel happy! *///:~ With regular expressions or Scanner objects, you can also split a string into parts using more complex patterns—something that’s difficult with StringTokenizer. It seems safe to say that the StringTokenizer is obsolete. 390 Thinking in Java Bruce Eckel Summary In the past, Java support for string manipulation was rudimentary, but in recent editions of the language we’ve seen far more sophisticated support adopted from other languages. At this point, the support for strings is reasonably complete, although you must sometimes pay attention to efficiency details such as the appropriate use of StringBuilder. Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for sale from www.MindView.net. Strings 391 Type Information Runtime type information (RTTI) allows you to discover and use type information while a program is running. It frees you from the constraint of doing type-oriented things only at compile time, and can enable some very powerful programs. The need for RTTI uncovers a plethora of interesting (and often perplexing) 0 0 design issues, and raises fundamental questions about how you should structure your programs. This chapter looks at the ways that Java allows you to discover information about objects and classes at run time. This takes two forms: "traditional" RTTI, which assumes that you have all the types available at compile time, and the reflection mechanism, which allows you to discover and use class information solely at run time. The need for RTTI Consider the now-familiar example of a class hierarchy that uses polymorphism. The generic type is the base class Shape, and the specific derived types are Circle, Square, and Triangle: This is a typical class hierarchy diagram, with the base class at the top and the derived classes growing downward. The normal goal in object-oriented programming is for your code to manipulate references to the base type (Shape, in this case), so if you decide to extend the program by adding a new class (such as Rhomboid, derived from Shape), the bulk of the code is not affected. In this example, the dynamically bound method in the Shape interface is draw( ), so the intent is for the client programmer to call draw( ) through a generic Shape reference. In all of the derived classes, draw( ) is overridden, and because it is a dynamically bound method, the proper behavior will occur even though it is called through a generic Shape reference. That’s polymorphism. Thus, you generally create a specific object (Circle, Square, or Triangle), upcast it to a Shape (forgetting the specific type of the object), and use that anonymous Shape reference in the rest of the program. You might code the Shape hierarchy as follows: //: typeinfo/Shapes.java import java.util.*; abstract class Shape { void draw() { System.out.println(this + ".draw()"); } abstract public String toString(); } class Circle extends Shape { public String toString() { return "Circle"; } } class Square extends Shape { public String toString() { return "Square"; } } class Triangle extends Shape { public String toString() { return "Triangle"; } } public class Shapes { public static void main(String[] args) { List shapeList = Arrays.asList( new Circle(), new Square(), new Triangle() ); for(Shape shape : shapeList) shape.draw(); } } /* Output: Circle.draw() Square.draw() Triangle.draw() *///:~ The base class contains a draw( ) method that indirectly uses toString( ) to print an identifier for the class by passing this to System.out.println( ) (notice that toString( ) is declared abstract to force inheritors to override it, and to prevent the instantiation of a plain Shape). If an object appears in a string concatenation expression (involving ‘+’ and String objects), the toString( ) method is automatically called to produce a String representation for that object. Each of the derived classes overrides the toString( ) method (from Object) so that draw( ) ends up (polymorphically) printing something different in each case. In this example, the upcast occurs when the shape is placed into the List. During the upcast to Shape, the fact that the objects are specific types of Shape is lost. To the array, they are just Shapes. At the point that you fetch an element out of the array, the container—which is actually holding everything as an Object—automatically casts the result back to a Shape. This is the most basic form of RTTI, because all casts are checked at run time for correctness. That’s what RTTI means: At run time, the type of an object is identified. In this case, the RTTI cast is only partial: The Object is cast to a Shape, and not all the way to a Circle, Square, or Triangle. That’s because the only thing you know at this point is that the List is full of Shapes. At compile time, this is enforced by the container and the Java generic system, but at run time the cast ensures it. Now polymorphism takes over and the exact code that’s executed for the Shape is determined by whether the reference is for a Circle, Square, or Triangle. And in general, this is how it should be; you want the bulk of your code to know as little as possible about specific types of objects, and to just deal with the general representation of a family of objects (in this case, Shape). As a result, your code will be easier to write, read, and maintain, and your designs will be easier to implement, understand, and change. So polymorphism is a general goal in object-oriented programming. 394 Thinking in Java Bruce Eckel But what if you have a special programming problem that’s easiest to solve if you know the exact type of a generic reference? For example, suppose you want to allow your users to highlight all the shapes of any particular type by turning them a special color. This way, they can find all the triangles on the screen by highlighting them. Or perhaps your method needs to "rotate" a list of shapes, but it makes no sense to rotate a circle so you’d like to skip the circles. With RTTI, you can ask a Shape reference the exact type that it’s referring to, and thus select and isolate special cases. The Class object To understand how RTTI works in Java, you must first know how type information is represented at run time. This is accomplished through a special kind of object called the Class object, which contains information about the class. In fact, the Class object is used to create all of the "regular" objects of your class. Java performs its RTTI using the Class object, even if you’re doing something like a cast. The class Class also has a number of other ways you can use RTTI. There’s one Class object for each class that is part of your program. That is, each time you write and compile a new class, a single Class object is also created (and stored, appropriately enough, in an identically named .class file). To make an object of that class, the Java Virtual Machine (JVM) that’s executing your program uses a subsystem called a class loader. The class loader subsystem can actually comprise a chain of class loaders, but there’s only one primordial class loader, which is part of the JVM implementation. The primordial class loader loads so-called trusted classes, including Java API classes, typically from the local disk. It’s usually not necessary to have additional class loaders in the chain, but if you have special needs (such as loading classes in a special way to support Web server applications, or downloading classes across a network), then you have a way to hook in additional class loaders. All classes are loaded into the JVM dynamically, upon the first use of a class. This happens when the program makes the first reference to a static member of that class. It turns out that the constructor is also a static method of a class, even though the static keyword is not used for a constructor. Therefore, creating a new object of that class using the new operator also counts as a reference to a static member of the class. Thus, a Java program isn’t completely loaded before it begins, but instead pieces of it are loaded when necessary. This is different from many traditional languages. Dynamic loading enables behavior that is difficult or impossible to duplicate in a statically loaded language like C++. The class loader first checks to see if the Class object for that type is loaded. If not, the default class loader finds the .class file with that name (an add-on class loader might, for example, look for the bytecodes in a database instead). As the bytes for the class are loaded, they are verified to ensure that they have not been corrupted and that they do not comprise bad Java code (this is one of the lines of defense for security in Java). Once the Class object for that type is in memory, it is used to create all objects of that type. Here’s a program to prove it: //: typeinfo/SweetShop.java // Examination of the way the class loader works. import static net.mindview.util.Print.*; class Candy { static { print("Loading Candy"); } } Type Information 395 class Gum { static { print("Loading Gum"); } } class Cookie { static { print("Loading Cookie"); } } public class SweetShop { public static void main(String[] args) { print("inside main"); new Candy(); print("After creating Candy"); try { Class.forName("Gum"); } catch(ClassNotFoundException e) { print("Couldn’t find Gum"); } print("After Class.forName(\"Gum\")"); new Cookie(); print("After creating Cookie"); } } /* Output: inside main Loading Candy After creating Candy Loading Gum After Class.forName("Gum") Loading Cookie After creating Cookie *///:~ Each of the classes Candy, Gum, and Cookie has a static clause that is executed as the class is loaded for the first time. Information will be printed to tell you when loading occurs for that class. In main( ), the object creations are spread out between print statements to help detect the time of loading. You can see from the output that each Class object is loaded only when it’s needed, and the static initialization is performed upon class loading. A particularly interesting line is: Class.forName("Gum"); All Class objects belong to the class Class. A Class object is like any other object, so you can get and manipulate a reference to it (that’s what the loader does). One of the ways to get a reference to the Class object is the static forName( ) method, which takes a String containing the textual name (watch the spelling and capitalization!) of the particular class you want a reference for. It returns a Class reference, which is being ignored here; the call to forName( ) is being made for its side effect, which is to load the class Gum if it isn’t already loaded. In the process of loading, Gum’s static clause is executed. In the preceding example, if Class.forName( ) fails because it can’t find the class you’re trying to load, it will throw a ClassNotFoundException. Here, we simply report the problem and move on, but in more sophisticated programs, you might try to fix the problem inside the exception handler. 396 Thinking in Java Bruce Eckel Anytime you want to use type information at run time, you must first get a reference to the appropriate Class object. Class.forName( ) is one convenient way to do this, because you don’t need an object of that type in order to get the Class reference. However, if you already have an object of the type you’re interested in, you can fetch the Class reference by calling a method that’s part of the Object root class: getClass( ). This returns the Class reference representing the actual type of the object. Class has many interesting methods; here are a few of them: //: typeinfo/toys/ToyTest.java // Testing class Class. package typeinfo.toys; import static net.mindview.util.Print.*; interface HasBatteries {} interface Waterproof {} interface Shoots {} class Toy { // Comment out the following default constructor // to see NoSuchMethodError from (*1*) Toy() {} Toy(int i) {} } class FancyToy extends Toy implements HasBatteries, Waterproof, Shoots { FancyToy() { super(1); } } public class ToyTest { static void printInfo(Class cc) { print("Class name: " + cc.getName() + " is interface? [" + cc.isInterface() + "]"); print("Simple name: " + cc.getSimpleName()); print("Canonical name : " + cc.getCanonicalName()); } public static void main(String[] args) { Class c = null; try { c = Class.forName("typeinfo.toys.FancyToy"); } catch(ClassNotFoundException e) { print("Can’t find FancyToy"); System.exit(1); } printInfo(c); for(Class face : c.getInterfaces()) printInfo(face); Class up = c.getSuperclass(); Object obj = null; try { // Requires default constructor: obj = up.newInstance(); } catch(InstantiationException e) { print("Cannot instantiate"); System.exit(1); } catch(IllegalAccessException e) { print("Cannot access"); System.exit(1); } printInfo(obj.getClass()); } } /* Output: Type Information 397 Class name: typeinfo.toys.FancyToy is interface? [false] Simple name: FancyToy Canonical name : typeinfo.toys.FancyToy Class name: typeinfo.toys.HasBatteries is interface? [true] Simple name: HasBatteries Canonical name : typeinfo.toys.HasBatteries Class name: typeinfo.toys.Waterproof is interface? [true] Simple name: Waterproof Canonical name : typeinfo.toys.Waterproof Class name: typeinfo.toys.Shoots is interface? [true] Simple name: Shoots Canonical name : typeinfo.toys.Shoots Class name: typeinfo.toys.Toy is interface? [false] Simple name: Toy Canonical name : typeinfo.toys.Toy *///:~ FancyToy inherits from Toy and implements the interfaces HasBatteries, Waterproof, and Shoots. In main( ), a Class reference is created and initialized to the FancyToy Class using forName( ) inside an appropriate try block. Notice that you must use the fully qualified name (including the package name) in the string that you pass to forName( ). printInfo( ) uses getName( ) to produce the fully qualified class name, and getSimpleName( ) and getCanonicalName( ) (introduced in Java SE5) to produce the name without the package, and the fully qualified name, respectively. As its name implies, islnterface( ) tells you whether this Class object represents an interface. Thus, with the Class object you can find out just about everything you want to know about a type. The Class.getlnterfaces( ) method called in main( ) returns an array of Class objects representing the interfaces that are contained in the Class object of interest. If you have a Class object, you can also ask it for its direct base class using getSuperclass( ). This returns a Class reference that you can further query. Thus you can discover an object’s entire class hierarchy at run time. The newlnstance( ) method of Class is a way to implement a "virtual constructor," which allows you to say, "I don’t know exactly what type you are, but create yourself properly anyway." In the preceding example, up is just a Class reference with no further type information known at compile time. And when you create a new instance, you get back an Object reference. But that reference is pointing to a Toy object. Of course, before you can send any messages other than those accepted by Object, you must investigate it a bit and do some casting. In addition, the class that’s being created with newlnstance( ) must have a default constructor. Later in this chapter, you’ll see how to dynamically create objects of classes using any constructor, with the Java reflection API. Exercise 1: (1) In ToyTest.java, comment out Toy’s default constructor and explain what happens. Exercise 2: (2) Incorporate a new kind of interface into ToyTest.java and verify that it is detected and displayed properly. Exercise 3: (2) Add Rhomboid to Shapes.java. Create a Rhomboid, upcast it to a Shape, then downcast it back to a Rhomboid. Try downcasting to a Circle and see what happens. Exercise 4: (2) Modify the previous exercise so that it uses instanceof to check the type before performing the downcast. 398 Thinking in Java Bruce Eckel Exercise 5: (3) Implement a rotate(Shape) method in Shapes.java, such that it checks to see if it is rotating a Circle (and, if so, doesn’t perform the operation). Exercise 6: (4) Modify Shapes.java so that it can "highlight" (set a flag in) all shapes of a particular type. The toString( ) method for each derived Shape should indicate whether that Shape is "highlighted." Exercise 7: (3) Modify SweetShop.java so that each type of object creation is controlled by a command-line argument. That is, if your command line is "Java Sweetshop Candy," then only the Candy object is created. Notice how you can control which Class objects are loaded via the commandline argument. Exercise 8: (5) Write a method that takes an object and recursively prints all the classes in that object’s hierarchy. Exercise 9: (5) Modify the previous exercise so that it uses Class.getDeclaredFields( ) to also display information about the fields in a class. Exercise 10: (3) Write a program to determine whether an array of char is a primitive type or a true Object. Class literals Java provides a second way to produce the reference to the Class object: the class literal. In the preceding program this would look like: FancyToy.class; which is not only simpler, but also safer since it’s checked at compile time (and thus does not need to be placed in a try block). Because it eliminates the forName( ) method call, it’s also more efficient. Class literals work with regular classes as well as interfaces, arrays, and primitive types. In addition, there’s a standard field called TYPE that exists for each of the primitive wrapper classes. The TYPE field produces a reference to the Class object for the associated primitive type, such that: ... is equivalent to ... boolean.class Boolean.TYPE char.class Character.TYPE byte.class Byte.TYPE short.class Short.TYPE int.class Integer.TYPE long.class Long.TYPE float.class Float.TYPE double.class Double.TYPE void.class Void.TYPE My preference is to use the ".class" versions if you can, since they’re more consistent with regular classes. Type Information 399 It’s interesting to note that creating a reference to a Class object using ".class" doesn’t automatically initialize the Class object. There are actually three steps in preparing a class for use: 1. Loading, which is performed by the class loader. This finds the bytecodes (usually, but not necessarily, on your disk in your classpath) and creates a Class object from those bytecodes. 2. Linking. The link phase verifies the bytecodes in the class, allocates storage for static fields, and if necessary, resolves all references to other classes made by this class. 3. Initialization. If there’s a superclass, initialize that. Execute static initializers and static initialization blocks. Initialization is delayed until the first reference to a static method (the constructor is implicitly static) or to a non-constant static field: //: typeinfo/ClassInitialization.java import java.util.*; class Initable { static final int staticFinal = 47; static final int staticFinal2 = ClassInitialization.rand.nextInt(1000); static { System.out.println("Initializing Initable"); } } class Initable2 { static int staticNonFinal = 147; static { System.out.println("Initializing Initable2"); } } class Initable3 { static int staticNonFinal = 74; static { System.out.println("Initializing Initable3"); } } public class ClassInitialization { public static Random rand = new Random(47); public static void main(String[] args) throws Exception { Class initable = Initable.class; System.out.println("After creating Initable ref"); // Does not trigger initialization: System.out.println(Initable.staticFinal); // Does trigger initialization: System.out.println(Initable.staticFinal2); // Does trigger initialization: System.out.println(Initable2.staticNonFinal); Class initable3 = Class.forName("Initable3"); System.out.println("After creating Initable3 ref"); System.out.println(Initable3.staticNonFinal); } } /* Output: After creating Initable ref 47 400 Thinking in Java Bruce Eckel Initializing Initable 258 Initializing Initable2 147 Initializing Initable3 After creating Initable3 ref 74 *///:~ Effectively, initialization is "as lazy as possible." From the creation of the initable reference, you can see that just using the .class syntax to get a reference to the class doesn’t cause initialization. However, Class.forName( ) initializes the class immediately in order to produce the Class reference, as you can see in the creation of initable3. If a static final value is a "compile-time constant," such as Initable.staticFinal, that value can be read without causing the Initable class to be initialized. Making a field static and final, however, does not guarantee this behavior: accessing Initable.staticFinal2 forces class initialization because it cannot be a compile-time constant. If a static field is not final, accessing it always requires linking (to allocate storage for the field) and initialization (to initialize that storage) before it can be read, as you can see in the access to Initable2.staticNonFinal. Generic class references A Class reference points to a Class object, which manufactures instances of classes and contains all the method code for those instances. It also contains the statics for that class. So a Class reference really does indicate the exact type of what it’s pointing to: an object of the class Class. However, the designers of Java SE5 saw an opportunity to make this a bit more specific by allowing you to constrain the type of Class object that the Class reference is pointing to, using the generic syntax. In the following example, both syntaxes are correct: //: typeinfo/GenericClassReferences.java public class GenericClassReferences { public static void main(String[] args) { Class intClass = int.class; Class genericIntClass = int.class; genericIntClass = Integer.class; // Same thing intClass = double.class; // genericIntClass = double.class; // Illegal } } ///:~ The ordinary class reference does not produce a warning. However, you can see that the ordinary class reference can be reassigned to any other Class object, whereas the generic class reference can only be assigned to its declared type. By using the generic syntax, you allow the compiler to enforce extra type checking. What if you’d like to loosen the constraint a little? Initially, it seems like you ought to be able to do something like: Class genericNumberClass = int.class; This would seem to make sense because Integer is inherited from Number. But this doesn’t work, because the Integer Class object is not a subclass of the Number Class Type Information 401 object (this may seem like a subtle distinction; we’ll look into it more deeply in the Generics chapter). To loosen the constraints when using generic Class references, I employ the wildcard, which is part of Java generics. The wildcard symbol is ‘?’, and it indicates "anything." So we can add wildcards to the ordinary Class reference in the above example and produce the same results: //: typeinfo/WildcardClassReferences.java public class WildcardClassReferences { public static void main(String[] args) { Class intClass = int.class; intClass = double.class; } } ///:~ In Java SE5, Class is preferred over plain Class, even though they are equivalent and the plain Class, as you saw, doesn’t produce a compiler warning. The benefit of Class is that it indicates that you aren’t just using a non-specific class reference by accident, or out of ignorance. You chose the non-specific version. In order to create a Class reference that is constrained to a type or any subtype, you combine the wildcard with the extends keyword to create a bound. So instead of just saying Class, you say: //: typeinfo/BoundedClassReferences.java public class BoundedClassReferences { public static void main(String[] args) { Class bounded = int.class; bounded = double.class; bounded = Number.class; // Or anything else derived from Number. } } ///:~ The reason for adding the generic syntax to Class references is only to provide compile-time type checking, so that if you do something wrong you find out about it a little sooner. You can’t actually go astray with ordinary Class references, but if you make a mistake you won’t find out until run time, which can be inconvenient. Here’s an example that uses the generic class syntax. It stores a class reference, and later produces a List filled with objects that it generates using newlnstance( ): //: typeinfo/FilledList.java import java.util.*; class CountedInteger { private static long counter; private final long id = counter++; public String toString() { return Long.toString(id); } } public class FilledList { private Class type; public FilledList(Class type) { this.type = type; } public List create(int nElements) { List result = new ArrayList(); try { 402 Thinking in Java Bruce Eckel for(int i = 0; i < nElements; i++) result.add(type.newInstance()); } catch(Exception e) { throw new RuntimeException(e); } return result; } public static void main(String[] args) { FilledList fl = new FilledList(CountedInteger.class); System.out.println(fl.create(15)); } } /* Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] *///:~ Notice that this class must assume that any type that it works with has a default constructor (one without arguments), and you’ll get an exception if that isn’t the case. The compiler does not issue any warnings for this program. An interesting thing happens when you use the generic syntax for Class objects: newlnstance( ) will return the exact type of the object, rather than just a basic Object as you saw in ToyTest.java. This is somewhat limited: //: typeinfo/toys/GenericToyTest.java // Testing class Class. package typeinfo.toys; public class GenericToyTest { public static void main(String[] args) throws Exception { Class ftClass = FancyToy.class; // Produces exact type: FancyToy fancyToy = ftClass.newInstance(); Class up = ftClass.getSuperclass(); // This won’t compile: // Class up2 = ftClass.getSuperclass(); // Only produces Object: Object obj = up.newInstance(); } } ///:~ If you get the superclass, the compiler will only allow you to say that the superclass reference is "some class that is a superclass of FancyToy" as seen in the expression Class . It will not accept a declaration of Class. This seems a bit strange because getSuperclass( ) returns the base class (not interface) and the compiler knows what that class is at compile time—in this case, Toy.class, not just "some superclass of FancyToy." In any event, because of the vagueness, the return value of up.newlnstance( ) is not a precise type, but just an Object. New cast syntax Java SE5 also adds a casting syntax for use with Class references, which is the cast( ) method: //: typeinfo/ClassCasts.java class Building {} class House extends Building {} Type Information 403 public class ClassCasts { public static void main(String[] args) { Building b = new House(); Class houseType = House.class; House h = houseType.cast(b); h = (House)b; // ... or just do this. } } ///:~ The cast( ) method takes the argument object and casts it to the type of the Class reference. Of course, if you look at the above code it seems like a lot of extra work compared to the last line in main( ), which does the same thing. The new casting syntax is useful for situations where you can’t just use an ordinary cast. This usually happens when you’re writing generic code (which you’ll learn about in the Generics chapter), and you’ve stored a Class reference that you want to use to cast with at a later time. It turns out to be a rare thing—I found only one instance where cast( ) was used in the entire Java SE5 library (it was in com.sun.mirror.util.DeclarationFilter). Another new feature had no usage in the Java SE5 library: Class.asSubclass( ). This allows you to cast the class object to a more specific type. Checking before a cast So far, you’ve seen forms of RTTI, including: 1. The classic cast; e.g., "(Shape)," which uses RTTI to make sure the cast is correct. This will throw a ClassCastException if you’ve performed a bad cast. 2. The Class object representing the type of your object. The Class object can be queried for useful runtime information. In C++, the classic cast "(Shape)" does not perform RTTI. It simply tells the compiler to treat the object as the new type. In Java, which does perform the type check, this cast is often called a "type-safe downcast." The reason for the term "downcast" is the historical arrangement of the class hierarchy diagram. If casting a Circle to a Shape is an upcast, then casting a Shape to a Circle is a downcast. However, because it knows that a Circle is also a Shape, the compiler freely allows an upcast assignment, without requiring any explicit cast syntax. The compiler cannot know, given a Shape, what that Shape actually is—it could be exactly a Shape, or it could be a subtype of Shape, such as a Circle, Square, Triangle or some other type. At compile time, the compiler only sees a Shape. Thus, it won’t allow you to perform a downcast assignment without using an explicit cast, to tell it that you have extra information that allows you to know that it is a particular type (the compiler will check to see if that downcast is reasonable, so it won’t let you downcast to a type that’s not actually a subclass). There’s a third form of RTTI in Java. This is the keyword instanceof, which tells you if an object is an instance of a particular type. It returns a boolean so you use it in the form of a question, like this: if(x instanceof Dog) ((Dog)x).bark(); The if statement checks to see if the object x belongs to the class Dog before casting x to a Dog. It’s important to use instanceof before a downcast when you don’t have other information that tells you the type of the object; otherwise, you’ll end up with a ClassCastException. 404 Thinking in Java Bruce Eckel Ordinarily, you might be hunting for one type (triangles to turn purple, for example), but you can easily tally all of the objects by using instanceof. For example, suppose you have a family of classes to describe Pets (and their people, a feature which will come in handy in a later example). Each Individual in the hierarchy has an id and an optional name. Although the classes that follow inherit from Individual, there are some complexities in the Individual class, so that code will be shown and explained in the Containers in Depth chapter. As you can see, it’s not really necessary to see the code for Individual at this point—you only need to know that you can create it with or without a name, and that each Individual has a method id( ) that returns a unique identifier (created by counting each object). There’s also a toString( ) method; if you don’t provide a name for an Individual, toString( ) only produces the simple type name. Here is the class hierarchy that inherits from Individual: //: typeinfo/pets/Person.java package typeinfo.pets; public class Person extends Individual { public Person(String name) { super(name); } } ///:~ //: typeinfo/pets/Pet.java package typeinfo.pets; public class Pet extends Individual { public Pet(String name) { super(name); } public Pet() { super(); } } ///:~ //: typeinfo/pets/Dog.java package typeinfo.pets; public class Dog extends Pet { public Dog(String name) { super(name); } public Dog() { super(); } } ///:~ //: typeinfo/pets/Mutt.java package typeinfo.pets; public class Mutt extends Dog { public Mutt(String name) { super(name); } public Mutt() { super(); } } ///:~ //: typeinfo/pets/Pug.java package typeinfo.pets; public class Pug extends Dog { public Pug(String name) { super(name); } public Pug() { super(); } } ///:~ //: typeinfo/pets/Cat.java package typeinfo.pets; public class Cat extends Pet { public Cat(String name) { super(name); } public Cat() { super(); } } ///:~ //: typeinfo/pets/EgyptianMau.java Type Information 405 package typeinfo.pets; public class EgyptianMau extends Cat { public EgyptianMau(String name) { super(name); } public EgyptianMau() { super(); } } ///:~ //: typeinfo/pets/Manx.java package typeinfo.pets; public class Manx extends Cat { public Manx(String name) { super(name); } public Manx() { super(); } } ///:~ //: typeinfo/pets/Cymric.java package typeinfo.pets; public class Cymric extends Manx { public Cymric(String name) { super(name); } public Cymric() { super(); } } ///:~ //: typeinfo/pets/Rodent.java package typeinfo.pets; public class Rodent extends Pet { public Rodent(String name) { super(name); } public Rodent() { super(); } } ///:~ //: typeinfo/pets/Rat.java package typeinfo.pets; public class Rat extends Rodent { public Rat(String name) { super(name); } public Rat() { super(); } } ///:~ //: typeinfo/pets/Mouse.java package typeinfo.pets; public class Mouse extends Rodent { public Mouse(String name) { super(name); } public Mouse() { super(); } } ///:~ //: typeinfo/pets/Hamster.java package typeinfo.pets; public class Hamster extends Rodent { public Hamster(String name) { super(name); } public Hamster() { super(); } } ///:~ Next, we need a way to randomly create different types of pets, and for convenience, to create arrays and Lists of pets. To allow this tool to evolve through several different implementations, we’ll define it as an abstract class: //: typeinfo/pets/PetCreator.java // Creates random sequences of Pets. package typeinfo.pets; 406 Thinking in Java Bruce Eckel import java.util.*; public abstract class PetCreator { private Random rand = new Random(47); // The List of the different types of Pet to create: public abstract List> types(); public Pet randomPet() { // Create one random Pet int n = rand.nextInt(types().size()); try { return types().get(n).newInstance(); } catch(InstantiationException e) { throw new RuntimeException(e); } catch(IllegalAccessException e) { throw new RuntimeException(e); } } public Pet[] createArray(int size) { Pet[] result = new Pet[size]; for(int i = 0; i < size; i++) result[i] = randomPet(); return result; } public ArrayList arrayList(int size) { ArrayList result = new ArrayList(); Collections.addAll(result, createArray(size)); return result; } } ///:~ The abstract getTypes( ) method defers to a derived class to get the List of Class objects (this is a variation of the Template Method design pattern). Notice that the type of class is specified to be "anything derived from Pet," so that newlnstance( ) produces a Pet without requiring a cast. randomPet( ) randomly indexes into the List and uses the selected Class object to generate a new instance of that class with Class.newlnstance( ). The createArray( ) method uses randomPet( ) to fill an array, and arrayList( ) uses createArray( ) in turn. You can get two kinds of exceptions when calling newlnstance( ). You can see these handled in the catch clauses following the try block. Again, the names of the exceptions are relatively useful explanations of what went wrong (IllegalAccessException relates to a violation of the Java security mechanism, in this case if the default constructor is private). When you derive a subclass of PetCreator, the only thing you need to supply is the List of the types of pet that you want to create using randomPet( ) and the other methods. The getTypes( ) method will normally just return a reference to a static List. Here’s an implementation using forName( ): //: typeinfo/pets/ForNameCreator.java package typeinfo.pets; import java.util.*; public class ForNameCreator extends PetCreator { private static List> types = new ArrayList>(); // Types that you want to be randomly created: private static String[] typeNames = { "typeinfo.pets.Mutt", "typeinfo.pets.Pug", "typeinfo.pets.EgyptianMau", "typeinfo.pets.Manx", "typeinfo.pets.Cymric", Type Information 407 "typeinfo.pets.Rat", "typeinfo.pets.Mouse", "typeinfo.pets.Hamster" }; @SuppressWarnings("unchecked") private static void loader() { try { for(String name : typeNames) types.add( (Class)Class.forName(name)); } catch(ClassNotFoundException e) { throw new RuntimeException(e); } } static { loader(); } public List> types() {return types;} } ///:~ The loader( ) method creates the List of Class objects using Class.forName( ). This may generate a ClassNotFoundException, which makes sense since you’re passing it a String which cannot be validated at compile time. Since the Pet objects are in package typeinfo, the package name must be used when referring to the classes. In order to produce a typed List of Class objects, a cast is required, which produces a compile-time warning. The loader( ) method is defined separately and then placed inside a static initialization clause because the @SuppressWarnings annotation cannot be placed directly onto the static initialization clause. To count Pets, we need a tool that keeps track of the quantities of various different types of Pets. A Map is perfect for this; the keys are the Pet type names and the values are Integers to hold the Pet quantities. This way, you can say, "How many Hamster objects are there?" We can use instanceof to count Pets: //: typeinfo/PetCount.java // Using instanceof. import typeinfo.pets.*; import java.util.*; import static net.mindview.util.Print.*; public class PetCount { static class PetCounter extends HashMap { public void count(String type) { Integer quantity = get(type); if(quantity == null) put(type, 1); else put(type, quantity + 1); } } public static void countPets(PetCreator creator) { PetCounter counter= new PetCounter(); for(Pet pet : creator.createArray(20)) { // List each individual pet: printnb(pet.getClass().getSimpleName() + " "); if(pet instanceof Pet) counter.count("Pet"); if(pet instanceof Dog) counter.count("Dog"); if(pet instanceof Mutt) counter.count("Mutt"); if(pet instanceof Pug) counter.count("Pug"); if(pet instanceof Cat) 408 Thinking in Java Bruce Eckel counter.count("Cat"); if(pet instanceof Manx) counter.count("EgyptianMau"); if(pet instanceof Manx) counter.count("Manx"); if(pet instanceof Manx) counter.count("Cymric"); if(pet instanceof Rodent) counter.count("Rodent"); if(pet instanceof Rat) counter.count("Rat"); if(pet instanceof Mouse) counter.count("Mouse"); if(pet instanceof Hamster) counter.count("Hamster"); } // Show the counts: print(); print(counter); } public static void main(String[] args) { countPets(new ForNameCreator()); } } /* Output: Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric {Pug=3, Cat=9, Hamster=1, Cymric=7, Mouse=2, Mutt=3, Rodent=5, Pet=20, Manx=7, EgyptianMau=7, Dog=6, Rat=2} *///:~ In countPets( ), an array is randomly filled with Pets using a PetCreator. Then each Pet in the array is tested and counted using instanceof. There’s a rather narrow restriction on instanceof: You can compare it to a named type only, and not to a Class object. In the preceding example you might feel that it’s tedious to write out all of those instanceof expressions, and you’re right. But there is no way to cleverly automate instanceof by creating an array of Class objects and comparing it to those instead (stay tuned—you’ll see an alternative). This isn’t as great a restriction as you might think, because you’ll eventually understand that your design is probably flawed if you end up writing a lot of instanceof expressions. Using class literals If we reimplement the PetCreator using class literals, the result is cleaner in many ways: //: typeinfo/pets/LiteralPetCreator.java // Using class literals. package typeinfo.pets; import java.util.*; public class LiteralPetCreator extends PetCreator { // No try block needed. @SuppressWarnings("unchecked") public static final List> allTypes = Collections.unmodifiableList(Arrays.asList( Pet.class, Dog.class, Cat.class, Rodent.class, Mutt.class, Pug.class, EgyptianMau.class, Manx.class, Cymric.class, Rat.class, Mouse.class,Hamster.class)); // Types for random creation: private static final List> types = allTypes.subList(allTypes.indexOf(Mutt.class), Type Information 409 allTypes.size()); public List> types() { return types; } public static void main(String[] args) { System.out.println(types); } } /* Output: [class typeinfo.pets.Mutt, class typeinfo.pets.Pug, class typeinfo.pets.EgyptianMau, class typeinfo.pets.Manx, class typeinfo.pets.Cymric, class typeinfo.pets.Rat, class typeinfo.pets.Mouse, class typeinfo.pets.Hamster] *///:~ In the upcoming PetCount3.java example, we need to pre-load a Map with all the Pet types (not just the ones that are to be randomly generated), so the allTypes List is necessary. The types list is the portion of allTypes (created using List.subList( )) that includes the exact pet types, so it is used for random Pet generation. This time, the creation of types does not need to be surrounded by a try block since it’s evaluated at compile time and thus won’t throw any exceptions, unlike Class.forName( ). We now have two implementations of PetCreator in the typeinfo.pets library. In order to provide the second one as a default implementation, we can create a Faqade that utilizes LiteralPetCreator: //: typeinfo/pets/Pets.java // Facade to produce a default PetCreator. package typeinfo.pets; import java.util.*; public class Pets { public static final PetCreator creator = new LiteralPetCreator(); public static Pet randomPet() { return creator.randomPet(); } public static Pet[] createArray(int size) { return creator.createArray(size); } public static ArrayList arrayList(int size) { return creator.arrayList(size); } } ///:~ This also provides indirection to randomPet( ), createArray( ) and arrayList( ). Because PetCount.countPets( ) takes a PetCreator argument, we can easily test the LiteralPetCreator (via the above Facade): //: typeinfo/PetCount2.java import typeinfo.pets.*; public class PetCount2 { public static void main(String[] args) { PetCount.countPets(Pets.creator); } } /* (Execute to see output) *///:~ The output is the same as that of PetCount.java. 410 Thinking in Java Bruce Eckel A dynamic instanceof The Class.islnstance( ) method provides a way to dynamically test the type of an object. Thus, all those tedious instanceof statements can be removed from PetCount.java: //: typeinfo/PetCount3.java // Using isInstance() import typeinfo.pets.*; import java.util.*; import net.mindview.util.*; import static net.mindview.util.Print.*; public class PetCount3 { static class PetCounter extends LinkedHashMap,Integer> { public PetCounter() { super(MapData.map(LiteralPetCreator.allTypes, 0)); } public void count(Pet pet) { // Class.isInstance() eliminates instanceofs: for(Map.Entry,Integer> pair : entrySet()) if(pair.getKey().isInstance(pet)) put(pair.getKey(), pair.getValue() + 1); } public String toString() { StringBuilder result = new StringBuilder("{"); for(Map.Entry,Integer> pair : entrySet()) { result.append(pair.getKey().getSimpleName()); result.append("="); result.append(pair.getValue()); result.append(", "); } result.delete(result.length()-2, result.length()); result.append("}"); return result.toString(); } } public static void main(String[] args) { PetCounter petCount = new PetCounter(); for(Pet pet : Pets.createArray(20)) { printnb(pet.getClass().getSimpleName() + " "); petCount.count(pet); } print(); print(petCount); } } /* Output: Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric {Pet=20, Dog=6, Cat=9, Rodent=5, Mutt=3, Pug=3, EgyptianMau=2, Manx=7, Cymric=5, Rat=2, Mouse=2, Hamster=1} *///:~ In order to count all the different types of Pet, the PetCounter Map is preloaded with the types from LiteralPetCreator.allTypes. This uses the net.mindview.util.MapData class, which takes an Iterable (the allTypes List) and a constant value (zero, in this case), and fills the Map with keys taken from allTypes and values of zero). Without pre-loading the Map, you would only end up counting the types that are randomly generated, and not the base types like Pet and Cat. Type Information 411 You can see that the isInstance( ) method has eliminated the need for the instanceof expressions. In addition, this means that you can add new types of Pet simply by changing the LiteralPetCreator.types array; the rest of the program does not need modification (as it did when using the instanceof expressions). The toString( ) method has been overloaded for easier-to-read output that still matches the typical output that you see when printing a Map. Counting recursively The Map in PetCount3.PetCounter was pre-loaded with all the different Pet classes. Instead of pre-loading the map, we can use Class.isAssignableFrom( ) and create a general-purpose tool that is not limited to counting Pets: //: net/mindview/util/TypeCounter.java // Counts instances of a type family. package net.mindview.util; import java.util.*; public class TypeCounter extends HashMap,Integer>{ private Class baseType; public TypeCounter(Class baseType) { this.baseType = baseType; } public void count(Object obj) { Class type = obj.getClass(); if(!baseType.isAssignableFrom(type)) throw new RuntimeException(obj + " incorrect type: " + type + ", should be type or subtype of " + baseType); countClass(type); } private void countClass(Class type) { Integer quantity = get(type); put(type, quantity == null ? 1 : quantity + 1); Class superClass = type.getSuperclass(); if(superClass != null && baseType.isAssignableFrom(superClass)) countClass(superClass); } public String toString() { StringBuilder result = new StringBuilder("{"); for(Map.Entry,Integer> pair : entrySet()) { result.append(pair.getKey().getSimpleName()); result.append("="); result.append(pair.getValue()); result.append(", "); } result.delete(result.length()-2, result.length()); result.append("}"); return result.toString(); } } ///:~ The count( ) method gets the Class of its argument, and uses isAssignableFrom( ) to perform a runtime check to verify that the object that you’ve passed actually belongs to the hierarchy of interest. countClass( ) first counts the exact type of the class. Then, if baseType is assignable from the superclass, countClass( ) is called recursively on the superclass. 412 Thinking in Java Bruce Eckel //: typeinfo/PetCount4.java import typeinfo.pets.*; import net.mindview.util.*; import static net.mindview.util.Print.*; public class PetCount4 { public static void main(String[] args) { TypeCounter counter = new TypeCounter(Pet.class); for(Pet pet : Pets.createArray(20)) { printnb(pet.getClass().getSimpleName() + " "); counter.count(pet); } print(); print(counter); } } /* Output: (Sample) Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric {Mouse=2, Dog=6, Manx=7, EgyptianMau=2, Rodent=5, Pug=3, Mutt=3, Cymric=5, Cat=9, Hamster=1, Pet=20, Rat=2} *///:~ As you can see from the output, both base types as well as exact types are counted. Exercise 11: (2) Add Gerbil to the typeinfo.pets library and modify all the examples in this chapter to adapt to this new class. Exercise 12: (3) Use TypeCounter with the CoffeeGenerator.java class in the Generics chapter. Exercise 13: (3) Use TypeCounter with the RegisteredFactories.java example in this chapter. Registered factories A problem with generating objects of the Pets hierarchy is the fact that every time you add a new type of Pet to the hierarchy you must remember to add it to the entries in LiteralPetCreator.java. In a system where you add more classes on a regular basis this can become problematic. You might think of adding a static initializer to each subclass, so that the initializer would add its class to a list somewhere. Unfortunately, static initializers are only called when the class is first loaded, so you have a chicken-and-egg problem: The generator doesn’t have the class in its list, so it can never create an object of that class, so the class won’t get loaded and placed in the list. Basically, you’re forced to create the list yourself, by hand (unless you want to write a tool that searches through and analyzes your source code, then creates and compiles the list). So the best you can probably do is to put the list in one central, obvious place. The base class for the hierarchy of interest is probably the best place. The other change we’ll make here is to defer the creation of the object to the class itself, using the Factory Method design pattern. A factory method can be called polymorphically, and creates an object of the appropriate type for you. In this very simple version, the factory method is the create( ) method in the Factory interface: Type Information 413 //: typeinfo/factory/Factory.java package typeinfo.factory; public interface Factory { T create(); } ///:~ The generic parameter T allows create( ) to return a different type for each implementation of Factory. This also makes use of covariant return types. In this example, the base class Part contains a List of factory objects. Factories for types that should be produced by the createRandom( ) method are "registered" with the base class by adding them to the partFactories List: //: typeinfo/RegisteredFactories.java // Registering Class Factories in the base class. import typeinfo.factory.*; import java.util.*; class Part { public String toString() { return getClass().getSimpleName(); } static List> partFactories = new ArrayList>(); static { // Collections.addAll() gives an "unchecked generic // array creation ... for varargs parameter" warning. partFactories.add(new FuelFilter.Factory()); partFactories.add(new AirFilter.Factory()); partFactories.add(new CabinAirFilter.Factory()); partFactories.add(new OilFilter.Factory()); partFactories.add(new FanBelt.Factory()); partFactories.add(new PowerSteeringBelt.Factory()); partFactories.add(new GeneratorBelt.Factory()); } private static Random rand = new Random(47); public static Part createRandom() { int n = rand.nextInt(partFactories.size()); return partFactories.get(n).create(); } } class Filter extends Part {} class FuelFilter extends Filter { // Create a Class Factory for each specific type: public static class Factory implements typeinfo.factory.Factory { public FuelFilter create() { return new FuelFilter(); } } } class AirFilter extends Filter { public static class Factory implements typeinfo.factory.Factory { public AirFilter create() { return new AirFilter(); } } } class CabinAirFilter extends Filter { public static class Factory implements typeinfo.factory.Factory { public CabinAirFilter create() { return new CabinAirFilter(); } 414 Thinking in Java Bruce Eckel } } class OilFilter extends Filter { public static class Factory implements typeinfo.factory.Factory { public OilFilter create() { return new OilFilter(); } } } class Belt extends Part {} class FanBelt extends Belt { public static class Factory implements typeinfo.factory.Factory { public FanBelt create() { return new FanBelt(); } } } class GeneratorBelt extends Belt { public static class Factory implements typeinfo.factory.Factory { public GeneratorBelt create() { return new GeneratorBelt(); } } } class PowerSteeringBelt extends Belt { public static class Factory implements typeinfo.factory.Factory { public PowerSteeringBelt create() { return new PowerSteeringBelt(); } } } public class RegisteredFactories { public static void main(String[] args) { for(int i = 0; i < 10; i++) System.out.println(Part.createRandom()); } } /* Output: GeneratorBelt CabinAirFilter GeneratorBelt AirFilter PowerSteeringBelt CabinAirFilter FuelFilter PowerSteeringBelt PowerSteeringBelt FuelFilter *///:~ Not all classes in the hierarchy should be instantiated; in this case Filter and Belt are just classifiers so you do not create an instance of either one, but only of their subclasses. If a class should be created by createRandom( ), it contains an inner Factory class. The only way to reuse the name Factory as seen above is by qualifying typeinfo.factory.Factory. Although you can use Collections.addAll( ) to add the factories to the list, the compiler expresses its unhappiness with a warning about a "generic array creation" (which is supposed Type Information 415 to be impossible, as you’ll see in the Generics chapter), so I reverted to calling add( ). The createRandom( ) method randomly selects a factory object from partFactories and calls its create( ) to produce a new Part. Exercise 14: (4) A constructor is a kind of factory method. Modify RegisteredFactories.java so that instead of using an explicit factory, the class object is stored in the List, and newlnstance( ) is used to create each object. Exercise 15: (4) Implement a new PetCreator using Registered Factories, and modify the Pets Facade so that it uses this one instead of the other two. Ensure that the rest of the examples that use Pets .Java still work correctly. Exercise 16: (4) Modify the Coffee hierarchy in the Generics chapter to use Registered Factories. instanceof vs. Class equivalence When you are querying for type information, there’s an important difference between either form of instanceof (that is, instanceof or islnstance( ), which produce equivalent results) and the direct comparison of the Class objects. Here’s an example that demonstrates the difference: //: typeinfo/FamilyVsExactType.java // The difference between instanceof and class package typeinfo; import static net.mindview.util.Print.*; class Base {} class Derived extends Base {} public class FamilyVsExactType { static void test(Object x) { print("Testing x of type " + x.getClass()); print("x instanceof Base " + (x instanceof Base)); print("x instanceof Derived "+ (x instanceof Derived)); print("Base.isInstance(x) "+ Base.class.isInstance(x)); print("Derived.isInstance(x) " + Derived.class.isInstance(x)); print("x.getClass() == Base.class " + (x.getClass() == Base.class)); print("x.getClass() == Derived.class " + (x.getClass() == Derived.class)); print("x.getClass().equals(Base.class)) "+ (x.getClass().equals(Base.class))); print("x.getClass().equals(Derived.class)) " + (x.getClass().equals(Derived.class))); } public static void main(String[] args) { test(new Base()); test(new Derived()); } } /* Output: Testing x of type class typeinfo.Base x instanceof Base true x instanceof Derived false Base.isInstance(x) true Derived.isInstance(x) false x.getClass() == Base.class true x.getClass() == Derived.class false 416 Thinking in Java Bruce Eckel x.getClass().equals(Base.class)) true x.getClass().equals(Derived.class)) false Testing x of type class typeinfo.Derived x instanceof Base true x instanceof Derived true Base.isInstance(x) true Derived.isInstance(x) true x.getClass() == Base.class false x.getClass() == Derived.class true x.getClass().equals(Base.class)) false x.getClass().equals(Derived.class)) true *///:~ The test( ) method performs type checking with its argument using both forms of instanceof. It then gets the Class reference and uses == and equals( ) to test for equality of the Class objects. Reassuringly, instanceof and islnstance( ) produce exactly the same results, as do equals( ) and ==. But the tests themselves draw different conclusions. In keeping with the concept of type, instanceof says, "Are you this class, or a class derived from this class?" On the other hand, if you compare the actual Class objects using ==, there is no concern with inheritance—it’s either the exact type or it isn’t. Reflection: runtime class information If you don’t know the precise type of an object, RTTI will tell you. However, there’s a limitation: The type must be known at compile time in order for you to detect it using RTTI and to do something useful with the information. Put another way, the compiler must know about all the classes you’re working with. This doesn’t seem like that much of a limitation at first, but suppose you’re given a reference to an object that’s not in your program space. In fact, the class of the object isn’t even available to your program at compile time. For example, suppose you get a bunch of bytes from a disk file or from a network connection, and you’re told that those bytes represent a class. Since this class shows up long after the compiler generates the code for your program, how can you possibly use such a class? In a traditional programming environment, this seems like a far-fetched scenario. But as we move into a larger programming world, there are important cases in which this happens. The first is component-based programming, in which you build projects using Rapid Application Development (RAD) in an Application Builder Integrated Development Environment, which I shall refer to simply as an IDE. This is a visual approach to creating a program by moving icons that represent components onto a form. These components are then configured by setting some of their values at program time. This design-time configuration requires that any component be instantiable, that it exposes parts of itself, and that it allows its properties to be read and modified. In addition, components that handle Graphical User Interface (GUI) events must expose information about appropriate methods so that the IDE can assist the programmer in overriding these event-handling methods. Reflection provides the mechanism to detect the available methods and produce the method names. Java provides a structure for component-based programming through JavaBeans (described in the Graphical User Interfaces chapter). Another compelling motivation for discovering class information at run time is to provide the ability to create and execute objects on remote platforms, across a network. This is called Remote Method Invocation (RMI), and it allows a Java program to have objects distributed across many machines. This distribution can happen for a number of reasons. For example, perhaps you’re doing a computation-intensive task, and in order to speed things up, you want to break it up and put pieces on machines that are idle. In other situations you might want to Type Information 417 place code that handles particular types of tasks (e.g., "Business Rules" in a multitier client/server architecture) on a particular machine, so the machine becomes a common repository describing those actions, and it can be easily changed to affect everyone in the system. (This is an interesting development, since the machine exists solely to make software changes easy!) Along these lines, distributed computing also supports specialized hardware that might be good at a particular task—matrix inversions, for example—but inappropriate or too expensive for generalpurpose programming. The class Class supports the concept of reflection, along with the java.lang.reflect library which contains the classes Field, Method, and Constructor (each of which implements the Member interface). Objects of these types are created by the JVM at run time to represent the corresponding member in the unknown class. You can then use the Constructors to create new objects, the get( ) and set( ) methods to read and modify the fields associated with Field objects, and the invoke( ) method to call a method associated with a Method object. In addition, you can call the convenience methods getFields( ), getMethods( ), getConstructors( ), etc., to return arrays of the objects representing the fields, methods, and constructors. (You can find out more by looking up the class Class in the JDK documentation.) Thus, the class information for anonymous objects can be completely determined at run time, and nothing need be known at compile time. It’s important to realize that there’s nothing magic about reflection. When you’re using reflection to interact with an object of an unknown type, the JVM will simply look at the object and see that it belongs to a particular class (just like ordinary RTTI). Before anything can be done with it, the Class object must be loaded. Thus, the .class file for that particular type must still be available to the JVM, either on the local machine or across the network. So the true difference between RTTI and reflection is that with RTTI, the compiler opens and examines the .class file at compile time. Put another way, you can call all the methods of an object in the "normal" way. With reflection, the .class file is unavailable at compile time; it is opened and examined by the runtime environment. A class method extractor Normally you won’t need to use the reflection tools directly, but they can be helpful when you need to create more dynamic code. Reflection is in the language to support other Java features, such as object serialization and JavaBeans (both covered later in the book). However, there are times when it’s quite useful to dynamically extract information about a class. Consider a class method extractor. Looking at a class definition source code or JDK documentation shows only the methods that are defined or overridden within that class definition. But there might be dozens more available to you that have come from base classes. To locate these is both tedious and time consuming.1 Fortunately, reflection provides a way to write a simple tool that will automatically show you the entire interface. Here’s the way it works: //: typeinfo/ShowMethods.java // Using reflection to show all the methods of a class, // even if the methods are defined in the base class. // {Args: ShowMethods} import java.lang.reflect.*; import java.util.regex.*; import static net.mindview.util.Print.*; public class ShowMethods { private static String usage = 1 Especially in the past. However, Sun has greatly improved its HTML Java documentation so that it’s easier to see baseclass methods. 418 Thinking in Java Bruce Eckel "usage:\n" + "ShowMethods qualified.class.name\n" + "To show all methods in class or:\n" + "ShowMethods qualified.class.name word\n" + "To search for methods involving ‘word’"; private static Pattern p = Pattern.compile("\\w+\\."); public static void main(String[] args) { if(args.length < 1) { print(usage); System.exit(0); } int lines = 0; try { Class c = Class.forName(args[0]); Method[] methods = c.getMethods(); Constructor[] ctors = c.getConstructors(); if(args.length == 1) { for(Method method : methods) print( p.matcher(method.toString()).replaceAll("")); for(Constructor ctor : ctors) print(p.matcher(ctor.toString()).replaceAll("")); lines = methods.length + ctors.length; } else { for(Method method : methods) if(method.toString().indexOf(args[1]) != -1) { print( p.matcher(method.toString()).replaceAll("")); lines++; } for(Constructor ctor : ctors) if(ctor.toString().indexOf(args[1]) != -1) { print(p.matcher( ctor.toString()).replaceAll("")); lines++; } } } catch(ClassNotFoundException e) { print("No such class: " + e); } } } /* Output: public static void main(String[]) public native int hashCode() public final native Class getClass() public final void wait(long,int) throws InterruptedException public final void wait() throws InterruptedException public final native void wait(long) throws InterruptedException public boolean equals(Object) public String toString() public final native void notify() public final native void notifyAll() public ShowMethods() *///:~ The Class methods getMethods( ) and getConstructors( ) return an array of Method and array of Constructor, respectively. Each of these classes has further methods to dissect the names, arguments, and return values of the methods they represent. But you can also just use toString( ), as is done here, to produce a String with the entire method signature. The rest of the code extracts the command-line information, determines if a particular signature matches your target string (using indexOf( )), and strips off the name qualifiers using regular expressions (introduced in the Strings chapter). Type Information 419 The result produced by Class.forName( ) cannot be known at compile time, and therefore all the method signature information is being extracted at run time. If you investigate the JDK reflection documentation, you’ll see that there is enough support to actually set up and make a method call on an object that’s totally unknown at compile time (there will be examples of this later in this book). Although initially this is something you may not think you’ll ever need, the value of full reflection can be quite surprising. The output above is produced from the command line: java ShowMethods ShowMethods You can see that the output includes a public default constructor, even though no constructor was defined. The constructor you see is the one that’s automatically synthesized by the compiler. If you then make ShowMethods a non-public class (that is, package access), the synthesized default constructor no longer shows up in the output. The synthesized default constructor is automatically given the same access as the class. Another interesting experiment is to invoke Java ShowMethods java.lang.String with an extra argument of char, int, String, etc. This tool can be a real time-saver while you’re programming, when you can’t remember if a class has a particular method and you don’t want to go hunting through the index or class hierarchy in the JDK documentation, or if you don’t know whether that class can do anything with, for example, Color objects. The Graphical User Interfaces chapter contains a GUI version of this program (customized to extract information for Swing components) so you can leave it running while you’re writing code, to allow quick lookups. Exercise 17: (2) Modify the regular expression in ShowMethods.java to additionally strip off the keywords native and final (hint: use the OR operator’|’)- Exercise 18: (1) Make ShowMethods a non-public class and verify that the synthesized default constructor no longer shows up in the output. Exercise 19: (4) In ToyTest.java, use reflection to create a Toy object using the non- default constructor. Exercise 20: (5) Look up the interface for java.lang.Class in the JDK documentation from http://java.sun.com. Write a program that takes the name of a class as a command-line argument, then uses the Class methods to dump all the information available for that class. Test your program with a standard library class and a class you create. Dynamic proxies Proxy is one of the basic design patterns. It is an object that you insert in place of the "real" object in order to provide additional or different operations—these usually involve communication with a "real" object, so a proxy typically acts as a go-between. Here’s a trivial example to show the structure of a proxy: //: typeinfo/SimpleProxyDemo.java import static net.mindview.util.Print.*; interface Interface { void doSomething(); void somethingElse(String arg); 420 Thinking in Java Bruce Eckel } class RealObject implements Interface { public void doSomething() { print("doSomething"); } public void somethingElse(String arg) { print("somethingElse " + arg); } } class SimpleProxy implements Interface { private Interface proxied; public SimpleProxy(Interface proxied) { this.proxied = proxied; } public void doSomething() { print("SimpleProxy doSomething"); proxied.doSomething(); } public void somethingElse(String arg) { print("SimpleProxy somethingElse " + arg); proxied.somethingElse(arg); } } class SimpleProxyDemo { public static void consumer(Interface iface) { iface.doSomething(); iface.somethingElse("bonobo"); } public static void main(String[] args) { consumer(new RealObject()); consumer(new SimpleProxy(new RealObject())); } } /* Output: doSomething somethingElse bonobo SimpleProxy doSomething doSomething SimpleProxy somethingElse bonobo somethingElse bonobo *///:~ Because consumer( ) accepts an Interface, it can’t know if it’s getting a RealObject or a SimpleProxy, because both implement Interface. But the SimpleProxy inserted between the client and the RealObject performs operations and then calls the identical method on a RealObject. A proxy can be helpful anytime you’d like to separate extra operations into a different place than the "real object," and especially when you want to easily change from not using the extra operations to using them, and vice versa (the point of design patterns is to encapsulate change—so you need to be changing things in order to justify the pattern). For example, what if you wanted to track calls to the methods in the RealObject, or to measure the overhead of such calls? This is not code you want to have incorporated in your application, so a proxy allows you to add and remove it easily. Java’s dynamic proxy takes the idea of a proxy one step further, by both creating the proxy object dynamically and handling calls to the proxied methods dynamically. All calls made on a dynamic proxy are redirected to a single invocation handler, which has the job of discovering what the call is and deciding what to do about it. Here’s SimpleProxyDemo.java rewritten to use a dynamic proxy: Type Information 421 //: typeinfo/SimpleDynamicProxy.java import java.lang.reflect.*; class DynamicProxyHandler implements InvocationHandler { private Object proxied; public DynamicProxyHandler(Object proxied) { this.proxied = proxied; } public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("**** proxy: " + proxy.getClass() + ", method: " + method + ", args: " + args); if(args != null) for(Object arg : args) System.out.println(" " + arg); return method.invoke(proxied, args); } } class SimpleDynamicProxy { public static void consumer(Interface iface) { iface.doSomething(); iface.somethingElse("bonobo"); } public static void main(String[] args) { RealObject real = new RealObject(); consumer(real); // Insert a proxy and call again: Interface proxy = (Interface)Proxy.newProxyInstance( Interface.class.getClassLoader(), new Class[]{ Interface.class }, new DynamicProxyHandler(real)); consumer(proxy); } } /* Output: (95% match) doSomething somethingElse bonobo **** proxy: class $Proxy0, method: public abstract void Interface.doSomething(), args: null doSomething **** proxy: class $Proxy0, method: public abstract void Interface.somethingElse(java.lang.String), args: [Ljava.lang.Object;@42e816 bonobo somethingElse bonobo *///:~ You create a dynamic proxy by calling the static method Proxy.newProxyInstance( ), which requires a class loader (you can generally just hand it a class loader from an object that has already been loaded), a list of interfaces (not classes or abstract classes) that you wish the proxy to implement, and an implementation of the interface InvocationHandler. The dynamic proxy will redirect all calls to the invocation handler, so the constructor for the invocation handler is usually given the reference to the "real" object so that it can forward requests once it performs its intermediary task. The invoke( ) method is handed the proxy object, in case you need to distinguish where the request came from—but in many cases you won’t care. However, be careful when calling methods on the proxy inside invoke( ), because calls through the interface are redirected through the proxy. 422 Thinking in Java Bruce Eckel In general you will perform the proxied operation and then use Method.invoke( ) to forward the request to the proxied object, passing the necessary arguments. This may initially seem limiting, as if you can only perform generic operations. However, you can filter for certain method calls, while passing others through: //: typeinfo/SelectingMethods.java // Looking for particular methods in a dynamic proxy. import java.lang.reflect.*; import static net.mindview.util.Print.*; class MethodSelector implements InvocationHandler { private Object proxied; public MethodSelector(Object proxied) { this.proxied = proxied; } public Object invoke(