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v Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 M T W R F S S 201 West 103rd Street Indianapolis, Indiana 46290 Laura Lemay Charles L. Perkins Teach Yourself JAVA in 21 Days 030-4 FM 1/29/96, 8:10 PM 5 i Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 W R F S M T W R About This Book This book teaches you all about the Java language and how to use it to create applets and applications. By the time you get through with this book, you’ll know enough about Java to do just about anything, inside an applet or out. Who Should Read This Book This book is intended for people with at least some basic programming back- ground, which includes people with years of programming experience or people with only a small amount of experience. If you understand what variables, loops, and functions are, you’ll be just fine for this book. The sorts of people who might want to read this book include you, if ■ You’re a real whiz at HTML, understand CGI programming (in perl, AppleScript, Visual Basic, or some other popular CGI language) pretty well, and want to move on to the next level in Web page design. ■ You had some Basic or Pascal in school and you have a basic grasp of what programming is, but you’ve heard Java is easy to learn, really powerful, and very cool. ■ You’ve programmed C and C++ for many years, you’ve heard this Java thing is becoming really popular and you’re wondering what all the fuss is all about. ■ You’ve heard that Java is really good for Web-based applets, and you’re curious about how good it is for creating more general applications. What if you know programming, but you don’t know object-oriented program- ming? Fear not. This book assumes no background in object-oriented design. If you know object-oriented programming, in fact, the first couple of days will be easy for you. How This Book Is Structured This book is intended to be read and absorbed over the course of three weeks. During each week, you’ll read seven chapters that present concepts related to the Java language and the creation of applets and applications. 030-4 FM 1/29/96, 8:10 PM 1 Teach Yourself JAVA in 21 Days M T W T F S S 21 ii P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Conventions Note: A Note box presents interesting pieces of information related to the surround- ing discussion. Technical Note: A Technical Note presents specific technical information related to the surrounding discussion. Tip: A Tip box offers advice or teaches an easier way to do something. Caution: A Caution box alerts you to a possible problem and gives you advice to avoid it. Warning: A Warning box advises you about potential problems and helps you steer clear of disaster. New terms are introduced in New Term boxes, with the term in italics. A type icon identifies some new HTML code that you can type in yourself. An Output icon highlights what the same HTML code looks like when viewed by either Netscape or Mosaic. An analysis icon alerts you to the author’s line-by-line analysis. !! Analysis Output Type NEW TERM ☛ 030-4 FM 1/29/96, 8:10 PM 2 Teach Yourself JAVA in 21 Days M T W T F S S 21 vi P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 To Eric, for all the usual reasons (moral support, stupid questions, comfort in dark times). LL For RKJP, ARL, and NMH the three most important people in my life. CLP Copyright ©1996 by Sams.net Publishing and its licensors FIRST EDITION All rights reserved. No part of this book shall be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without written permission from the publisher. No patent liability is assumed with respect to the use of the information contained herein. Although every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions. Neither is any liability assumed for damages resulting from the use of the information contained herein. For information, address Sams.net Publishing, 201 W. 103rd St., Indianapolis, IN 46290. International Standard Book Number: 1-57521-030-4 Library of Congress Catalog Card Number: 95-78866 99 98 97 96 4 3 2 1 Interpretation of the printing code: the rightmost double-digit number is the year of the book’s printing; the rightmost single-digit, the number of the book’s printing. For example, a printing code of 96-1 shows that the first printing of the book occurred in 1996. Composed in AGaramond and MCPdigital by Macmillan Computer Publishing Printed in the United States of America All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Sams.net Publishing cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark. President, Sams Publishing: Richard K. Swadley Publisher, Sams.net Publishing: George Bond Publishing Manager: Mark Taber Managing Editor: Cindy Morrow Marketing Manager: John Pierce Acquisitions Editor Mark Taber Development Editor Fran Hatton Software Development Specialist Merle Newlon Production Editor Nancy Albright Technical Reviewer Patrick Chan Editorial Coordinator Bill Whitmer Technical Edit Coordinator Lynette Quinn Formatter Frank Sinclair Editorial Assistant Carol Ackerman Cover Designer Tim Amrhein Book Designer Alyssa Yesh Production Team Supervisor Brad Chinn Production Michael Brumitt Jason Hand Cheryl Moore Ayanna Lacey Nancy Price Bobbi Satterfield Tim Taylor Susan Van Ness Mark Walchle Todd Wente Indexer Tim Griffin 030-4 FM 1/29/96, 8:10 PM 6 vii Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 M T W R F S S Overview Introduction xxi Week 1 at a Glance Day 1 An Introduction to Java Programming 3 2 Object-Oriented Programming and Java 19 3 Java Basics 41 4 Working with Objects 61 5 Arrays, Conditionals, and Loops 79 6 Creating Classes and Applications in Java 95 7 More About Methods 111 Week 2 at a Glance Day 8 Java Applet Basics 129 9 Graphics, Fonts, and Color 149 10 Simple Animation and Threads 173 11 More Animation, Images, and Sound 195 12 Managing Simple Events and Interactivity 217 13 User Interfaces with the Java Abstract Windowing Toolkit 237 14 Windows, Networking, and Other Tidbits 279 Week 3 at a Glance Day 15 Modifiers 305 16 Packages and Interfaces 323 17 Exceptions 341 18 Multithreading 353 19 Streams 375 20 Native Methods and Libraries 403 21 Under the Hood 421 Appendixes A Language Summary 473 B The Java Class Library 483 C How Java Differs from C and C++ 497 D How Java Differs from C and C++ 507 Index 511 030-4 FM 1/29/96, 8:11 PM 7 ix Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 M T W R F S S Contents Introduction xxi Week 1 at a Glance 1 Day 1 An Introduction to Java Programming 3 What Is Java? .............................................................................................. 4 Java’s Past, Present, and Future ................................................................... 6 Why Learn Java? ......................................................................................... 7 Java Is Platform-Independent................................................................. 7 Java Is Object-Oriented ......................................................................... 9 Java Is Easy to Learn .............................................................................. 9 Getting Started with Programming in Java .............................................................................. 10 Getting the Software ............................................................................ 10 Applets and Applications ..................................................................... 11 Creating a Java Application .................................................................. 11 Creating a Java Applet ......................................................................... 13 Summary .................................................................................................. 16 Q&A ........................................................................................................ 16 Day 2 Object-Oriented Programming and Java 19 Thinking in Objects: An Analogy ............................................................. 20 Objects and Classes .................................................................................. 21 Behavior and Attributes ............................................................................ 23 Attributes ............................................................................................. 23 Behavior .............................................................................................. 24 Creating a Class ................................................................................... 24 Inheritance, Interfaces, and Packages ........................................................ 28 Inheritance........................................................................................... 29 Creating a Class Hierarchy................................................................... 30 How Inheritance Works ...................................................................... 32 Single and Multiple Inheritance ........................................................... 34 Interfaces and Packages ........................................................................ 34 Creating a Subclass ................................................................................... 35 Summary .................................................................................................. 38 Q&A ........................................................................................................ 39 Day 3 Java Basics 41 Statements and Expressions ...................................................................... 42 Variables and Data Types ......................................................................... 43 Declaring Variables .............................................................................. 43 Notes on Variable Names .................................................................... 44 030-4 FM 1/29/96, 8:11 PM 9 Teach Yourself JAVA in 21 Days M T W T F S S 21 x P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Variable Types ..................................................................................... 45 Assigning Values to Variables ............................................................... 46 Comments ................................................................................................ 47 Literals ...................................................................................................... 47 Number Literals ................................................................................... 47 Boolean Literals ................................................................................... 48 Character Literals ................................................................................. 48 String Literals ...................................................................................... 49 Expressions and Operators ........................................................................ 50 Arithmetic ........................................................................................... 50 More About Assignment ...................................................................... 52 Incrementing and Decrementing ......................................................... 52 Comparisons ........................................................................................ 54 Logical Operators ................................................................................ 55 Bitwise Operators ................................................................................ 55 Operator Precedence ............................................................................ 56 String Arithmetic ...................................................................................... 57 Summary .................................................................................................. 58 Q&A ........................................................................................................ 60 Day 4 Working with Objects 61 Creating New Objects .............................................................................. 62 Using new ............................................................................................ 63 What new Does ................................................................................... 64 A Note on Memory Management ........................................................ 64 Accessing and Setting Class and Instance Variables ................................... 65 Getting Values ..................................................................................... 65 Changing Values .................................................................................. 65 Class Variables ..................................................................................... 66 Calling Methods ....................................................................................... 67 Class Methods ..................................................................................... 69 References to Objects ............................................................................... 70 Casting and Converting Objects and Primitive Types .............................. 71 Casting Primitive Types ...................................................................... 71 Casting Objects .................................................................................. 72 Converting Primitive Types to Objects and Vice Versa ................................................................. 73 Odds and Ends ......................................................................................... 73 Comparing Objects ............................................................................. 74 Copying Objects .................................................................................. 75 Determining the Class of an Object ..................................................... 76 The Java Class Libraries ............................................................................ 76 Summary .................................................................................................. 77 Q&A ........................................................................................................ 78 030-4 FM 1/29/96, 8:11 PM 10 xi Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Day 5 Arrays, Conditionals, and Loops 79 Arrays ....................................................................................................... 80 Declaring Array Variables .................................................................... 80 Creating Array Objects ........................................................................ 81 Accessing Array Elements ..................................................................... 81 Changing Array Elements .................................................................... 82 Multidimensional Arrays...................................................................... 83 Block Statements ...................................................................................... 83 if Conditionals .......................................................................................... 83 The Conditional Operator ................................................................... 84 switch Conditionals ................................................................................... 85 for Loops .................................................................................................. 86 while and do Loops ................................................................................... 88 while Loops .......................................................................................... 88 do...while Loops ................................................................................... 89 Breaking Out of Loops ............................................................................. 89 Labeled Loops ...................................................................................... 90 Summary .................................................................................................. 91 Q&A ........................................................................................................ 92 Day 6 Creating Classes and Applications in Java 95 Defining Classes ....................................................................................... 96 Creating Instance and Class Variables ....................................................... 96 Defining Instance Variables ................................................................. 97 Constants ............................................................................................. 97 Class Variables ..................................................................................... 98 Creating Methods ..................................................................................... 99 Defining Methods ............................................................................... 99 The this Keyword ............................................................................... 101 Variable Scope and Method Definitions ............................................ 101 Passing Arguments to Methods .......................................................... 102 Class Methods ................................................................................... 104 Creating Java Applications ...................................................................... 105 Java Applications and Command-Line Arguments ................................. 106 Passing Arguments to Java Programs .................................................. 106 Handling Arguments in Your Java Program ....................................... 106 Summary ................................................................................................ 108 Q&A ...................................................................................................... 109 Day 7 More About Methods 111 Creating Methods with the Same Name, Different Arguments ............... 112 Constructor Methods ............................................................................. 115 Basic Constructors ............................................................................. 116 Calling Another Constructor ............................................................. 117 Overloading Constructors .................................................................. 117 030-4 FM 1/29/96, 8:11 PM 11 Teach Yourself JAVA in 21 Days M T W T F S S 21 xii P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Overriding Methods ............................................................................... 119 Creating Methods that Override Existing Methods ............................ 119 Calling the Original Method ............................................................. 121 Overriding Constructors .................................................................... 122 Finalizer Methods ................................................................................... 123 Summary ................................................................................................ 124 Q&A ...................................................................................................... 124 Week 2 at a Glance 127 Day 8 Java Applet Basics 129 How Applets and Applications Are Different .......................................... 130 Creating Applets ..................................................................................... 131 Major Applet Activities ...................................................................... 132 A Simple Applet................................................................................. 134 Including an Applet on a Web Page ........................................................ 136 The <APPLET> Tag.......................................................................... 136 Testing the Result .............................................................................. 137 Making Java Applets Available to the Web......................................... 137 More About the <APPLET> Tag ............................................................ 138 ALIGN .............................................................................................. 138 HSPACE and VSPACE ...................................................................... 140 CODE and CODEBASE .................................................................... 141 Passing Parameters to Applets ................................................................. 141 Summary ................................................................................................ 146 Q&A ...................................................................................................... 147 Day 9 Graphics, Fonts, and Color 149 The Graphics Class ................................................................................. 150 The Graphics Coordinate System ...................................................... 151 Drawing and Filling ............................................................................... 151 Lines .................................................................................................. 152 Rectangles .......................................................................................... 152 Polygons ............................................................................................ 155 Ovals ................................................................................................. 156 Arc ..................................................................................................... 157 A Simple Graphics Example............................................................... 161 Copying and Clearing ........................................................................ 163 Text and Fonts ....................................................................................... 163 Creating Font Objects ....................................................................... 163 Drawing Characters and Strings ......................................................... 164 Finding Out Information About a Font ............................................. 166 Color ...................................................................................................... 168 Using Color Objects .......................................................................... 168 Testing and Setting the Current Colors ............................................. 169 A Single Color Example ..................................................................... 170 Summary ................................................................................................ 171 Q&A ...................................................................................................... 171 030-4 FM 1/29/96, 8:11 PM 12 xiii Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Day 10 Simple Animation and Threads 173 Creating Animation in Java .................................................................... 174 Painting and Repainting .................................................................... 174 Starting and Stopping an Applet’s Execution ...................................................................... 175 Putting It Together ............................................................................ 175 Threads: What They Are and Why You Need Them ................................................................... 177 The Problem with the Digital Clock Applet ...................................... 178 Writing Applets with Threads ............................................................ 179 Fixing The Digital Clock ................................................................... 180 Reducing Animation Flicker ................................................................... 182 Flicker and How to Avoid It .............................................................. 182 How to Override Update ................................................................... 183 Solution One: Don’t Clear the Screen ............................................... 183 Solution Two: Redraw Only What You Have To ................................................................ 186 Summary ................................................................................................ 192 Q&A ...................................................................................................... 192 Day 11 More Animation, Images, and Sound 195 Retrieving and Using Images .................................................................. 196 Getting Images .................................................................................. 196 Drawing Images ................................................................................. 198 Modifying Images .............................................................................. 201 Creating Animation Using Images .......................................................... 201 An Example: Neko............................................................................. 201 Retrieving and Using Sounds .................................................................. 209 Sun’s Animator Applet ........................................................................... 211 More About Flicker: Double-Buffering................................................... 212 Creating Applets with Double-Buffering............................................ 212 An Example: Checkers Revisited ........................................................ 213 Summary ................................................................................................ 214 Q&A ...................................................................................................... 215 Day 12 Managing Simple Events and Interactivity 217 Mouse Clicks .......................................................................................... 218 mouseDown and mouseUp................................................................... 219 An Example: Spots ............................................................................. 220 Mouse Movements ................................................................................. 223 mouseDrag and mouseMove ................................................................. 223 mouseEnter and mouseExit .................................................................. 223 An Example: Drawing Lines .............................................................. 224 Keyboard Events ..................................................................................... 228 The keyDown Method ....................................................................... 228 Default Keys ...................................................................................... 229 030-4 FM 1/29/96, 8:11 PM 13 Teach Yourself JAVA in 21 Days M T W T F S S 21 xiv P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 An Example: Entering, Displaying, and Moving Characters .............. 229 Testing for Modifier Keys .................................................................. 232 The AWT Event Handler ....................................................................... 233 Summary ................................................................................................ 235 Q&A ...................................................................................................... 235 Day 13 The Java Abstract Windowing Toolkit 237 An AWT Overview................................................................................. 238 The Basic User Interface Components .................................................... 240 Labels ................................................................................................ 241 Buttons .............................................................................................. 242 Checkboxes ........................................................................................ 243 Radio Buttons .................................................................................... 244 Choice Menus ................................................................................... 245 Text Fields ......................................................................................... 247 Panels and Layout ................................................................................... 249 Layout Managers ............................................................................... 249 Insets ................................................................................................. 254 Handling UI Actions and Events ............................................................ 255 Nesting Panels and Components ............................................................ 258 Nested Panels .................................................................................... 258 Events and Nested Panels .................................................................. 258 More UI Components ............................................................................ 259 Text Areas .......................................................................................... 259 Scrolling Lists .................................................................................... 261 Scrollbars and Sliders ......................................................................... 262 Canvases ............................................................................................ 265 More UI Events ...................................................................................... 265 A Complete Example: RGB to HSB Converter ....................................................................... 266 Create the Applet Layout ................................................................... 267 Create the Panel Layout ..................................................................... 267 Define the Subpanels ......................................................................... 269 Handle the Actions ............................................................................ 272 Update the Result .............................................................................. 272 The Complete Source Code ............................................................... 274 Summary ................................................................................................ 277 Q&A ...................................................................................................... 277 Day 14 Windows, Networking, and Other Tidbits 279 Windows, Menus, and Dialog Boxes ...................................................... 280 Frames ............................................................................................... 280 Menus ............................................................................................... 282 Dialog Boxes ...................................................................................... 285 File Dialogs ........................................................................................ 287 Window Events ................................................................................. 288 Using AWT Windows in Stand-Alone Applications .......................... 288 030-4 FM 1/29/96, 8:11 PM 14 xv Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Networking in Java ................................................................................. 289 Creating Links Inside Applets ............................................................ 290 Opening Web Connections ............................................................... 292 openStream() ...................................................................................... 293 The URLconnection Class ................................................................... 296 Sockets ............................................................................................... 296 Other Applet Hints ................................................................................ 297 The showStatus Method ..................................................................... 297 Applet Information ............................................................................ 298 Communicating Between Applets ...................................................... 298 Summary ................................................................................................ 299 Q&A ...................................................................................................... 300 Week 3 at a Glance 303 Day 15 Modifiers 305 Method and Variable Access Control ...................................................... 307 The Four P’s of Protection ................................................................. 307 The Conventions for Instance Variable Access ................................... 312 Class Variables and Methods .................................................................. 314 The final Modifier .................................................................................. 316 final Classes ....................................................................................... 316 final Variables .................................................................................... 317 final Methods .................................................................................... 317 abstract Methods and Classes .................................................................. 319 Summary ................................................................................................ 320 Q&A ...................................................................................................... 320 Day 16 Packages and Interfaces 323 Packages ................................................................................................. 324 Programming in the Large ................................................................. 324 Programming in the Small ................................................................. 327 Hiding Classes ................................................................................... 329 Interfaces ................................................................................................ 331 Programming in the Large ................................................................. 331 Programming in the Small ................................................................. 335 Summary ................................................................................................ 338 Q&A ...................................................................................................... 339 Day 17 Exceptions 341 Programming in the Large ...................................................................... 342 Programming in the Small ...................................................................... 345 The Limitations Placed on the Programmer ........................................... 348 The finally Clause ................................................................................... 349 Summary ................................................................................................ 350 Q&A ...................................................................................................... 351 030-4 FM 1/29/96, 8:11 PM 15 Teach Yourself JAVA in 21 Days M T W T F S S 21 xvi P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Day 18 Multithreading 353 The Problem with Parallelism ................................................................. 354 Thinking Multithreaded ......................................................................... 355 Points About Points ........................................................................... 357 Protecting a Class Variable ................................................................. 360 Creating and Using Threads ................................................................... 361 The Runnable Interface ...................................................................... 362 ThreadTester ...................................................................................... 363 NamedThreadTester ........................................................................... 365 Knowing When a Thread has Stopped ................................................... 366 Thread Scheduling ................................................................................. 367 Preemptive Versus Nonpreemptive .................................................... 367 Testing Your Scheduler ...................................................................... 368 Summary ................................................................................................ 371 Q&A ...................................................................................................... 372 Day 19 Streams 375 Input Streams ......................................................................................... 377 The abstract Class InputStream ........................................................... 377 ByteArrayInputStream ......................................................................... 381 FileInputStream .................................................................................. 382 FilterInputStream ............................................................................... 383 PipedInputStream ............................................................................... 389 SequenceInputStream .......................................................................... 389 StringBufferInputStream ..................................................................... 390 Output Streams ...................................................................................... 391 The abstract Class OutputStream ........................................................ 391 ByteArrayOutputStream ...................................................................... 392 FileOutputStream ............................................................................... 393 FilterOutputStream............................................................................. 394 PipedOutputStream ............................................................................ 399 Related Classes ....................................................................................... 399 Summary ................................................................................................ 399 Q&A ...................................................................................................... 400 Day 20 Native Methods and Libraries 403 Disadvantages of native Methods ............................................................ 404 The Illusion of Required Efficiency ........................................................ 405 Built-In Optimizations ...................................................................... 407 Simple Optimization Tricks............................................................... 407 Writing native Methods .......................................................................... 408 The Example Class ............................................................................ 409 Generating Header and Stub Files ..................................................... 410 Creating SimpleFileNative.c .............................................................. 414 030-4 FM 1/29/96, 8:11 PM 16 xvii Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 A Native Library ..................................................................................... 417 Linking It All ..................................................................................... 418 Using Your Library ............................................................................ 418 Summary ................................................................................................ 418 Q&A ...................................................................................................... 419 Day 21 Under the Hood 421 The Big Picture ...................................................................................... 422 Why It’s a Powerful Vision ................................................................ 423 The Java Virtual Machine ....................................................................... 423 An Overview ...................................................................................... 424 The Fundamental Parts ...................................................................... 426 The Constant Pool ............................................................................ 430 Limitations ........................................................................................ 430 Bytecodes in More Detail ....................................................................... 431 The Bytecode Interpreter ................................................................... 431 The “Just-in-Time” Compiler ............................................................ 432 The java2c Translator ........................................................................ 433 The Bytecodes Themselves ................................................................ 434 The _quick Bytecodes ........................................................................ 450 The .class File Format ............................................................................. 452 Method Signatures ................................................................................. 454 The Garbage Collector ........................................................................... 455 The Problem ...................................................................................... 455 The Solution ...................................................................................... 456 Java’s Parallel Garbage Collector ........................................................ 459 The Security Story .................................................................................. 459 Why You Should Worry .................................................................... 459 Why You Might Not Have To .......................................................... 460 Java’s Security Model ......................................................................... 460 Summary ................................................................................................ 470 Q&A ...................................................................................................... 470 A Language Summary 473 Reserved Words ...................................................................................... 474 Comments .............................................................................................. 475 Literals .................................................................................................... 475 Variable Declaration ............................................................................... 476 Variable Assignment ............................................................................... 476 Operators ............................................................................................... 477 Objects ................................................................................................... 478 Arrays ..................................................................................................... 478 Loops and Conditionals .......................................................................... 478 Class Definitions .................................................................................... 479 Method and Constructor Definitions ..................................................... 479 Packages, Interfaces, and Importing ........................................................ 480 Exceptions and Guarding ....................................................................... 481 030-4 FM 1/29/96, 8:12 PM 17 Teach Yourself JAVA in 21 Days M T W T F S S 21 xviii P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 B Class Hierarchy Diagrams 483 About These Diagrams ........................................................................... 495 C The Java Class Library 497 java.lang ................................................................................................. 498 Interfaces ........................................................................................... 498 Classes ............................................................................................... 498 java.util .................................................................................................. 499 Interfaces ........................................................................................... 499 Classes ............................................................................................... 499 java.io ..................................................................................................... 500 Interfaces ........................................................................................... 500 Classes ............................................................................................... 500 java.net ................................................................................................... 501 Interfaces ........................................................................................... 501 Classes ............................................................................................... 502 java.awt .................................................................................................. 502 Interfaces ........................................................................................... 502 Classes ............................................................................................... 502 java.awt.image ........................................................................................ 504 Interfaces ........................................................................................... 504 Classes ............................................................................................... 504 java.awt.peer ........................................................................................... 505 java.applet ............................................................................................... 505 Interfaces ........................................................................................... 505 Classes ............................................................................................... 505 D How Java Differs from C and C++ 507 Pointers .................................................................................................. 508 Arrays ..................................................................................................... 508 Strings .................................................................................................... 508 Memory Management ............................................................................ 509 Data Types ............................................................................................. 509 Operators ............................................................................................... 509 Control Flow .......................................................................................... 510 Arguments .............................................................................................. 510 Other Differences ................................................................................... 510 Index 511 030-4 FM 1/29/96, 8:12 PM 18 xix Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Acknowledgments From Laura Lemay: To Sun’s Java team, for all their hard work on Java the language and on the browser, and particularly to Jim Graham, who demonstrated Java and HotJava to me on very short notice in May and planted the idea for this book. To everyone who bought my previous books, and liked them. Buy this one too. From Charles L. Perkins: To Patrick Naughton, who first showed me the power and the promise of OAK (Java) in early 1993. To Mark Taber, who shepherded this lost sheep through his first book. 030-4 FM 1/29/96, 8:12 PM 19 Teach Yourself JAVA in 21 Days M T W T F S S 21 xx P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 About the Authors Laura Lemay is a technical writer and a nerd. After spending six years writing software documentation for various computer companies in Silicon Valley, she decided writing books would be much more fun (but has still not yet made up her mind). In her spare time she collects computers, e-mail addresses, interesting hair colors, and nonrunning motorcycles. She is also the perpetrator of Teach Yourself Web Publishing with HTML in 14 Days. You can reach her by e-mail at lemay@lne.com, or visit her home page at http://www.lne.com/ lemay/. Charles L. Perkins is the founder of Virtual Rendezvous, a company building what it spent two years designing: a software layer above Java that will foster socially focused, computer-mediated, real-time filtered interactions between people’s personas in the virtual environments of the near future. In previous lives, he has evangelized NeXTSTEP, Smalltalk, and UNIX, and has degrees in both physics and computer science. Before attempting this book, he was an amateur columnist and author. He’s done research in speech recognition, neural nets, gestural user interfaces, computer graphics, and language theory, but had the most fun working at Thinking Machines and Xerox PARC’s Smalltalk group. In his spare time, he reads textbooks for fun. You can reach him via e-mail at virtual@rendezvous.com, or visit his Java page at http:// rendezvous.com/java. 030-4 FM 1/29/96, 8:12 PM 20 xxi Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 Introduction The World Wide Web, for much of its existence, has been a method for distributing passive information to a widely distributed number of people. The Web has, indeed, been exceptionally good for that purpose. With the addition of forms and image maps, Web pages began to become interactive—but the interaction was often simply a new way to get at the same information. The limitations of Web distribution were all too apparent once designers began to try to stretch the boundaries of what the Web can do. Even other innovations, such as Netscape’s server push to create dynamic animations, were merely clever tricks layered on top of a framework that wasn’t built to support much other than static documents with images and text. Enter Java, and the capability for Web pages of containing Java applets. Applets are small programs that create animations, multimedia presentations, real-time (video) games, multi-user networked games, and real interactivity—in fact, most anything a small program can do, Java applets can. Downloaded over the net and executed inside a Web page by a browser that supports Java, applets are an enormous step beyond standard Web design. The disadvantage of Java is that to create Java applets right now, you need to write them in the Java language. Java is a programming language, and as such, creating Java applets is more difficult than creating a Web page or a form using HTML. Soon there will be tools and programs that will make creating Java applets easier—they may be available by the time you read this. For now, however, the only way to delve into Java is to learn the language and start playing with the raw Java code. Even when the tools come out, you may want to do more with Java than the tools can provide, and you’re back to learning the language. That’s where Teach Yourself Java in 21 Days comes in. This book teaches you all about the Java language and how to use it to create not only applets, but also applications, which are more general Java programs that don’t need to run inside a Web browser. By the time you get through with this book, you’ll know enough about Java to do just about anything, inside an applet or out. Who Should Read This Book Teach Yourself Java in 21 Days is intended for people with at least some basic programming background—which includes people with years of programming experience and people with only a small amount of experience. If you understand what variables, loops, and functions are, you’ll be just fine for this book. The sorts of people who might want to read this book include you, if one or more of the following is true: ■ You’re a real whiz at HTML, understand CGI programming (in perl, AppleScript, Visual Basic, or some other popular CGI language) pretty well, and want to move onto the next level in Web page design. 030-4 FM 1/29/96, 8:12 PM 21 Teach Yourself JAVA in 21 Days M T W T F S S 21 xxii P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 ■ You had some Basic or Pascal in school, you’ve got a basic grasp of what programming is, but you’ve heard Java is easy to learn, really powerful, and very cool. ■ You’ve programmed C and C++ for many years, you’ve heard this Java thing is becoming really popular, and you’re wondering what all the fuss is all about. ■ You’ve heard that Java is really good for Web-based applets, and you’re curious about how good it is for creating more general applications. What if you know programming, but you don’t know object-oriented programming? Fear not. Teach Yourself Java in 21 Days assumes no background in object-oriented design. If you know object-oriented programming, the first couple of days will be easy for you. What if you’re a rank beginner? This book might move a little fast for you. Java is a good language to start with, though, and if you take it slow and work through all the examples, you may still be able to pick up Java and start creating your own applets. How This Book Is Organized Teach Yourself Java in 21 Days describes Java primarily in its current state—what’s known as the beta API (Application Programming Interface). This is the version of Java that Netscape and other browsers, such as Spyglass’s Mosaic, support. A previous version of Java, the alpha API, was significantly different from the version described in this book, and the two versions are not compatible with each other. There are other books that describe only the alpha API, and there may still be programs and browsers out there that can only run using alpha Java programs. Teach Yourself Java in 21 Days uses primarily Java beta because that is the version that is most current and is the version that will continue to be used in the future. The alpha API is obsolete and will eventually die out. If you learn Java using beta API, you’ll be much better prepared for any future changes (which will be minor) than if you have to worry about both APIs at once. Java is still in development. “Beta” means that Java is not complete and that things may change between the time this book is being written and the time you read this. Keep this in mind as you work with Java and with the software you’ll use to create and compile programs. If things aren’t behaving the way you expect, check the Web sites mentioned at the end of this introduction for more information. Teach Yourself Java in 21 Days covers the Java language and its class libraries in 21 days, organized as three separate weeks. Each week covers a different broad area of developing Java applets and applications. In the first week, you’ll learn about the Java language itself: ■ Day 1 is the basic introduction: what Java is, why it’s cool, and how to get the software. You’ll also create your first Java applications and applets. 030-4 FM 1/29/96, 8:12 PM 22 xxiii Sams.netLearningCenterabcd P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 ■ On Day 2, you’ll explore basic object-oriented programming concepts as they apply to Java. ■ On Day 3, you start getting down to details with the basic Java building blocks: data types, variables, and expressions such as arithmetic and comparisons. ■ Day 4 goes into detail about how to deal with objects in Java: how to create them, how to access their variables and call their methods, and how to compare and copy them. You’ll also get your first glance at the Java class libraries. ■ On Day 5, you’ll learn more about Java with arrays, conditional statements. and loops. ■ Day 6 is the best one yet. You’ll learn how to create classes, the basic building blocks of any Java program, as well as how to put together a Java application (an application being a Java program that can run on its own without a Web browser). ■ Day 7 builds on what you learned on Day 6. On Day 7, you’ll learn more about how to create and use methods, including overriding and overloading methods and creating constructors. Week 2 is dedicated to applets and the Java class libraries: ■ Day 8 provides the basics of applets—how they’re different from applications, how to create them, and the most important parts of an applet’s life cycle. You’ll also learn how to create HTML pages that contain Java applets. ■ On Day 9, you’ll learn about the Java classes for drawing shapes and characters to the screen—in black, white, or any other color. ■ On Day 10, you’ll start animating those shapes you learned about on Day 9, includ- ing learning what threads and their uses are. ■ Day 11 covers more detail about animation, adding bitmap images and audio to the soup. ■ Day 12 delves into interactivity—handling mouse and keyboard clicks from the user in your Java applets. ■ Day 13 is ambitious; on that day you’ll learn about using Java’s Abstract Windowing Toolkit to create a user interface in your applet including menus, buttons, checkboxes, and other elements. ■ On Day 14, you explore the last of the main Java class libraries for creating applets: windows and dialogs, networking, and a few other tidbits. Week 3 finishes up with advanced topics, for when you start doing larger and more complex Java programs, or when you want to learn more: ■ On Day 15, you’ll learn more about the Java language’s modifiers—for abstract and final methods and classes as well as for protecting a class’s private information from the prying eyes of other classes. 030-4 FM 1/29/96, 8:12 PM 23 Teach Yourself JAVA in 21 Days M T W T F S S 21 xxiv P2/V4SQC6 TY Java in 21 Days 030-4 louisa 12.31.95 FM LP#4 ■ Day 16 covers interfaces and packages, useful for abstracting protocols of methods to aid reuse and for the grouping and categorization of classes. ■ Day 17 covers exceptions: errors and warnings and other abnormal conditions, generated either by the system or by you in your programs. ■ Day 18 builds on the thread basics you learned on Day 10 to give a broad overview of multithreading and how to use it to allow different parts of your Java programs to run in parallel. ■ On Day 19, you’ll learn all about the input and output streams in Java’s I/O library. ■ Day 20 teaches you about native code—how to link C code into your Java programs to provide missing functionality or to gain performance. ■ Finally, on Day 21, you’ll get an overview of some of the “behind-the-scenes” techni- cal details of how Java works: the bytecode compiler and interpreter, the techniques Java uses to ensure the integrity and security of your programs, and the Java garbage collector. Conventions Used in This Book Text that you type and text that should appear on your screen is presented in monospace type: It will look like this. to mimic the way text looks on your screen. Variables and placeholders will appear in monospace italic. The end of each chapter offers common questions asked about that day’s subject matter with answers from the authors. Web Sites for Further Information Before, while, and after you read this book, there are two Web sites that may be of interest to you as a Java developer. The official Java web site is at http://java.sun.com/. At this site, you’ll find the Java development software, the HotJava web browser, and online documentation for all aspects of the Java language. It has several mirror sites that it lists online, and you should probably use the site “closest” to you on the Internet for your downloading and Java Web browsing. There is also a site for developer resources, called Gamelan, at http://www.gamelan.com/. This book also has a companion Web site at http://www.lne.com/Web/Java/. Information at that site includes examples, more information and background for this book, corrections to this book, and other tidbits that were not included here. 030-4 FM 1/29/96, 8:12 PM 24 1 Sams.netLearningCenterabcd P2/V4/sqc8 TY Java in 21 Days 030-4 Everly 12.11.95 AAG1 LP#2 M T W R F S S ■ An Introduction to Java Programming Platform independence The Java compiler and the java interpreter ■ Object-Oriented Programming and Java Objects and classes Encapsulation Modularity ■ Java Basics Java statements and expressions Variables and data types Comparisons and logical operators ■ Working with Objects Testing and modifying instance variables Converting objects ■ Arrays, Conditionals, and Loops Conditional tests Iteration Block statements WEEK AT A GLANCE2 3 4 5 6 7 1 1 030-4s AAG 01 1/29/96, 8:13 PM 1 2 Week 1 at a Glance M T W R F S S WEEK 1 P2/V4/sqc8 TY Java in 21 Days 030-4 Everly 12.11.95 AAG1 LP#2 ■ Creating Classes and Applications in Java Defining constants, instance and class variables, and methods ■ More About Methods Overloading methods Constructor methods Overriding methods 030-4s AAG 01 1/29/96, 8:13 PM 2 3 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 M T W R F S S An Introduction to Java Programming by Laura Lemay WEEK 1 1 030-4s CH01.i 1/29/96, 8:34 PM 3 4 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Hello and welcome to Teach Yourself Java in 21 Days! Starting today and for the next three weeks you’ll learn all about the Java language and how to use it to create applets, as well as how to create stand-alone Java applications that you can use for just about anything. An applet is a dynamic and interactive program that can run inside a Web page displayed by a Java-capable browser such as HotJava or Netscape 2.0. The HotJava browser is a World Wide Web browser used to view Web pages, follow links, and submit forms. It can also download and play applets on the reader’s system. That’s the overall goal for the next three weeks. Today, the goals are somewhat more modest, and you’ll learn about the following: ■ What exactly Java and HotJava are, and their current status ■ Why you should learn Java—its various features and advantages over other program- ming languages ■ Getting started programming in Java—what you’ll need in terms of software and background, as well as some basic terminology ■ How to create your first Java programs—to close this day, you’ll create both a simple Java application and a simple Java applet! What Is Java? Java is an object-oriented programming language developed by Sun Microsystems, a company best known for its high-end Unix workstations. Modeled after C++, the Java language was designed to be small, simple, and portable across platforms and operating systems, both at the source and at the binary level (more about this later). Java is often mentioned in the same breath as HotJava, a World Wide Web browser from Sun like Netscape or Mosaic (see Figure 1.1). What makes HotJava different from most other browsers is that, in addition to all its basic Web features, it can also download and play applets on the reader’s system. Applets appear in a Web page much in the same way as images do, but unlike images, applets are dynamic and interactive. Applets can be used to create animations, figures, or areas that can respond to input from the reader, games, or other interactive effects on the same Web pages among the text and graphics. Although HotJava was the first World Wide Web browser to be able to play Java applets, Java support is rapidly becoming available in other browsers. Netscape 2.0 provides support for Java applets, and other browser developers have also announced support for Java in forthcoming products. NEW TERM ☛ 030-4s CH01.i 1/29/96, 8:34 PM 4 5 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 To create an applet, you write it in the Java language, compile it using a Java compiler, and refer to that applet in your HTML Web pages. You put the resulting HTML and Java files on a Web site much in the same way that you make ordinary HTML and image files available. Then, when someone using the HotJava browser (or other Java-aware browser) views your page with the embedded applet, that browser downloads the applet to the local system and executes it, and then the reader can view and interact with your applet in all its glory (readers using other browsers won’t see anything). You’ll learn more about how applets, browsers, and the World Wide Web work together further on in this book. The important thing to understand about Java is that you can do so much more with it besides create applets. Java was written as a full-fledged programming language in which you can accomplish the same sorts of tasks and solve the same sorts of problems that you can in other programming languages, such as C or C++. HotJava itself, including all the networking, display, and user interface elements, is written in Java. Figure 1.1. The HotJava browser. 030-4s CH01.i 1/29/96, 8:35 PM 5 6 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Java’s Past, Present, and Future The Java language was developed at Sun Microsystems in 1991 as part of a research project to develop software for consumer electronics devices—television sets, VCRs, toasters, and the other sorts of machines you can buy at any department store. Java’s goals at that time were to be small, fast, efficient, and easily portable to a wide range of hardware devices. It is those same goals that made Java an ideal language for distributing executable programs via the World Wide Web, and also a general-purpose programming language for developing programs that are easily usable and portable across different platforms. The Java language was used in several projects within Sun, but did not get very much commercial attention until it was paired with HotJava. HotJava was written in 1994 in a matter of months, both as a vehicle for downloading and running applets and also as an example of the sort of complex application that can be written in Java. At the time this book is being written, Sun has released the beta version of the Java Developer’s Kit (JDK), which includes tools for developing Java applets and applications on Sun systems running Solaris 2.3 or higher for Windows NT and for Windows 95. By the time you read this, support for Java development may have appeared on other platforms, either from Sun or from third-party companies. Note that because the JDK is currently in beta, it is still subject to change between now and when it is officially released. Applets and applications you write using the JDK and using the examples in this book may require some changes to work with future versions of the JDK. However, because the Java language has been around for several years and has been used for several projects, the language itself is quite stable and robust and most likely will not change excessively. Keep this beta status in mind as you read through this book and as you develop your own Java programs. Support for playing Java programs is a little more confusing at the moment. Sun’s HotJava is not currently included with the Beta JDK; the only available version of HotJava is an older alpha version, and, tragically, applets written for the alpha version of Java do not work with the beta JDK, and vice versa. By the time you read this, Sun may have released a newer version of HotJava which will enable you to view applets. The JDK does include an application called appletviewer that allows you to test your Java applets as you write them. If an applet works in the appletviewer, it should work with any Java-capable browser. You’ll learn more about applet viewer later today. What’s in store for the future? In addition to the final Java release from Sun, other companies have announced support for Java in their own World Wide Web browsers. Netscape Commu- nications Corporation has already incorporated Java capabilities into the 2.0 version of their very popular Netscape Navigator Web browser—pages with embedded Java applets can be viewed and played with Netscape. With support for Java available in as popular a browser as Netscape, 030-4s CH01.i 1/29/96, 8:35 PM 6 7 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 tools to help develop Java applications (debuggers, development environments, and so on) most likely will be rapidly available as well. Why Learn Java? At the moment, probably the most compelling reason to learn Java—and probably the reason you bought this book—is that HotJava applets are written in Java. Even if that were not the case, Java as a language has significant advantages over other languages and other programming environments that make it suitable for just about any programming task. This section describes some of those advantages. Java Is Platform-Independent Platform independence is one of the most significant advantages that Java has over other programming languages, particularly for systems that need to work on many different platforms. Java is platform-independent at both the source and the binary level. Platform-independence is a program’s capability of moving easily from one computer system to another. At the source level, Java’s primitive data types have consistent sizes across all development platforms. Java’s foundation class libraries make it easy to write code that can be moved from platform to platform without the need to rewrite it to work with that platform. Platform-independence doesn’t stop at the source level, however. Java binary files are also platform-independent and can run on multiple problems without the need to recompile the source. How does this work? Java binary files are actually in a form called bytecodes. Bytecodes are a set of instructions that looks a lot like some machine codes, but that is not specific to any one processor. Normally, when you compile a program written in C or in most other languages, the compiler translates your program into machine codes or processor instructions. Those instructions are specific to the processor your computer is running—so, for example, if you compile your code on a Pentium system, the resulting program will run only on other Pentium systems. If you want to use the same program on another system, you have to go back to your original source, get a compiler for that system, and recompile your code. Figure 1.2 shows the result of this system: multiple executable programs for multiple systems. Things are different when you write code in Java. The Java development environment has two parts: a Java compiler and a Java interpreter. The Java compiler takes your Java program and instead of generating machine codes from your source files, it generates bytecodes. NEW TERM ☛ NEW TERM ☛ 030-4s CH01.i 1/29/96, 8:35 PM 7 8 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 To run a Java program, you run a program called a bytecode interpreter, which in turn executes your Java program (see Figure 1.3). You can either run the interpreter by itself, or—for applets— there is a bytecode interpreter built into HotJava and other Java-capable browsers that runs the applet for you. Figure 1.2. Traditional compiled programs. Your Code Compiler (Pentium) Binary File (Pentium) Binary File (PowerPC) Binary File (SPARC) Compiler (PowerPC) Compiler (SPARC) Figure 1.3. Java programs. Java Code Java Compiler (Pentium) Java Bytecode (Platform- Independent) Java Compiler (PowerPC) Java Compiler (SPARC) Java Interpreter (Pentium) Java Interpreter (PowerPC) Java Interpreter (SPARC) Window Window Window 030-4s CH01.i 1/29/96, 8:35 PM 8 9 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Why go through all the trouble of adding this extra layer of the bytecode interpreter? Having your Java programs in bytecode form means that instead of being specific to any one system, your programs can be run on any platform and any operating or window system as long as the Java interpreter is available. This capability of a single binary file to be executable across platforms is crucial to what enables applets to work, because the World Wide Web itself is also platform- independent. Just as HTML files can be read on any platform, so applets can be executed on any platform that is a Java-capable browser. The disadvantage of using bytecodes is in execution speed. Because system-specific programs run directly on the hardware for which they are compiled, they run significantly faster than Java bytecodes, which must be processed by the interpreter. For many Java programs, the speed may not be an issue. If you write programs that require more execution speed than the Java interpreter can provide, you have several solutions available to you, including being able to link native code into your Java program or using tools to convert your Java bytecodes into native code. Note that by using any of these solutions, you lose the portability that Java bytecodes provide. You’ll learn about each of these mechanisms on Day 20. Java Is Object-Oriented To some, object-oriented programming (OOP) technique is merely a way of organizing programs, and it can be accomplished using any language. Working with a real object-oriented language and programming environment, however, enables you to take full advantage of object- oriented methodology and its capabilities of creating flexible, modular programs and reusing code. Many of Java’s object-oriented concepts are inherited from C++, the language on which it is based, but it borrows many concepts from other object-oriented languages as well. Like most object-oriented programming languages, Java includes a set of class libraries that provide basic data types, system input and output capabilities, and other utility functions. These basic classes are part of the Java development kit, which also has classes to support networking, common Internet protocols, and user interface toolkit functions. Because these class libraries are written in Java, they are portable across platforms as all Java applications are. You’ll learn more about object-oriented programming and Java tomorrow. Java Is Easy to Learn In addition to its portability and object-orientation, one of Java’s initial design goals was to be small and simple, and therefore easier to write, easier to compile, easier to debug, and, best of all, easy to learn. Keeping the language small also makes it more robust because there are fewer chances for programmers to make difficult-to-find mistakes. Despite its size and simple design, however, Java still has a great deal of power and flexibility. 030-4s CH01.i 1/29/96, 8:35 PM 9 10 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Java is modeled after C and C++, and much of the syntax and object-oriented structure is borrowed from the latter. If you are familiar with C++, learning Java will be particularly easy for you, because you have most of the foundation already. Although Java looks similar to C and C++, most of the more complex parts of those languages have been excluded from Java, making the language simpler without sacrificing much of its power. There are no pointers in Java, nor is there pointer arithmetic. Strings and arrays are real objects in Java. Memory management is automatic. To an experienced programmer, these omissions may be difficult to get used to, but to beginners or programmers who have worked in other languages, they make the Java language far easier to learn. Getting Started with Programming in Java Enough background! Let’s finish off this day by creating two real Java programs: a stand-alone Java application and an applet that you can view in either in the appletviewer (part of the JDK) or in a Java-capable browser. Although both these programs are extremely simple, they will give you an idea of what a Java program looks like and how to compile and run it. Getting the Software In order to write Java programs, you will, of course, need a Java development environment. At the time this book is being written, Sun’s Java Development Kit provides everything you need to start writing Java programs. The JDK is available for Sun SPARC systems running Solaris 2.2 or higher and for Windows NT and Windows 95. You can get the JDK from several places: ■ The CD-ROM that came with this book contains the full JDK distribution. See the CD information for installation instructions. ■ The JDK can be downloaded from Sun’s Java FTP site at ftp://java.sun.com/pub/ or from a mirror site (ftp://www.blackdown.org/pub/Java/pub/is one). Note: The Java Development Kit is currently in beta release. By the time you read this, The JDK may be available for other platforms, or other organizations may be selling Java development tools as well. Although Netscape and other Java-aware browsers provide an environment for playing Java applets, they do not provide a mechanism for developing Java applications. For that, you need separate tools—merely having a browser is not enough. 030-4s CH01.i 1/29/96, 8:35 PM 10 11 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Applets and Applications Java applications fall into two main groups: applets and applications. Applets, as you have learned, are Java programs that are downloaded over the World Wide Web and executed by a Web browser on the reader’s machine. Applets depend on a Java-capable browser in order to run (although they can also be viewed using a tool called the appletviewer, which you’ll learn about later today). Java applications are more general programs written in the Java language. Java applications don’t require a browser to run, and in fact, Java can be used to create most other kinds of applications that you would normally use a more conventional programming language to create. HotJava itself is a Java application. A single Java program can be an applet or an application or both, depending on how you write that program and the capabilities that program uses. Throughout this first week, you’ll be writing mostly HotJava applications; then you’ll apply what you’ve learned to write applets in Week 2. If you’re eager to get started with applets, be patient. Everything that you learn while you’re creating simple Java applications will apply to creating applets, and it’s easier to start with the basics before moving onto the hard stuff. You’ll be creating plenty of applets in Week 2. Creating a Java Application Let’s start by creating a simple Java application: the classic Hello World example that all language books use to begin. As with all programming languages, your Java source files are created in a plain text editor, or in an editor that can save files in plain ASCII without any formatting characters. On Unix, emacs, ped, or vi will work; on Windows, Notepad or DOS Edit are both text editors. Fire up your editor of choice, and enter the Java program shown in Listing 1.1. Type this program, as shown, in your text editor. Be careful that all the parentheses, braces, and quotes are there. Listing 1.1. Your first Java application. 1: class HelloWorld { 2: public static void main (String args[]) { 3: System.out.println(“Hello World!”); 4: } 5: } Type 030-4s CH01.i 1/29/96, 8:35 PM 11 12 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 !! Warning: The numbers before each line are part of the listing and not part of the program; they’re there so I can refer to specific line numbers when I explain what’s going on in the program. Do not include them in your own file. This program has two main parts: ■ All the program is enclosed in a class definition—here, a class called HelloWorld. ■ The body of the program (here, just the one line) is contained in a routine called main(). In Java applications, as in a C or C++ program, main() is the first routine that is run when the program is executed. You’ll learn more about both these parts of a Java application as the book progresses. Once you finish typing the program, save the file. Conventionally, Java source files are named the same name as the class they define, with an extension of .java. This file should therefore be called HelloWorld.java. Now, let’s compile the source file using the Java compiler. In Sun’s JDK, the Java compiler is called javac. To compile your Java program, Make sure the javac program is in your execution path and type javac followed by the name of your source file: javac HelloWorld.java Note: In these examples, and in all the examples throughout this book, we’ll be using Sun’s Java compiler, part of the JDK. If you have a third-party development environment, check with the documentation for that program to see how to compile your Java programs. The compiler should compile the file without any errors. If you get errors, go back and make sure that you’ve typed the program exactly as it appears in Listing 1.1. When the program compiles without errors, you end up with a file called HelloWorld.class, in the same directory as your source file. This is your Java bytecode file. You can then run that bytecode file using the Java interpreter. In the JDK, the Java interpreter is called simply java. Make sure the java program is in your path and type java followed by the name of the file without the .class extension: java HelloWorld Analysis 030-4s CH01.i 1/29/96, 8:35 PM 12 13 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 If your program was typed and compiled correctly, you should get the string “Hello World!” printed to your screen as a response. Note: Remember, the Java compiler and the Java interpreter are different things. You use the Java compiler (javac) for your Java source files to create .class files, and you use the Java interpreter (java)to actually run your class files. Creating a Java Applet Creating applets is different from creating a simple application, because Java applets run and are displayed inside a Web page with other page elements and as such have special rules for how they behave. Because of these special rules for applets in many cases (particularly the simple ones), creating an applet may be more complex than creating an application. For example, to do a simple Hello World applet, instead of merely being able to print a message, you have to create an applet to make space for your message and then use graphics operations to paint the message to the screen. Note: Actually, if you run the Hello World application as an applet, the Hello World message prints to a special window or to a log file, depending on how the browser has screen messages set up. It will not appear on the screen unless you write your applet to put it there. In the next example, you create that simple Hello World applet, place it inside a Web page, and view the result. First, you set up an environment so that your Java-capable browser can find your HTML files and your applets. Much of the time, you’ll keep your HTML files and your applet code in the same directory. Although this isn’t required, it makes it easier to keep track of each element. In this example, you use a directory called HTML that contains all the files you’ll need. mkdir HTML Now, open up that text editor and enter Listing 1.2. 030-4s CH01.i 1/29/96, 8:36 PM 13 14 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Listing 1.2. The Hello World applet. 1: import java.awt.Graphics; 2: 3: class HelloWorldApplet extends java.applet.Applet { 4: 5: public void paint(Graphics g) { 6: g.drawString(“Hello world!”, 5, 25); 7: } 8:} Save that file inside your HTML directory. Just like with Java applications, give your file a name that has the same name as the class. In this case, the filename would be HelloWorldApplet.java. Features to note about applets? There are a couple I’d like to point out: ■ The import line at the top of the file is somewhat analogous to an #include statement in C; it enables this applet to interact with the JDK classes for creating applets and for drawing graphics on the screen. ■ The paint() method displays the content of the applet onto the screen. Here, the string Hello World gets drawn. Applets use several standard methods to take the place of main(), which include init() to initialize the applet, start() to start it running, and paint() to display it to the screen. You’ll learn about all of these in Week 2. Now, compile the applet just as you did the application, using javac, the Java compiler. javac HelloWorldApplet.java Again, just as for applications, you should now have a file called HelloWorldApplet.class in your HTML directory. To include an applet in a Web page, you refer to that applet in the HTML code for that Web page. Here, you create a very simple HTML file in the HTML directory (see Listing 1.3). Listing 1.3. The HTML with the applet in it. 1: <HTML> 2: <HEAD> 3: <TITLE>Hello to Everyone!</TITLE> 4: </HEAD><BODY> 5: <P>My Java applet says: 6: <APPLET CODE=”HelloWorldApplet.class” WIDTH=150 HEIGHT=25> 7: </BODY> 8: </HTML> Type Type 030-4s CH01.i 1/29/96, 8:36 PM 14 15 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 You refer to an applet in your HTML files with the <APPLET> tag. You’ll learn more about <APPLET> later on, but here are two things to note: ■ Use the CODE attribute to indicate the name of the class that contains your applet. ■ Use the WIDTH and HEIGHT attributes to indicate the size of the applet. The browser uses these values to know how big a chunk of space to leave for the applet on the page. Here, a box 150 pixels wide and 25 pixels high is created. Save the HTML file in your HTML directory, with a descriptive name (for example, you might name your HTML file the same name as your applet—HellowWorldApplet.html). And now, you’re ready for the final test—actually viewing the result of your applet. To view the applet, you need one of the following: ■ A browser that supports Java applets, such as Netscape 2.0. ■ The appletviewer application, which is part of the JDK. The appletviewer is not a Web browser and won’t enable you to see the entire Web page, but it’s acceptable for testing to see how an applet will look and behave if there is nothing else available. Note: Do not use the alpha version of HotJava to view your applets; applets developed with the beta JDK and onward cannot be viewed by the alpha HotJava. If, by the time you read this, there is a more recent version of HotJava, you can use that one instead. If you’re using a Java-capable browser such as Netscape to view your applet files, you can use the Open Local... item under the File menu to navigate to the HTML file containing the applet (make sure you open the HTML file and not the class file). You don’t need to install anything on a Web server yet; all this works on your local system. If you don’t have a Web browser with Java capabilities built into it, you can use the appletviewer program to view your Java applet. To run appletviewer, just indicate the path to the HTML file on the command line: appletviewer HTML/HelloWorldApplet.html Tip: Although you can start appletviewer from the same directory as your HTML and class files, you may not be able to reload that applet without quitting appletviewer first. If you start appletviewer from some other directory (as in the previous command line), you can modify and recompile your Java applets and then just use the Reload menu item to view the newer version. Analysis 030-4s CH01.i 1/29/96, 8:36 PM 15 16 An Introduction to Java Programming M T W R F S S DAY 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 Now, if you use the browser to view the applet, you see something similar to the image shown in Figure 1.4. If you’re using appletviewer, you won’t see the text around the applet (My Java applet says...), but you will see the Hello World itself. Figure 1.4. The Hello World applet. Summary Today, you got a basic introduction to the Java language and its goals and features. Java is a programming language, similar to C or C++, in which you can develop a wide range of programs. The most common use of Java at the moment is in creating applets for HotJava, an advanced World Wide Web browser also written in Java. Applets are Java programs that are downloaded and run as part of a Web page. Applets can create animations, games, interactive programs, and other multimedia effects on Web pages. Java’s strengths lie in its portability—both at the source and at the binary level, in its object- oriented design—and in its simplicity. Each of these features help make applets possible, but they also make Java an excellent language for writing more general-purpose programs that do not require HotJava or other Java-capable browser to run. These general-purpose Java programs are called applications. HotJava itself is a Java application. To end this day, you experimented with an example applet and an example application, getting a feel for the differences between the two and how to create, compile, and run Java programs— or, in the case of applets, how to include them in Web pages. From here, you now have the foundation to create more complex applications and applets. Q&A Q I’d like to use HotJava as my regular Web browser. You haven’t mentioned much about HotJava today. A The focus of this book is primarily on programming in Java and in the HotJava classes, rather than on using HotJava itself. Documentation for using the HotJava browser comes with the HotJava package. Q I know a lot about HTML, but not much about computer programming. Can I still write Java programs? 030-4s CH01.i 1/29/96, 8:36 PM 16 17 Sams.netLearningCenterabcd 1 P2/V4sqc7 TY Java in 21 Days 030-4 sdv 12.22.95 Ch01 LP#4 A If you have no programming experience whatsoever, you most likely will find pro- gramming Java significantly more difficult. However, Java is an excellent language to learn programming with, and if you patiently work through the examples and the exercises in this book, you should be able to learn enough to get started with Java. Q According to today’s lesson, Java applets are downloaded via HotJava and run on the reader’s system. Isn’t that an enormous security hole? What stops someone from writing an applet that compromises the security of my system—or worse, that damages my system? A Sun’s Java team has thought a great deal about the security of applets within Java- capable browsers and has implemented several checks to make sure applets cannot do nasty things: ■ Java applets cannot read or write to the disk on the local system. ■ Java applets cannot execute any programs on the local system. ■ Java applets cannot connect to any machines on the Web except for the server from which they are originally downloaded. In addition, the Java compiler and interpreter check both the Java source code and the Java bytecodes to make sure that the Java programmer has not tried any sneaky tricks (for example, overrunning buffers or stack frames). These checks obviously cannot stop every potential security hole, but they can significantly reduce the potential for hostile applets. You’ll learn more about security issues later on in this book. Q I followed all the directions you gave for creating a Java applet. I loaded it into HotJava, but Hello World didn’t show up. What did I do wrong? A I’ll bet you’re using the alpha version of HotJava to view the applet. Unfortunately, between alpha and beta, significant changes were made as to how applets are written. The result is that you can’t view beta applets (as this one was) in the alpha version of HotJava, nor can you view alpha applets in browsers that expect beta applets. To view the applet, either use a different browser, or use the appletviewer application that comes with the JDK. 030-4s CH01.i 1/29/96, 8:36 PM 17 19 Sams.netLearningCenterabcd P2/V4sqc6 TY Java in 21 Days 030-4 ayanna 12.15.95 Ch02 LP#3 2 M T W R F S S Object-Oriented Programming and Java by Laura Lemay WEEK 1 2 030-4S CH02.i 1/29/96, 8:37 PM 19 20 Object-Oriented Programming and Java M T W R F S S DAY 2 P2/V4sqc6 TY Java in 21 Days 030-4 ayanna 12.15.95 Ch02 LP#3 Object-oriented programming (OOP) is one of the bigger programming buzzwords of recent years, and you can spend years learning all about object-oriented programming methodologies and how they can make your life easier than The Old Way of programming. It all comes down to organizing your programs in ways that echo how things are put together in the real world. Today, you’ll get an overview of object-oriented programming concepts in Java and how they relate to how you structure your own programs: ■ What classes and objects are, and how they relate to each other ■ The two main parts of a class or object: its behaviors and its attributes ■ Class inheritance and how inheritance affects the way you design your programs ■ Some information about packages and interfaces If you’re already familiar with object-oriented programming, much of today’s lesson will be old hat to you. You may want to skim it and go to a movie today instead. Tomorrow, you’ll get into more specific details. Thinking in Objects: An Analogy Consider, if you will, Legos. Legos, for those who do not spend much time with children, are small plastic building blocks in various colors and sizes. They have small round bits on one side that fit into small round holes on other Legos so that they fit together snugly to create larger shapes. With different Lego bits (Lego wheels, Lego engines, Lego hinges, Lego pulleys), you can put together castles, automobiles, giant robots that swallow cities, or just about anything else you can create. Each Lego bit is a small object that fits together with other small objects in predefined ways to create other larger objects. Here’s another example. You can walk into a computer store and, with a little background and often some help, assemble an entire PC computer system from various components: a motherboard, a CPU chip, a video card, a hard disk, a keyboard, and so on. Ideally, when you finish assembling all the various self-contained units, you have a system in which all the units work together to create a larger system with which you can solve the problems you bought the computer for in the first place. Internally, each of those components may be vastly complicated and engineered by different companies with different methods of design. But you don’t need to know how the component works, what every chip on the board does, or how, when you press the A key, an “A” gets sent to your computer. As the assembler of the overall system, each component you use is a self- contained unit, and all you are interested in is how the units interact with each other. Will this video card fit into the slots on the motherboard and will this monitor work with this video card? Will each particular component speak the right commands to the other components it interacts with so that each part of the computer is understood by every other part? Once you know what 030-4S CH02.i 1/29/96, 8:37 PM 20 21 Sams.netLearningCenterabcd P2/V4sqc6 TY Java in 21 Days 030-4 ayanna 12.15.95 Ch02 LP#3 2 the interactions are between the components and can match the interactions, putting together the overall system is easy. What does this have to do with programming? Everything. Object-oriented programming works in exactly this same way. Using object-oriented programming, your overall program is made up of lots of different self-contained components (objects), each of which has a specific role in the program and all of which can talk to each other in predefined ways. Objects and Classes Object-oriented programming is modeled on how, in the real world, objects are often made up of many kinds of smaller objects. This capability of combining objects, however, is only one very general aspect of object-oriented programming. Object-oriented programming provides several other concepts and features to make creating and using objects easier and more flexible, and the most important of these features is that of classes. A class is a template for multiple objects with similar features. Classes embody all the features of a particular set of objects. When you write a program in an object-oriented language, you don’t define actual objects. You define classes of objects. For example, you might have a Tree class that describes the features of all trees (has leaves and roots, grows, creates chlorophyll). The Tree class serves as an abstract model for the concept of a tree—to reach out and grab, or interact with, or cut down a tree you have to have a concrete instance of that tree. Of course, once you have a tree class, you can create lots of different instances of that tree, and each different tree instance can have different features (short, tall, bushy, drops leaves in Autumn), while still behaving like and being immediately recognizable as a tree (see Figure 2.1). An instance of a class is another word for an actual object. If classes are an abstract representation of an object, an instance is its concrete representation. So what, precisely, is the difference between an instance and an object? Nothing, really. Object is the more general term, but both instances and objects are the concrete representation of a class. In fact, the terms instance and object are often used interchangeably in OOP language. An instance of a tree and a tree object are both the same thing. In an example closer to the sort of things you might want to do in Java programming, you might create a class for the user interface element called a button. The Button class defines the features of a button (its label, its size, its appearance) and how it behaves (does it need a single click or a double click to activate it, does it change color when it’s clicked, what does it do when it’s activated?). Once you define the Button class, you can then easily create instances of that button—that is, button objects—that all take on the basic features of the button as defined by NEW TERM ☛ NEW TERM ☛ 030-4S CH02.i 1/29/96, 8:37 PM 21
