Fluent Essays

Read pages 1 through 108 (intro, observer, decorator sections ) as an introduction to design patterns and meet your first design patterns. Program an observer pattern independent from the example in the book using a problem space that is interesting to you. Download at WoweBook.Com Praise for Head First Design Patterns “I received the book yesterday and started to read it on the way home... and I couldn’t stop. I took it to the gym and I expect people saw me smiling a lot while I was exercising and reading. This is tres ‘cool’. It is fun but they cover a lot of ground and they are right to the point. I’m really impressed.” — Erich Gamma, IBM Distinguished Engineer, and co-author of Design Patterns “‘Head First Design Patterns’ manages to mix fun, belly-laughs, insight, technical depth and great practical advice in one entertaining and thought provoking read. Whether you are new to design patterns, or have been using them for years, you are sure to get something from visiting Objectville.” — Richard Helm, coauthor of “Design Patterns” with rest of the Gang of Four - Erich Gamma, Ralph Johnson and John Vlissides “I feel like a thousand pounds of books have just been lifted off of my head.” — Ward Cunningham, inventor of the Wiki and founder of the Hillside Group “This book is close to perfect, because of the way it combines expertise and readability. It speaks with authority and it reads beautifully. It’s one of the very few software books I’ve ever read that strikes me as indispensable. (I’d put maybe 10 books in this category, at the outside.)” — David Gelernter, Professor of Computer Science, Yale University and author of “Mirror Worlds” and “Machine Beauty” “A Nose Dive into the realm of patterns, a land where complex things become simple, but where simple things can also become complex. I can think of no better tour guides than the Freemans.” — Miko Matsumura, Industry Analyst, The Middleware Company Former Chief Java Evangelist, Sun Microsystems “I laughed, I cried, it moved me.” — Daniel Steinberg, Editor-in-Chief, java.net “My first reaction was to roll on the floor laughing. After I picked myself up, I realized that not only is the book technically accurate, it is the easiest to understand introduction to design patterns that I have seen.” — Dr. Timothy A. Budd, Associate Professor of Computer Science at Oregon State University and author of more than a dozen books, including “C++ for Java Programmers” “Jerry Rice runs patterns better than any receiver in the NFL, but the Freemans have out run him. Seriously...this is one of the funniest and smartest books on software design I’ve ever read.” — Aaron LaBerge, VP Technology, ESPN.com Download at WoweBook.Com More Praise for Head First Design Patterns “Great code design is, first and foremost, great information design. A code designer is teaching a com- puter how to do something, and it is no surprise that a great teacher of computers should turn out to be a great teacher of programmers. This book’s admirable clarity, humor and substantial doses of clever make it the sort of book that helps even non-programmers think well about problem-solving.” — Cory Doctorow, co-editor of Boing Boing and author of “Down and Out in the Magic Kingdom” and “Someone Comes to Town, Someone Leaves Town” “There’s an old saying in the computer and videogame business – well, it can’t be that old because the discipline is not all that old – and it goes something like this: Design is Life. What’s particularly curious about this phrase is that even today almost no one who works at the craft of creating electronic games can agree on what it means to “design” a game. Is the designer a software engineer? An art director? A storyteller? An architect or a builder? A pitch person or a visionary? Can an individual indeed be in part all of these? And most importantly, who the %$!#&* cares? It has been said that the “designed by” credit in interactive entertainment is akin to the “directed by” credit in filmmaking, which in fact allows it to share DNA with perhaps the single most controversial, overstated, and too often entirely lacking in humility credit grab ever propagated on commercial art. Good company, eh? Yet if Design is Life, then perhaps it is time we spent some quality cycles thinking about what it is. Eric and Elisabeth Freeman have intrepidly volunteered to look behind the code curtain for us in “Head First Design Patterns.” I’m not sure either of them cares all that much about the PlayStation or X-Box, nor should they. Yet they do address the notion of design at a significantly honest level such that anyone looking for ego reinforcement of his or her own brilliant auteurship is best advised not to go digging here where truth is stunningly revealed. Sophists and circus barkers need not apply. Next generation literati please come equipped with a pencil.” — Ken Goldstein, Executive Vice President & Managing Director, Disney Online “Just the right tone for the geeked-out, casual-cool guru coder in all of us. The right reference for practical development strategies—gets my brain going without having to slog through a bunch of tired, stale professor-speak.” — Travis Kalanick, Founder of Scour and Red Swoosh Member of the MIT TR100 “This book combines good humors, great examples, and in-depth knowledge of Design Patterns in such a way that makes learning fun. Being in the entertainment technology industry, I am intrigued by the Hollywood Principle and the home theater Facade Pattern, to name a few. The understanding of Design Patterns not only helps us create reusable and maintainable quality software, but also helps sharpen our problem-solving skills across all problem domains. This book is a must read for all com- puter professionals and students.” — Newton Lee, Founder and Editor-in-Chief, Association for Computing Machinery’s (ACM) Computers in Entertainment (acmcie.org) Download at WoweBook.Com More Praise for Head First Design Patterns “If there’s one subject that needs to be taught better, needs to be more fun to learn, it’s design patterns. Thank goodness for Head First Design Patterns. From the awesome Head First Java folks, this book uses every conceivable trick to help you understand and remember. Not just loads of pictures: pictures of humans, which tend to interest other humans. Surprises everywhere. Stories, because humans love narrative. (Stories about things like pizza and chocolate. Need we say more?) Plus, it’s darned funny. It also covers an enormous swath of concepts and techniques, including nearly all the patterns you’ll use most (observer, decorator, factory, singleton, command, adapter, façade, template method, iterator, composite, state, proxy). Read it, and those won’t be ‘just words’: they’ll be memories that tickle you, and tools you own.” — Bill Camarda, READ ONLY “After using Head First Java to teach our freshman how to start programming, I was eagerly waiting to see the next book in the series. Head First Design Patterns is that book and I am delighted. I am sure it will quickly become the standard first design patterns book to read, and is already the book I am recommending to students.” — Ben Bederson, Associate Professor of Computer Science & Director of the Human-Computer Interaction Lab, University of Maryland “Usually when reading through a book or article on design patterns I’d have to occasionally stick myself in the eye with something just to make sure I was paying attention. Not with this book. Odd as it may sound, this book makes learning about design patterns fun. While other books on design patterns are saying, ‘Buehler... Buehler... Buehler...’ this book is on the float belting out ‘Shake it up, baby!’” — Eric Wuehler “I literally love this book. In fact, I kissed this book in front of my wife.” — Satish Kumar Praise for the Head First approach “Java technology is everywhere—in mobile phones, cars, cameras, printers, games, PDAs, ATMs, smart cards, gas pumps, sports stadiums, medical devices, Web cams, servers, you name it. If you develop software and haven’t learned Java, it’s definitely time to dive in—Head First.” — Scott McNealy, Sun Microsystems Chairman, President and CEO “It’s fast, irreverent, fun, and engaging. Be careful—you might actually learn something!” — Ken Arnold, former Senior Engineer at Sun Microsystems Co-author (with James Gosling, creator of Java), “The Java Programming Language” Download at WoweBook.Com Make it Stick Learning Java Java in a Nutshell Java Enterprise in a Nutshell Java Examples in a Nutshell Java Cookbook J2EE Design Patterns Be watching for more books in the Head First series! Other related books from O’Reilly Head First Java Head First Servlets & JSP Head First EJB Head First Object-Oriented Analysis & Design Head First HTML with CSS & XHTML Head Rush Ajax Head First PMP Head First SQL (2007) Head First C# (2007) Head First Software Development (2007) Head First JavaScript (2007) Other books in O'Reilly's Head First series Download at WoweBook.Com Head First Design Patterns Beijing • Cambridge • Köln • Paris • Sebastopol • Taipei • Tokyo Wouldn’t it be dreamy if there was a Design Patterns book that was more fun than going to the dentist, and more revealing than an IRS form? It’s probably just a fantasy... Eric Freeman Elisabeth Freeman with Kathy Sierra Bert Bates Download at WoweBook.Com ISBN-10: 0-596-00712-4 ISBN-13: 978-0-596-00712-6 [M] [7/07] Download at WoweBook.Com To the Gang of Four, whose insight and expertise in capturing and communicating Design Patterns has changed the face of software design forever, and bettered the lives of developers throughout the world. But seriously, when are we going to see a second edition? After all, it’s been only ten years! Download at WoweBook.Com Download at WoweBook.Com Cre ators of the He ad First serie s (and co-conspirators on this book) Kathy Sierra Kathy has been interested in learning theory since her days as a game designer (she wrote games for Virgin, MGM, and Amblin’). She developed much of the Head First format while teaching New Media Authoring for UCLA Extension’s Entertainment Studies program. More recently, she’s been a master trainer for Sun Microsystems, teaching Sun’s Java instructors how to teach the latest Java technologies, and developing several of Sun’s certifi cation exams. Together with Bert Bates, she has been actively using the Head First concepts to teach throusands of developers. Kathy is the founder of javaranch.com, which won a 2003 and 2004 Software Development magazine Jolt Cola Productivity Award. You might catch her teaching Java on the Java Jam Geek Cruise (geekcruises.com). She recently moved from California to Colorado, where she’s had to learn new words like, “ice scraper” and “fl eece”, but the lightning there is fantastic. Likes: runing, skiing, skateboarding, playing with her Icelandic horse, and weird science. Dislikes: entropy. You can fi nd her on javaranch, or occasionally blogging on java.net. Write to her at [email protected] Bert is a long-time software developer and architect, but a decade-long stint in artifi cial intelligence drove his interest in learning theory and technology-based training. He’s been helping clients becoming better programmers ever since. Recently, he’s been heading up the development team for several of Sun’s Java Certifi cation exams. He spent the fi rst decade of his software career travelling the world to help broadcast clients like Radio New Zealand, the Weather Channel, and the Arts & Entertainment Network (A & E). One of his all-time favorite projects was building a full rail system simulation for Union Pacifi c Railroad. Bert is a long-time, hopelessly addicted go player, and has been working on a go program for way too long. He’s a fair guitar player and is now trying his hand at banjo. Look for him on javaranch, on the IGS go server, or you can write to him at [email protected] Bert Bates Download at WoweBook.Com x Intro Your brain on Design Patterns. Here you are trying to learn something, while here your brain is doing you a favor by making sure the learning doesn’t stick. Your brain’s thinking, “Better leave room for more important things, like which wild animals to avoid and whether naked snowboarding is a bad idea.” So how do you trick your brain into thinking that your life depends on knowing Design Patterns? Who is this book for? xxvi We know what your brain is thinking xxvii Metacognition xxix Bend your brain into submission xxxi Technical reviewers xxxiv Acknowledgements xxxv Table of Contents (summary) Intro xxv 1 Welcome to Design Patterns: an introduction 1 2 Keeping your Objects in the know: the Observer Pattern 37 3 Decorating Objects: the Decorator Pattern 79 4 Baking with OO goodness: the Factory Pattern 109 5 One of a Kind Objects: the Singleton Pattern 169 6 Encapsulating Invocation: the Command Pattern 191 7 Being Adaptive: the Adapter and Facade Patterns 235 8 Encapsulating Algorithms: theTemplate Method Pattern 275 9 Well-managed Collections: the Iterator and Composite Patterns 315 10 The State of Things: the State Pattern 385 11 Controlling Object Access: the Proxy Pattern 429 12 Patterns of Patterns: Compound Patterns 499 13 Patterns in the Real World: Better Living with Patterns 577 14 Appendix: Leftover Patterns 611 Table of Contents (the real thing) table of contents Download at WoweBook.Com xi 1 Welcome to Design PatternsSomeone has already solved your problems. In this chapter, you’ll learn why (and how) you can exploit the wisdom and lessons learned by other developers who’ve been down the same design problem road and survived the trip. Before we’re done, we’ll look at the use and benefi ts of design patterns, look at some key OO design principles, and walk through an example of how one pattern works. The best way to use patterns is to load your brain with them and then recognize places in your designs and existing applications where you can apply them. Instead of code reuse, with patterns you get experience reuse. intro to Design Patterns Your BR AIN Your Code, now new and improved wit h design patterns! A Bu nc h of P at te rn s swim() display() performQ uack() performF ly() setFlyBe havior() setQuack Behavior () // OTHER duck-like methods ... Duck FlyBeha vior flyB ehavior; QuackBe havior q uackBeh avior; <<interfa ce>> FlyBeha vior fly() fly() { // implem ents duck flying } FlyWithW ings fly() { // implem ents duck flying } FlyWithW ings // implem ents duck flying fly() { // do no thing - ca n’t fly! } FlyNoWa y fly() { // do no thing - ca n’t fly! } FlyNoWa y <<interfa ce>> QuackBe havior quack() quack) { // implem ents duck quacking } Quack // implem ents duck quacking Quack // implem ents duck quacking quack() { // rubbe r duckie s queak } Squeak quack() { // rubbe r duckie s queak } Squeak // rubbe r duckie s queak quack() { // do no thing - ca n’t quack ! } MuteQua ck quack() { // do no thing - ca n’t quack ! } MuteQua ck quack) { // implem ents duck quacking } quack) { // implem ents duck quacking } Quack display() { // looks li ke a deco y duck } Decoy D uck // looks li ke a deco y duck } display() { // looks li ke a mall ard } Mallard D uck // looks li ke a mall ard } display() { // looks li ke a redh ead } Redhead Duck Decoy D uck display() { // looks li ke a deco y duck } Decoy D uck // looks li ke a redh ead } display() { // looks li ke a rubb erduck } Rubber D uck display() { // looks li ke a deco y duck }d isplay() { // looks li ke a deco y duck } display() { // looks li ke a rubb erduck } Rubber D uck Encaps ulated fl y beh avior Encaps ulated quack behavi or Client View Con troll er Mod el Requ est MVC Subject Obje ct 8 int Dog Obje ct Mouse Obj ec t Cat Objec t Duck Obj ec t Observers 8 8 8 8 Automatic update/notification Object that holds state D ep en de nt O bj ec ts OBSERVER Remember, knowing concepts like abstraction, inheritance, and polymorphism do not make you a good object oriented designer. A design guru thinks about how to create fl exible designs that are maintainable and that can cope with change. The SimUDuck app 2 Joe thinks about inheritance... 5 How about an interface? 6 The one constant in software development 8 Separating what changes from what stays the same 10 Designing the Duck Behaviors 11 Testing the Duck code 18 Setting behavior dynamically 20 The Big Picture on encapsulated behaviors 22 HAS-A can be better than IS-A 23 The Strategy Pattern 24 The power of a shared pattern vocabulary 28 How do I use Design Patterns? 29 Tools for your Design Toolbox 32 Exercise Solutions 34 Download at WoweBook.Com xii The Weather Monitoring application 39 Meet the Observer Pattern 44 Publishers + Subscribers = Observer Pattern 45 Five minute drama: a subject for observation 48 The Observer Pattern defined 51 The power of Loose Coupling 53 Designing the Weather Station 56 Implementing the Weather Station 57 Using Java’s built-in Observer Pattern 64 The dark side of java.util.Observable 71 Tools for your Design Toolbox 74 Exercise Solutions 78 2 Keeping your Objects in the KnowDon’t miss out when something interesting happens! We’ve got a pattern that keeps your objects in the know when something they might care about happens. Objects can even decide at runtime whether they want to be kept informed. The Observer Pattern is one of the most heavily used patterns in the JDK, and it’s incredibly useful. Before we’re done, we’ll also look at one to many relationships and loose coupling (yeah, that’s right, we said coupling). With Observer, you’ll be the life of the Patterns Party. the Observer Pattern Subject Obje ct 8 int Dog Obje ct Mouse Obj ec t Cat Objec t Duck Obj ec t Observers 8 8 8 8 ONE TO MANY RELATIONSHIP Automatic update/notification Object that holds state D ep en de nt O bj ec ts Abstraction Encapsulatio n Polymorphism Inheritence OO Basics Abstraction Encapsulatio n Polymorphism Inheritence Encapsulate what varies Favor Compo sition over in heri- tance Program to Interfaces, n ot implementati ons Strive for lo osely coupled designs betw een objects t hat interact OO Principles table of contents Download at WoweBook.Com xiii 3 Decorating ObjectsJust call this chapter “Design Eye for the Inheritance Guy.” We’ll re-examine the typical overuse of inheritance and you’ll learn how to decorate your classes at runtime using a form of object composition. Why? Once you know the techniques of decorating, you’ll be able to give your (or someone else’s) objects new responsibilities without making any code changes to the underlying classes. the Decorator Pattern I used to think real men subclassed everything. That was until I learned the power of extension at runtime, rather than at compile time. Now look at me ! Welcome to Starbuzz Coffee 80 The Open-Closed Principle 86 Meet the Decorator Pattern 88 Constructing a Drink Order with Decorators 89 The Decorator Pattern Defined 91 Decorating our Beverages 92 Writing the Starbuzz code 95 Real World Decorators: Java I/O 100 Writing your own Java I/O Decorator 102 Tools for your Design Toolbox 105 Exercise Solutions 106 Download at WoweBook.Com xiv 4 Baking with OO GoodnessGet ready to cook some loosely coupled OO designs. There is more to making objects than just using the new operator. You’ll learn that instantiation is an activity that shouldn’t always be done in public and can often lead to coupling problems. And you don’t want that, do you? Find out how Factory Patterns can help save you from embarrasing dependencies. the Factory Pattern <<interface>> Clams <<interface>> Cheese <<interface>> Sauce <<interface>> Dough createPizza() NYPizzaStore ThinCrustDough MarinaraSauce ReggianoCheese FrozenClams <<interface>> Sauce <<interface>> Sauce ThinCrustDoughThickCrustDough <<interface>> Cheese <<interface>> Cheese MarinaraSaucePlumTomatoSauce <<interface>> Clams <<interface>> Clams ReggianoCheeseMozzarella Cheese FreshClams Each factory produces a different implementation for the family of products. The abstract PizzaIngredientFactory is the interface that defines how to make a family of related products - everything we need to make a pizza. The clients of the Abstract Factory are the two instances of our PizzaStore, NYPizzaStore and ChicagoStylePizzaSore. The job of the concrete pizza factories is to make pizza ingredients. Each factory knows how to create the right objects for their region. createDough() createSauce() createCheese() createVeggies() createPepperoni() createClam() <<interface>> PizzaIngredientFactory createDough() createSauce() createCheese() createVeggies() createPepperoni() createClam() NYPizzaIngredientFactory createDough() createSauce() createCheese() createVeggies() createPepperoni() createClam() ChicagoPizzaIngredientFactory table of contents When you see “new”, think “concrete” 110 Objectville Pizza 112 Encapsulating object creation 114 Building a simple pizza factory 115 The Simple Factory defined 117 A Framework for the pizza store 120 Allowing the subclasses to decide 121 Let’s make a PizzaStore 123 Declaring a factory method 125 Meet the Factory Method Pattern 131 Parallel class hierarchies 132 Factory Method Pattern def ined 134 A very dependent PizzaStore 137 Looking at object dependencies 138 The Dependency Inversion Principle 139 Meanwhile, back at the PizzaStore... 144 Families of ingredients... 145 Building our ingredient factories 146 Looking at the Abstract Factory 153 Behind the scenes 154 Abstract Factory Pattern defi ned 156 Factory Method and Abstract Factory compared 160 Tools for your Design Toolbox 162 Exercise Solutions 164 Download at WoweBook.Com xv 5 One of a Kind ObjectsThe Singleton Pattern: your ticket to creating one-of-a-kind objects, for which there is only one instance. You might be happy to know that of all patterns, the Singleton is the simplest in terms of its class diagram; in fact the diagram holds just a single class! But don’t get too comfortable; despite its simplicity from a class design perspective, we’ll encounter quite a few bumps and potholes in its implementation. So buckle up—this one’s not as simple as it seems... the Singleton Pattern Strategy - d efines a fam ily of algorit hms, encapsulates each one, and makes them inter- changeable. Strategy let s the algorit hm vary independentl y from client s that use it . OO Patterns Strategy encapsulates each one, and makes them inter- changeable. Strategy let s the algorit hm vary independentl y from client s that use it . OO Patterns Observer - de fines a one- to-many dependency b etween objec ts so that when one obj ect changes state, all its dependents a re notified a nd updated automatically changeable. Strategy let s the algorit hm vary independentl y from client s that use it . OO Patterns Observer dependency b etween objec ts so that when one obj ect changes state, all its dependents a re notified a nd updated when one obj ect changes state, all its dependents a re notified a nd updated dependents a re notified a nd updated when one obj ect changes state, all its automatically Decorator - Attach addi tional responsibilitie s to an objec t dynami- cally. Decor ators provid e a flexible alternative t o subclassing for extendin g functionality . OO Patterns Observer de fines a one- to-many dependents a re notified a nd updated automatically Decorator responsibilitie s to an objec t dynami- cally. Decor ators provid e a flexible responsibilitie s to an objec t dynami- cally. Decor ators provid e a flexible cally. Decor ators provid e a flexible responsibilitie s to an objec t dynami- alternative t o subclassing for extendin g cally. Decor ators provid e a flexible alternative t o subclassing for extendin g alternative t o subclassing for extendin g cally. Decor ators provid e a flexible functionality . Abstract Fa ctory - Provid e an interface fo r creating f amilies of related or d epedent obje cts without specifying th eir concrete classes. OO Patterns Observer de fines a one- to-many Decorator functionality . Abstract Fa ctory interface fo r creating f amilies of Abstract Fa ctory interface fo r creating f amilies of interface fo r creating f amilies of Abstract Fa ctory related or d epedent obje cts without specifying th eir concrete classes.related or d epedent obje cts without specifying th eir concrete classes. specifying th eir concrete classes.related or d epedent obje cts without Facto ry Method - Define an interface fo r creating an object, but let subclasses decide whic h class to in- stantiate. F actory Meth od lets a cla ss defer instan tiation to th e subclasses. related or d epedent obje cts without specifying th eir concrete classes.related or d epedent obje cts without specifying th eir concrete classes. specifying th eir concrete classes.related or d epedent obje cts without Observer de fines a one- to-many DecoratorAbstract Fa ctory Factory Met hod - Define an interface fo r creating an object, but Factory Met hod interface fo r creating an object, but interface fo r creating an object, but Factory Met hod let subclasses decide whic h class to in- stantiate. F actory Meth od lets a cla ss defer instan tiation to th e subclasses.stantiate. F actory Meth od lets a cla ss defer instan tiation to th e subclasses. defer instan tiation to th e subclasses.stantiate. F actory Meth od lets a cla ss Singleton - Ensure a cla ss only has one instance and provide a global poin t of access to it. One and only one object 170 The Little Singleton 171 Dissecting the classic Singleton Pattern 173 Confessions of a Singleton 174 The Chocolate Factory 175 Singleton Pattern defined 177 Houston, we have a problem... 178 BE the JVM 179 Dealing with multithreading 180 Singleton Q&A 184 Tools for your Design Toolbox 186 Exercise Solutions 188 Hershey, PA Download at WoweBook.Com xvi 6 Encapsulating InvocationIn this chapter we take encapsulation to a whole new level: we’re going to encapsulate method invocation. That’s right, by encapsulating invocation we can crystallize pieces of computation so that the object invoking the computation doesn’t need to worry about how to do things; it just uses our crystallized method to get it done. We can also do some wickedly smart things with these encapsulated method invocations, like save them away for logging or reuse them to implement undo in our code. the Command Pattern I’ll have a Burger with Cheese and a Malt Shake Burger with C heese Malt Shake createOrder() takeOrder() Burger with C heese Malt Shake or de rU p( ) makeBurger(), makeShake() ou tp ut The Order consis ts of an order slip and the cust omer’s menu items that are w ritten on it. The customer knows what he wants and creates an order. The Waitress takes the Order, and when she gets around to it, she calls its orderUp() method to begin the Order’s preparation. The Or der has all the …