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On Java 8 Bruce Eckel MindView LLC 2017 ©MindView LLC All Rights Reserved On Java 8 Copyright ©2017 by Bruce Eckel, President, MindView LLC. Version: 7 ISBN 978-0-9818725-2-0 This book is available for purchase at www.OnJava8.com, where you’ll also find supporting materials. All rights reserved. Pro...

On Java 8 Bruce Eckel MindView LLC 2017 ©MindView LLC All Rights Reserved On Java 8 Copyright ©2017 by Bruce Eckel, President, MindView LLC. Version: 7 ISBN 978-0-9818725-2-0 This book is available for purchase at www.OnJava8.com, where you’ll also find supporting materials. All rights reserved. Produced in the United States of America. This publication is protected by copyright, and permission must be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed with initial capital letters or in all capitals. Java is a trademark of Oracle, Inc. Windows 95, Windows NT, Windows 2000, Windows XP, Windows 7, Windows 8 and Windows 10 are trademarks of Microsoft Corporation. All other product names and company names mentioned herein are the property of their respective owners. The author and publisher have taken care in the preparation of this book, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information or programs contained herein. This book was created as an eBook for tablets and computers. That is, it was not first created for print and then converted. It is an eBook first —all layout and formatting is designed to optimize your viewing experience on the various eBook reading platforms and systems. Cover design by Daniel Will-Harris, www.Will-Harris.com Preface This book teaches the most modern form of Java programming using the features in the 8th version of that language. My previous Java book, Thinking in Java, 4th Edition (Prentice Hall 2006), is still useful for programming in Java 5, the version of the language used for Android programming. But especially with the advent of Java 8, the language has changed significantly enough that new Java code feels and reads differently. This justified the two-year effort of creating a new book. On Java 8 is designed for someone with a basic foundation in programming. For beginners, web sites like Code.org and Khan Academy can provide at least some of that background, along with the Thinking in C seminar freely available at the OnJava8 Site. Services like YouTube, blogs and StackOverflow have made finding answers ridiculously easy compared to just a few years ago when we relied on print media. Combine these with perseverance, and you can use this book as your first programming text. It’s also intended for professional programmers who want to expand their knowledge. I am grateful for all the benefits from Thinking in Java, mostly in the form of speaking engagements all over the world. It has proved invaluable in creating connections with people and companies for my Reinventing Business project. One of the reasons I finally wrote this book is to support my Reinventing Business research, and it seems the next logical step is to actually create a so-called Teal Organization. I hope this book can become a kind of crowdfunding for that project. Goals Each chapter teaches a concept, or a group of associated concepts, without relying on features that haven’t yet been introduced. That way you can digest each piece in the context of your current knowledge before moving on. My goals in this book are to: 1. Present the material one step at a time so you can easily incorporate each idea before moving on, and to carefully sequence the presentation of features so you’re exposed to a topic before you see it in use. This isn’t always possible; in those situations, a brief introductory description is given. 2. Use examples that are as simple and short as possible. This sometimes prevents me from tackling “real world” problems, but I’ve found that beginners are usually happier when they can understand every detail of an example rather than being impressed by the scope of the problem it solves. For this I might receive criticism for using “toy examples,” but I’m willing to accept that in favor of producing something pedagogically useful. 3. Give you what I think is important for you to understand about the language, rather than everything I know. I believe there is an information importance hierarchy, and there are some facts that 95 percent of programmers will never need to know—details that just confuse people and increase their perception of the complexity of the language. If you must think about it, it will also confuse the reader/maintainer of that code, so I advocate choosing a simpler approach. 4. Provide you with a solid foundation so you understand the issues well enough to move on to more difficult coursework and books. Language Design Errors Every language has design errors. New programmers experience deep uncertainty and frustration when they must wade through features and guess at what they should use and what they shouldn’t. It’s embarrassing to admit mistakes, but this bad beginner experience is a lot worse than the discomfort of acknowledging you were wrong about something. Alas, every failed language/library design experiment is forever embedded in the Java distribution. The Nobel laureate economist Joseph Stiglitz has a philosophy of life that applies here, called The Theory of Escalating Commitment: “The cost of continuing mistakes is borne by others, while the cost of admitting mistakes is borne by yourself.” If you’ve read my past writings, you’ll know that when I find design errors in a language, I tend to point them out. Java has developed a particularly avid following, folks who treat the language more like a country of origin and less like a programming tool. Because I’ve written about Java, they assume I am a fellow patriot. When I criticize the errors I find, it tends to have two effects: 1. Initially, a lot of “my-country-right-or-wrong” furor, which typically dies down to isolated pockets. Eventually—this can take years—the error is acknowledged and seen as just part of the history of Java. 2. More importantly, new programmers don’t go through the struggle of wondering why “they” did it this way, especially the self-doubt that comes from finding something that just doesn’t seem right and naturally assuming I must be doing it wrong or I just don’t get it. Worse, those who teach the language often go right along with the misconceptions rather than delving in and analyzing the issue. By understanding the language design errors, new programmers can understand that something was a mistake, and move ahead. Understanding language and library design errors is essential because of the impact they have on programmer productivity. Some companies and teams choose to avoid certain features because, while seductive on the surface, those features can block your progress when you least expect it. Design errors also inform the creation and adoption of new languages. It’s fun to explore what can be done with a language, but design errors tell you what can’t be done with that language. For many years, I honestly felt a lack of care from the Java designers regarding their users. Some of these errors seemed so blatant, so poorly thought-out, that it appeared the designers had some other motivation in mind instead of serving their users. There was a lot of notoriety around the Java language for a long time, and perhaps that’s where the seduction was. This seeming lack of respect for programmers is the major reason I moved away from Java and didn’t want anything to do with it for such a long time. When I did start looking into Java again, something about Java 8 felt very different, as if a fundamental shift had occurred in the designers’ attitude about the language and its users. Many features and libraries that had been warts on the language were fixed after years of ignoring user complaints. New features felt very different, as if there were new folks on board who were extremely interested in programmer experience. These features were—finally—working to make the language better rather than just quickly adding ideas without delving into their implications. And some of the new features are downright elegant (or at least, as elegant as possible given Java constraints). I can only guess that some person or people have departed the language group and this has changed the perspective. Because of this new focus by the language developers—and I don’t think I’m imagining it—writing this book has been dramatically better than past experiences. Java 8 contains fundamental and important improvements. Alas, because of Java’s rigid backwards-compatibility promise, these improvements required great effort so it’s unlikely we’ll see anything this dramatic again (I hope I’m wrong about this). Nonetheless, I applaud those who have turned the ship as much as they have and set the language on a better course. For the first time I can ever recall, I found myself saying “I love that!” about some of the Java code I’ve been able to write in Java 8. Ultimately, the timing for this book seems good, because Java 8 introduces important features that strongly affect the way code is written, while—so far—Java 9 seems to focus on the understory of the language, bringing important infrastructure features but not those that affect the kind of coding focused on in this book. However, because it’s an eBook, if I discover something I think requires an update or an addition, I can push the new version to existing customers. Tested Examples The code examples in this book compile with Java 8 and the Gradle build tool. All the examples are in a freely-accessible Github repository. Without a built-in test framework with tests that run every time you do a build of your system, you have no way of knowing whether your code is reliable. To accomplish this in the book, I created a test system to display and validate the output of most examples. The output from running an example is attached, as a block comment, at the end of examples that produce output. In some cases only the first few lines are shown, or first and last lines. Embedded output improves the reading and learning experience, and provides yet another way to verify the correctness of the examples. Popularity Java’s popularity has significant implications. If you learn it, getting a job will probably be easier. There are a lot more training materials, courses, and other learning resources available. If you’re starting a company and you choose to work in Java, it’s much easier to find programmers, and that’s a compelling argument. Short-term thinking is almost always a bad idea. Don’t use Java if you really don’t like it—using it just to get a job is an unhappy life choice. As a company, think hard before choosing Java just because you can hire people. There might be another language that makes fewer employees far more productive for your particular need. But if you do enjoy it, if Java does call to you, then welcome. I hope this book will enrich your programming experience. Android Programmers I’ve made this book as “Java 8 as possible,” so if you want to program for Android devices, you must study Java 5, which I cover in Thinking in Java, 4th edition. At the time of publishing of On Java 8, Thinking in Java, 4th Edition has become a free download, available through www.OnJava8.com. Thinking in Java, 4th Edition is available in print from Prentice-Hall. In addition, there are many other resources that specialize in Android programming. This is Only an eBook On Java 8 is only available as an eBook, and only via www.OnJava8.com. Any other source or delivery mechanism is illegitimate. There is no print version. This is copyrighted work. Do not post or share it in any way without permission via [email protected]. You may use the examples for teaching, as long as they are not republished without permission and attribution. See the Copyright.txt file in the example distribution for full details. This book is far too large to publish as a single print volume, and my intent has always been to only publish it as an eBook. Color syntax highlighting for code listings is, alone, worth the cost of admission. Searchability, font resizing or text-to-voice for the vision-impaired, the fact you can always keep it with you—there are so many benefits to eBooks it’s hard to name them all. Anyone buying this book needs a computer to run the programs and write code, and the eBook reads nicely on a computer (I was also surprised to discover that it even reads tolerably well on a phone). However, the best reading experience is on a tablet computer. Tablets are inexpensive enough that you can now buy one for less than you’d pay for an equivalent print version of this book. It’s much easier to read a tablet in bed (for example) than trying to manage the pages of a physical book, especially one this big. When working at your computer, you don’t have to hold the pages open when using a tablet at your side. It might feel different at first, but I think you’ll find the benefits far outweigh the discomfort of adapting. I’ve done the research, and Google Play Books works on, and provides a very nice reading experience, every platform, including Linux and iOS devices. As an experiment, I’ve decided to try publishing exclusively through Google Books. Note: At the time of this writing, reading the book through the Google Play Books web browser app was—although tolerable—the least satisfying viewing experience. I strongly advocate using a tablet computer instead. Colophon This book was written with Pandoc-flavored Markdown, and produced into ePub version 3 format using Pandoc. The body font is Georgia and the headline font is Verdana. The code font is Ubuntu Mono, because it is especially compact and allows more characters on a line without wrapping. I chose to place the code inline (rather than make listings into images, as I’ve seen some books do) because it was important to me that the reader be able to resize the font of the code listings when they resize the body font (otherwise, really, what’s the point?). The build process for the book was automated, as well as the process to extract, compile and test the code examples. All automation was achieved through fairly extensive programs I wrote in Python 3. Cover Design The cover of On Java 8 is from a mosaic created through the Works Progress Administration (WPA, a huge project during the US Great Depression from 1935-1943 which put millions of out-of-work-people back to work). It also reminds me of the illustrations from The Wizard of Oz series of books. My friend and designer, Daniel Will-Harris (www.will-harris.com) and I just liked the image. Thanks Thanks to Eric Evans (author of Domain-Driven Design) for suggesting the book title, and to everyone else in the conference newsgroups for their help in finding the title. Thanks to James Ward for starting me with the Gradle build tool for this book, and for his help and friendship over the years. Thanks to Ben Muschko for his work polishing the build files, and Hans Dockter for giving Ben the time. Jeremy Cerise and Bill Frasure came to the developer retreat for the book and followed up with valuable help. Thanks to all who have taken the time and effort to come to my conferences, workshops, developer retreats, and other events in my town of Crested Butte, Colorado. Your contributions might not be easily seen, but they are deeply important. Dedication For my beloved father, E. Wayne Eckel. April 1, 1924—November 23, 2016. Introduction “The limits of my language are the limits of my world”—Wittgenstein This is true of both spoken/written languages and programming languages. It’s often subtle: A language gently guides you into certain modes of thought and away from others. Java is particularly opinionated. Java is a derived language. The original language designers didn’t want to use C++ for a project, so created a new language which unsurprisingly looked a lot like C++, but with improvements (their original project never came to fruition). The core changes were the incorporation of a virtual machine and garbage collection, both of which are described in detail in this book. Java is also responsible for pushing the industry forward in other ways; for example, most languages are now expected to include documentation markup syntax and a tool to produce HTML documentation. One of the most predominant Java concepts came from the SmallTalk language, which insists that the “object” (described in the next chapter) is the fundamental unit of programming, so everything must be an object. Time has tested this belief and found it overenthusiastic. Some folks even declare that objects are a complete failure and should be discarded. Personally, I find that making everything an object is not only an unnecessary burden but also pushes many designs in a poor direction. However, there are still situations where objects shine. Requiring that everything be an object (especially all the way down to the lowest level) is a design mistake, but banning objects altogether seems equally draconian. Other Java language decisions haven’t panned out as promised. Throughout this book I attempt to explain these so you not only understand those features, but also why they might not feel quite right to you. It’s not about declaring that Java is a good language or a bad one. If you understand the flaws and limitations of the language you will: 1. Not get stymied when you encounter a feature that seems “off.” 2. Design and code better by knowing where the boundaries are. Programming is about managing complexity: the complexity of the problem, laid upon the complexity of the machine. Because of this complexity, most of our programming projects fail. Many language design decisions are made with complexity in mind, but at some point other issues are considered essential. Inevitably, those other issues are what cause programmers to eventually “hit the wall” with a language. For example, C++ had to be backwardcompatible with C (to allow easy migration for C programmers), as well as efficient. Those are both useful goals and account for much of the success of C++, but they also expose extra complexity that prevent some projects from finishing. Certainly, you can blame programmers and management, but if a language can help by catching your mistakes, why shouldn’t it? Visual BASIC (VB) was tied to BASIC, which wasn’t really designed as an extensible language. All the extensions piled upon VB have produced some truly un-maintainable syntax. Perl is backwardcompatible with awk, sed, grep, and other Unix tools it was meant to replace, and as a result it is often accused of producing “write-only code” (that is, you can’t read your own code). On the other hand, C++, VB, Perl, and other languages such as SmallTalk had some of their design efforts focused on the issue of complexity and as a result are remarkably successful in solving certain types of problems. The communication revolution enables all of us to communicate with each other more easily: one-on-one as well as in groups and as a planet. I’ve heard it suggested that the next revolution is the formation of a kind of global mind that results from enough people and enough interconnectedness. Java might or might not be one of the tools for that revolution, but at least the possibility has made me feel like I’m doing something meaningful by attempting to teach the language. Prerequisites This book assumes you have some programming familiarity, so you understand: A program is a collection of statements The idea of a subroutine/function/macro Control statements such as “if” and looping constructs such as “while” Etc. You might have learned this in many places, typically school, books, or the Internet. As long as you you feel comfortable with the basic ideas of programming, you can work through this book. The Thinking in C multimedia seminar freely downloadable from OnJava8.com will bring you up to speed on the fundamentals necessary to learn Java. On Java 8 does introduce the concepts of object-oriented programming (OOP) and Java’s basic control mechanisms. Although I make references to C and C++ language features, these are not intended to be insider comments, but instead to help all programmers put Java in perspective with those languages, from which, after all, Java is descended. I attempt to make these references simple and to explain anything that might be unfamiliar to a nonC/C++ programmer. JDK HTML Documentation The Java Development Kit (JDK) from Oracle (a free download) comes with documentation in electronic form, readable through your Web browser. Unless necessary, this book will not repeat that documentation, because it’s usually much faster to find the class descriptions with your browser than to look them up in a book (also, the online documentation is current). I’ll simply refer to “the JDK documentation.” I’ll provide extra descriptions of the classes only when it’s necessary to supplement that documentation so you understand a particular example. Thinking in C The Thinking in C multimedia seminar is freely downloadable from www.OnJava8.com. This gives an introduction to the C syntax, operators, and functions that are the foundation of Java syntax. Thinking in C also provides a gentle introduction to coding, assuming even less about the student’s programming background than does this book. I commissioned Chuck Allison to create Thinking in C as a standalone product, which was later included in book CDs, and finally reworked as a free download. By freely providing this seminar online, I can ensure that everyone begins with adequate preparation. Source Code All the source code for this book is available as copyrighted freeware, distributed via Github. To ensure you have the most current version, this is the official code distribution site. You may use this code in classroom and other educational situations. The primary goal of the copyright is to ensure that the source of the code is properly cited, and to prevent you from republishing the code without permission. (As long as this book is cited, using examples from the book in most media is generally not a problem.) In each source-code file you find a reference to the following copyright notice: // Copyright.txt This computer source code is Copyright ©2017 MindView LLC. All Rights Reserved. Permission to use, copy, modify, and distribute this computer source code (Source Code) and its documentation without fee and without a written agreement for the purposes set forth below is hereby granted, provided that the above copyright notice, this paragraph and the following five numbered paragraphs appear in all copies. 1. Permission is granted to compile the Source Code and to include the compiled code, in executable format only, in personal and commercial software programs. 2. Permission is granted to use the Source Code without modification in classroom situations, including in presentation materials, provided that the book "On Java 8" is cited as the origin. 3. Permission to incorporate the Source Code into printed media may be obtained by contacting: MindView LLC, PO Box 969, Crested Butte, CO 81224 [email protected] 4. The Source Code and documentation are copyrighted by MindView LLC. The Source code is provided without express or implied warranty of any kind, including any implied warranty of merchantability, fitness for a particular purpose or non-infringement. MindView LLC does not warrant that the operation of any program that includes the Source Code will be uninterrupted or error-free. MindView LLC makes no representation about the suitability of the Source Code or of any software that includes the Source Code for any purpose. The entire risk as to the quality and performance of any program that includes the Source Code is with the user of the Source Code. The user understands that the Source Code was developed for research and instructional purposes and is advised not to rely exclusively for any reason on the Source Code or any program that includes the Source Code. Should the Source Code or any resulting software prove defective, the user assumes the cost of all necessary servicing, repair, or correction. 5. IN NO EVENT SHALL MINDVIEW LLC, OR ITS PUBLISHER BE LIABLE TO ANY PARTY UNDER ANY LEGAL THEORY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, BUSINESS INTERRUPTION, LOSS OF BUSINESS INFORMATION, OR ANY OTHER PECUNIARY LOSS, OR FOR PERSONAL INJURIES, ARISING OUT OF THE USE OF THIS SOURCE CODE AND ITS DOCUMENTATION, OR ARISING OUT OF THE INABILITY TO USE ANY RESULTING PROGRAM, EVEN IF MINDVIEW LLC, OR ITS PUBLISHER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. MINDVIEW LLC SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOURCE CODE AND DOCUMENTATION PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, WITHOUT ANY ACCOMPANYING SERVICES FROM MINDVIEW LLC, AND MINDVIEW LLC HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS. Please note that MindView LLC maintains a Web site which is the sole distribution point for electronic copies of the Source Code, https://github.com/BruceEckel/OnJava8-examples, where it is freely available under the terms stated above. If you think you've found an error in the Source Code, please submit a correction at: https://github.com/BruceEckel/OnJava8-examples/issues You may use the code in your projects and in the classroom (including your presentation materials) as long as the copyright notice that appears in each source file is retained. Coding Standards In the text of this book, identifiers (keywords, methods, variables, and class names) are set in bold, fixed-width code font. Some keywords, such as class, are used so much that the bolding can become tedious. Those which are distinctive enough are left in normal font. I use a particular coding style for the examples in this book. As much as possible within the book’s formatting constraints, this follows the style that Oracle itself uses in virtually all code you find at its site, and seems to be supported by most Java development environments. As the subject of formatting style is good for hours of hot debate, I’ll just say I’m not trying to dictate correct style via my examples; I have my own motivation for using the style I do. Because Java is a free-form programming language, continue to use whatever style you’re comfortable with. One solution to the coding style issue is to use an IDE ( integrated development environment) tool like IntelliJ IDEA, Eclipse or NetBeans to change formatting to that which suits you. The code files in this book are tested with an automated system, and should work without compiler errors (except those specifically tagged) in the latest version of Java. This book focuses on and is tested with Java 8. If you must learn about earlier releases of the language not covered here, the 4th edition of Thinking in Java is freely downloadable at www.OnJava8.com. Bug Reports No matter how many tools a writer uses to detect errors, some always creep in and these often leap off the page for a fresh reader. If you discover anything you believe to be an error, please submit the error along with your suggested correction, for either the book’s prose or examples, here. Your help is appreciated. Mailing List For news and notifications, you can subscribe to the low-volume email list at www.OnJava8.com. I don’t use ads and strive to make the content as appropriate as possible. What About User Interfaces? Graphical user interfaces and desktop programming in Java have had a tumultuous—some would say tragic—history. The original design goal of the graphical user interface (GUI) library in Java 1.0 was to enable the programmer to build a GUI to look good on all platforms. That goal was not achieved. Instead, the Java 1.0 Abstract Windowing Toolkit (AWT) produced a GUI that looked equally mediocre on all systems. In addition, it was restrictive; you could use only four fonts and you could not access any of the more sophisticated GUI elements that exist in your operating system. The Java 1.0 AWT programming model was also awkward and non-objectoriented. A student in one of my seminars (who had been at Sun during the creation of Java) explained why: The original AWT had been conceived, designed, and implemented in a month. Certainly a marvel of productivity, and also an object lesson in why design is important. The situation improved with the Java 1.1 AWT event model, which took a much clearer, object-oriented approach, along with the addition of JavaBeans, a component programming model (now dead) oriented toward the easy creation of visual programming environments. Java 2 (Java 1.2) finished the transformation away from the old Java 1.0 AWT by essentially replacing everything with the Java Foundation Classes (JFC), the GUI portion of which is called “Swing.” These are a rich set of JavaBeans that create a reasonable GUI. The revision 3 rule of the software industry (“a product isn’t good until revision 3”) seems to hold true with programming languages as well. It seemed that Swing was the final GUI library for Java. This assumption turned out to be wrong—Sun made a final attempt, called JavaFX. When Oracle bought Sun they changed the original ambitious project (which included a scripting language) into a library, and now it appears to be the only UI toolkit getting development effort (see the Wikipedia article on JavaFX)—but even that effort has diminished. JavaFX, too, seems eventually doomed. Swing is still part of the Java distribution (but it only receives maintenance, no new development), and with Java now an opensource project it should always be available. Also, Swing and JavaFX have some limited interactivity, presumably to aid the transition to JavaFX. Ultimately, desktop Java never took hold, and never even touched the designers’ ambitions. Other pieces, such as JavaBeans, were given much fanfare (and many unfortunate authors spent a lot of effort writing books solely on Swing and even books just on JavaBeans) but never gained any traction. Most usage you’ll see for desktop Java is for integrated development environments (IDEs) and some in-house corporate applications. People do develop user interfaces in Java, but it’s safe to consider that a niche usage of the language. If you must learn Swing, it’s covered in the freely-downloadable Thinking in Java, 4th Edition (available at www.OnJava8.com), and in books dedicated to the topic. What is an Object? “We do not realize what tremendous power the structure of an habitual language has. It is not an exaggeration to say that it enslaves us through the mechanism of semantic reactions and that the structure which a language exhibits, and impresses upon us unconsciously, is automatically projected upon the world around us.”—Alfred Korzybski (1930) The genesis of the computer revolution was in a machine. Our programming languages thus tend to look like that machine. But computers are not so much machines as they are mind amplification tools (“bicycles for the mind,” as Steve Jobs was fond of saying) and a different kind of expressive medium. As a result, tools are beginning to look less like machines and more like parts of our minds. Programming languages are the fabric of thought for creating applications. Languages take inspiration from other forms of expression such as writing, painting, sculpture, animation, and filmmaking. Object-oriented programming (OOP) is one experiment in using the computer as an expressive medium. Many people feel uncomfortable wading into object-oriented programming without understanding the big picture, so the concepts introduced here give you an overview of OOP. Others might not understand such an overview until they are exposed to the mechanism, becoming lost without seeing code. If you’re part of this latter group and are eager to get to the specifics of the language, feel free to jump past this chapter—skipping it now will not prevent you from writing programs or learning the language. However, come back here eventually to fill in your knowledge so you understand why objects are important and how to design with them. This chapter assumes you have some programming experience, although not necessarily in C. If you need more preparation in programming before tackling this book, work through the Thinking in C multimedia seminar, freely downloadable from www.OnJava8.com. The Progress of Abstraction All programming languages are abstractions. It can be argued that the complexity of the problems you’re able to solve is directly related to the kind and quality of abstraction. By “kind” I mean, “What is it you are abstracting?” Assembly language is a minimal abstraction of the underlying machine. Many so-called “imperative” languages (such as FORTRAN, BASIC, and C) were themselves abstractions of assembly language. Although they were big improvements, their primary abstraction still requires you to think in terms of the structure of the computer rather than the structure of the problem you are trying to solve. The programmer must establish the association between the machine model (the “solution space,” the place where you’re implementing that solution, such as a computer) and the model of the problem that is actually solved (the “problem space,” the place where the problem exists, such as a business). The effort required to perform this mapping, and the fact it is extrinsic to the programming language, produces programs that are difficult to write and expensive to maintain. The alternative to modeling the machine is to model the problem you’re trying to solve. Early languages such as LISP and APL chose particular views of the world (“All problems are ultimately lists” or “All problems are algorithmic,” respectively). Prolog casts all problems into chains of decisions. Languages have been created for constraint-based programming and for programming exclusively by manipulating graphical symbols. Each of these approaches can be a good solution to the particular class of problem they’re designed to solve, but when you step outside of that domain they become awkward. The object-oriented approach goes a step further by providing tools for the programmer to represent elements in the problem space. This representation is general enough that the programmer is not constrained to any particular type of problem. We refer to the elements in the problem space and their representations in the solution space as “objects.” (Note that some objects don’t have problem-space analogs.) The idea is that the program adapts itself to the lingo of the problem by adding new types of objects. When you read the code describing the solution, you’re reading words that also express the problem. This is a more flexible and powerful language abstraction than what we’ve had before. Thus, OOP describes the problem in terms of the problem, rather than in terms of the computer where the solution will run. There’s still a connection, because objects look somewhat like little computers: Each has state and performs operations. This is similar to objects in the real world—they all have characteristics and behaviors. Alan Kay summarized five basic characteristics of SmallTalk, the first successful object-oriented language and a language that inspired Java. These characteristics represent a pure approach to object-oriented programming: 1. Everything is an object. Think of an object as a fancy variable; it stores data, but you can “make requests”, asking it to perform operations on itself. You can usually take any conceptual component in the problem you’re trying to solve (dogs, buildings, services, etc.) and represent it as an object in your program. 2. A program is a bunch of objects telling each other what to do by sending messages. When you “send a message” to an object, it’s a request to call a method that belongs to that object. 3. Each object has its own memory made up of other objects. Put another way, you create a new kind of object by packaging existing objects. This hides the complexity of a program behind the simplicity of objects. 4. Every object has a type. Each object is an instance of a class, where “class” is (approximately) synonymous with “type.” The most important distinguishing characteristic of a class is “What messages can you send to it?” 5. All objects of a particular type can receive the same messages. This is a loaded statement, as you will see later. Because an object of type “circle” is also an object of type “shape,” a circle is guaranteed to accept shape messages. This means you can write code that talks to shapes and automatically handles anything that fits the description of a shape. This substitutability is a foundation of OOP. Grady Booch offers an even more succinct description of an object: An object has state, behavior and identity This means an object can have internal data (which gives it state), methods (to produce behavior), and each object is uniquely distinguished from every other object—that is, every object has a unique address in memory. 1 An Object Has an Interface Aristotle was probably the first to begin a careful study of the concept of type; he spoke of “the class of fishes and the class of birds.” The idea that all objects, while unique, are also part of a class of objects that have characteristics and behaviors in common was used directly in the first object-oriented language, Simula-67, with its fundamental keyword class that introduces a new type into a program. Simula, as its name implies, was created for developing simulations such as the classic “bank teller problem.” In this, you have numerous tellers, customers, accounts, transactions, and units of money—many “objects.” Objects that are identical except for their state are grouped together into “classes of objects,” and that’s where the keyword class arose. Creating abstract data types (classes) is a fundamental concept in object-oriented programming. Abstract data types work almost exactly like built-in types: You create variables of a type (called objects or instances in object-oriented parlance) and manipulate those variables (called sending messages or requests; you send a message and the object figures out what to do with it). The members (elements) of each class share some commonality: Every account has a balance, every teller can accept a deposit, etc. At the same time, each member has its own state: Each account has a different balance, each teller has a name. Thus, the tellers, customers, accounts, transactions, etc., can each be represented with a unique entity in the program. This entity is the object, and each object belongs to a particular class that defines its characteristics and behaviors. So, although what we really do in object-oriented programming is create new data types, virtually all object-oriented programming languages use the “class” keyword. When you see the word “type” think “class” and vice versa.2 Since a class describes a set of objects that have identical characteristics (data elements) and behaviors (functionality), a class is really a data type because a floating point number, for example, also has a set of characteristics and behaviors. The difference is that a programmer defines a class to fit a problem rather than being forced to use an existing data type that was designed to represent a unit of storage in a machine. You extend the programming language by adding new data types specific to your needs. The programming system welcomes the new classes and gives them the same care and type checking it gives to built-in types. The object-oriented approach is not limited to building simulations. Whether or not you agree that any program is a simulation of the system you’re designing, OOP techniques help reduce a large set of problems to a simpler solution. Once a class is established, make as many objects of that class as you like, then manipulate those objects as if they are the elements that exist in your problem. Indeed, one of the challenges of object-oriented programming is creating a one-to-one mapping between the elements in the problem space and objects in the solution space. How do you get an object to do useful work? You make a request of that object—complete a transaction, draw something on the screen, turn on a switch. Each object accepts only certain requests, defined by its interface The type determines the interface. As a simple example, consider a representation for a light bulb: Light lt = new Light(); lt.on(); The interface determines the requests you can make for a particular object. However, there must be code somewhere to satisfy that request. This, along with the hidden data, comprises the implementation. A type has a method associated with each possible request, and when you make a particular request to an object, that method is called. This process is usually summarized by saying you “send a message” (make a request) to an object, and the object figures out what to do with that message (it executes code). Here, the name of the class is Light, the name of this particular Light object is lt, and the requests you can make of a Light object are to turn it on, turn it off, make it brighter, or make it dimmer. You create a Light object by defining a “reference” (lt) for that object and calling new to request a new object of that type. To send a message to the object, you state the name of the object and connect it to the message request with a period (dot). From the standpoint of the user of a predefined class, that’s pretty much all there is to programming with objects. The preceding diagram follows the format of the Unified Modeling Language (UML). Each class is represented by a box, with the type name in the top portion of the box, any data members you care to describe in the middle portion of the box, and the methods (the functions that belong to this object, which receive any messages you send to that object) in the bottom portion of the box. Often, only the name of the class and the public methods are shown in UML design diagrams, so the middle portion is not shown, as in this case. If you’re interested only in the class name, the bottom portion doesn’t need to be shown, either. Objects Provide Services When trying to develop or understand a program design, an excellent way to think about objects is as “service providers.” Your program itself will provide services to the user, and it will accomplish this by using the services offered by other objects. Your goal is to produce (or better, locate in existing code libraries) a set of objects providing the ideal services to solve your problem. A way to start doing this is to ask, “If I could magically pull them out of a hat, what objects would solve my problem right away?” For example, suppose you are creating a bookkeeping program. You might imagine objects that contain predefined bookkeeping input screens, other objects that perform bookkeeping calculations, and an object that handles printing of checks and invoices on all different kinds of printers. Maybe some of these objects already exist, and for the ones that don’t, what would they look like? What services would those objects provide, and what objects would they need to fulfill their obligations? If you keep doing this, you eventually reach a point where you say either, “That object seems simple enough to sit down and write” or “I’m sure that object must exist already.” This is a reasonable way to decompose a problem into a set of objects. Thinking of an object as a service provider has an additional benefit: It helps improve the cohesiveness of the object. High cohesion is a fundamental quality of software design: It means the various aspects of a software component (such as an object, although this could also apply to a method or a library of objects) “fit together” well. One problem people have when designing objects is cramming too much functionality into one object. For example, in your check printing module, you might decide you need an object that knows all about formatting and printing. You’ll probably discover this is too much for one object, and that what you need is three or more objects. One object might be a catalog of all the possible check layouts, which can be queried for information about how to print a check. One object or set of objects can be a generic printing interface that knows all about different kinds of printers (but nothing about bookkeeping—that is a candidate for buying rather than writing yourself). A third object uses the services of the other two to accomplish the task. Thus, each object has a cohesive set of services it offers. In good object-oriented design, each object does one thing well, but doesn’t try to do too much. This not only discovers objects that might be purchased (the printer interface object), but it also produces new objects that might be reused somewhere else (the catalog of check layouts). Treating objects as service providers is useful not only during the design process, but also when someone else is trying to understand your code or reuse an object. If they can see the value of the object based on what service it provides, it makes it much easier to fit it into the design. The Hidden Implementation We can break up the playing field into class creators (those who create new data types) and client programmers 3 (the class consumers who use the data types in their applications). The goal of the client programmer is to collect a toolbox full of classes to use for rapid application development. The goal of the class creator is to build a class that exposes only what’s necessary to the client programmer and keeps everything else hidden. Why? Because if it’s hidden, the client programmer can’t access it, which means the class creator can change the hidden portion at will without worrying about the impact on anyone else. The hidden portion usually represents the tender insides of an object that could easily be corrupted by a careless or uninformed client programmer, so hiding the implementation reduces program bugs. All relationships need boundaries, respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is also a programmer, but one who is putting together an application by using your library, possibly to build a bigger library. If all members of a class are available to everyone, the client programmer can do anything with that class and there’s no way to enforce rules. Even though you might prefer that the client programmer not directly manipulate some of the members of your class, without access control there’s no way to prevent it. Everything’s naked to the world. So the first reason for access control is to keep client programmers’ hands off portions they shouldn’t touch—parts necessary for the internal operation of the data type but not part of the interface that users need to solve their particular problems. This is actually a service to client programmers because they can easily see what’s important and what they can ignore. (Notice this is also a philosophical decision. Some programming languages assume that if a programmer wishes to access the internals, they should be allowed.) The second reason for access control is to enable the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. For example, you might implement a particular class in a simple fashion to ease development, then later discover you must rewrite it to make it run faster. If the interface and implementation are clearly separated and protected, you can accomplish this easily. Java has three explicit keywords to set the boundaries in a class: public, private, and protected. These access specifiers determine who can use the definitions that follow. public means the element is available to everyone. private means no one can access that element except you, the creator of the type, inside methods of that type. private is a brick wall between you and the client programmer. Anyone trying to access a private member gets a compile-time error. protected acts like private, with the exception that an inheriting class may access protected members, but not private members. Inheritance is introduced shortly. Java also has a “default” access, which comes into play if you don’t use one of the aforementioned specifiers. This is usually called package access because classes can access the members of other classes in the same package (library component), but outside the package those same members appear to be private. Reusing the Implementation Once a class is tested, it should (ideally) represent a useful unit of code. This reusability is not nearly so easy to achieve as many hope; it takes experience and insight to produce a reusable object design. But once you have such a design, it begs for reuse. Code reuse is an argument for object-oriented programming languages. The simplest way to reuse a class is to use an object of that class directly, but you can also place an object of that class inside a new class. Your new class can be made up of any number and type of other objects, in any combination, to produce the desired functionality. Because you compose a new class from existing classes, this concept is called composition (if composition is dynamic, it’s usually called aggregation). Composition is often called a has-a relationship, as in “A car has an engine.” (This diagram indicates composition with the filled diamond, which states there is one car. I typically use a simpler form: just a line, without the diamond, to indicate an association.4 Composition comes with a great deal of flexibility. The member objects of your new class are typically private, making them inaccessible to client programmers who use the class. This means changing those members doesn’t disturb existing client code. You can also change the member objects at run time, to dynamically change the behavior of your program. Inheritance, described next, does not have this flexibility since the compiler must place compile-time restrictions on classes created using inheritance. Inheritance is is often highly emphasized in object-oriented programming. A new programmer can get the impression that inheritance should be used everywhere. This can result in awkward and overly complicated designs. Instead, first look to composition when creating new classes, since it is simpler, more flexible, and produces cleaner designs. Once you’ve had some experience, it is reasonably obvious when you need inheritance. Inheritance By itself, the idea of an object is a convenient tool. Objects package data and functionality together by concept and represent an appropriate problem-space idea rather than being forced to use the idioms of the underlying machine. These concepts are expressed as fundamental units in the programming language by using the class keyword. It seems a pity, however, to go to all the trouble to create a class, then be forced to create a brand new one that might have similar functionality. It’s nicer if we can take the existing class, clone it, then make additions and modifications to the clone. This is effectively what you get with inheritance, with the exception that if the original class (called the base class or superclass or parent class) is changed, the modified “clone” (called the derived class or inherited class or subclass or child class) also reflects those changes. The arrow in this diagram points from the derived class to the base class. As you will see, there is commonly more than one derived class. A type does more than describe the constraints on a set of objects; it also relates to other types. Two types can have characteristics and behaviors in common, but one type might contain more characteristics than another and might also handle more messages (or handle them differently). Inheritance expresses this similarity through the concept of base types and derived types. A base type contains all characteristics and behaviors shared among the types derived from it. You create a base type to represent the core of your ideas. From the base type, you derive other types to express the different ways this core can be realized. For example, a trash-recycling machine sorts pieces of trash. The base type is “trash.” Each piece of trash has a weight, a value, and so on, and can be shredded, melted, or decomposed. From this, more specific types of trash are derived with additional characteristics (a bottle has a color, a steel can is magnetic) or behaviors (you can crush an aluminum can). In addition, some behaviors can be different (the value of paper depends on its type and condition). Using inheritance, you build a type hierarchy that expresses the problem you’re trying to solve in terms of its types. A second example is the common “shape” example, perhaps used in a computer-aided design system or game simulation. The base type is “shape,” and each shape has a size, a color, a position, and so on. Each shape can be drawn, erased, moved, colored, etc. From this, specific types of shapes are derived (inherited)—circle, square, triangle, and so on—each of which can have additional characteristics and behaviors. Certain shapes can be flipped, for example. Some behaviors might be different, such as when you calculate the area of a shape. The type hierarchy embodies both the similarities and differences between the shapes. Casting the solution in the same terms as the problem is useful because you don’t need intermediate models to get from a description of the problem to a description of the solution. With objects, the type hierarchy is an important aspect of the model, so you go directly from the description of the system in the real world to the description of the system in code. Indeed, sometimes people who are trained to look for complex solutions have difficulty with the simplicity of object-oriented design. Inheriting from an existing type creates a new type. This new type contains not only all the members of the existing type (although the private ones are hidden away and inaccessible), but more importantly it duplicates the interface of the base class. That is, all messages accepted by base-class objects are also accepted by derivedclass objects. We know the type of a class by the messages it accepts, so the derived class is the same type as the base class. In the previous example, “A circle is a shape.” This type equivalence via inheritance is one of the fundamental gateways in understanding the meaning of object-oriented programming. Since both base class and derived class have the same fundamental interface, there must be some implementation to go along with that interface. That is, there must be executable code when an object receives a particular message. If you inherit a class and don’t do anything else, the methods from the base-class interface come right along into the derived class. That means objects of the derived class have not only the same type, they also have the same behavior, which isn’t particularly interesting. There are two ways to differentiate your new derived class from the original base class. The first is straightforward: add brand new methods to the derived class. These new methods are not part of the base-class interface. This means the base class didn’t do as much as you wanted, so you added more methods. This simple and primitive use for inheritance is, at times, the perfect solution to your problem. However, look closely for the possibility that your base class might also need these additional methods (or that you should be using composition instead). This process of discovery and iteration of your design happens regularly in object-oriented programming. Although inheritance can sometimes imply (especially in Java, where the keyword for inheritance is extends) that you are going to add new methods to the interface, that’s not necessarily true. The second and more important way to differentiate your new class is to change the behavior of an existing base-class method. This is called overriding that method. To override a method, you create a new definition for the method in the derived class. You’re saying, “I’m using the same interface method here, but I want it to do something different for my new type.” Is-a vs. Is-Like-a Relationships There’s a certain debate that can occur about inheritance: Should inheritance override only base-class methods (and not add new methods that aren’t in the base class)? This would mean that the derived class is exactly the same type as the base class since it has exactly the same interface. As a result, you can perfectly substitute an object of the derived class for an object of the base class. This can be thought of as pure substitution, and it’s often called the substitution principle 5. In a sense, this is the ideal way to treat inheritance. We often refer to the relationship between the base class and derived classes in this case as an is-a relationship, because you can say, “A circle is a shape.” A test for inheritance is to see if the is-a relationship makes sense for your classes. Sometimes you add new interface elements to a derived type, thus extending the interface. The new type can still substitute for the base type, but the substitution isn’t perfect because your new methods are not accessible from the base type. This can be described as an is-like-a relationship (my term). The new type has the interface of the old type but it also contains other methods, so you can’t really say it’s exactly the same. For example, consider an air conditioner. Suppose your house is wired with all the controls for cooling; that is, it has an interface that to control cooling. Imagine that the air conditioner breaks down and you replace it with a heat pump, which can both heat and cool. The heat pump is-like-an air conditioner, but it can do more. Because the control system of your house is designed only to control cooling, it is restricted to communication with the cooling part of the new object. The interface of the new object is extended, and the existing system only knows about the original interface. Once you see this design it becomes clear that the base class “cooling system” is not general enough, and should be renamed to “temperature control system” so it can also include heating—at which point the substitution principle will work. However, this diagram shows what can happen with design in the real world. When you see the substitution principle it’s easy to feel like this approach (pure substitution) is the only way to do things, and in fact it is nice if your design works out that way. But you’ll find there are times when it’s equally clear you must add new methods to the interface of a derived class (extension). With inspection both cases should be reasonably obvious. Interchangeable Objects with Polymorphism When dealing with type hierarchies, you often treat an object not as the specific type it is, but as its base type. This way you can write code that doesn’t depend on specific types. In the shape example, methods manipulate generic shapes, unconcerned about whether they’re circles, squares, triangles, or some shape that hasn’t even been defined yet. All shapes can be drawn, erased, and moved, so these methods send a message to a shape object without worrying how the object copes with the message. Such code is unaffected by the addition of new types, and adding new types is a common way to extend an object-oriented program to handle new situations. For example, you can derive a new subtype of shape called “pentagon” without modifying methods that deal only with generic shapes. This ability to easily extend a design by deriving new subtypes is one of the essential ways to encapsulate change. This improves designs while reducing the cost of software maintenance. There’s a problem when attempting to treat derived-type objects as their generic base types (circles as shapes, bicycles as vehicles, cormorants as birds, etc.). If a method tells a generic shape to draw itself, or a generic vehicle to steer, or a generic bird to move, the compiler cannot know at compile time precisely what piece of code is executed. That’s the whole point—when the message is sent, the programmer doesn’t want to know what piece of code is executed; the draw method can be applied equally to a circle, a square, or a triangle, and the object will execute the proper code depending on its specific type. If you don’t need to know what piece of code is executed, when you add a new subtype, the code it executes can be different without requiring changes to the code that calls it. But what does the compiler do when it cannot know precisely what piece of code is executed? For example, in the following diagram the BirdController object just works with generic Bird objects and does not know what exact type they are. This is convenient from BirdControllers perspective because it doesn’t require special code to determine the exact type of Bird it’s working with or that Birds behavior. So how does it happen that, when move() is called while ignoring the specific type of Bird, the right behavior will occur (a Goose walks, flies, or swims, and a Penguin walks or swims)? The answer is the primary twist of inheritance: The compiler cannot make a function call in the traditional sense. The function call generated by a non-OOP compiler produces what is called early binding, a term you might not have heard because you’ve never thought about it any other way. It means the compiler generates a call to a specific function name, which resolves to the absolute address of the code to be executed. With inheritance, the program cannot determine the address of the code until run time, so some other scheme is necessary when a message is sent to an object. To solve the problem, object-oriented languages use the concept of late binding. When you send a message to an object, the code called isn’t determined until run time. The compiler does ensure that the method exists and performs type checking on the arguments and return value, but it doesn’t know the exact code to execute. To perform late binding, Java uses a special bit of code in lieu of the absolute call. This code calculates the address of the method body, using information stored in the object (this process is covered in great detail in the Polymorphism chapter). Thus, each object behaves differently according to the contents of that special bit of code. When you send a message to an object, the object actually does figure out what to do with that message. In some languages you must explicitly grant a method the flexibility of late-binding properties. For example, C++ uses the virtual keyword. In such languages, methods are not dynamically bound by default. In Java, dynamic binding is the default behavior and you don’t need extra keywords to produce polymorphism. Consider the shape example. The family of classes (all based on the same uniform interface) was diagrammed earlier in this chapter. To demonstrate polymorphism, we write a single piece of code that ignores specific details of type and talks only to the base class. That code is decoupled from type-specific information and thus is simpler to write and easier to understand. And, if a new type—a Hexagon, for example—is added through inheritance, code works just as well for the new type of Shape as it did on the existing types. Thus, the program is extensible. If you write a method in Java (you will soon learn how): void doSomething(Shape shape) { shape.erase(); //... shape.draw(); } This method speaks to any Shape, so it is independent of the specific type of object it’s drawing and erasing. If some other part of the program uses the doSomething() method: Circle circle = new Circle(); Triangle triangle = new Triangle(); Line line = new Line(); doSomething(circle); doSomething(triangle); doSomething(line); The calls to doSomething() automatically work correctly, regardless of the exact type of the object. This is a rather amazing trick. Consider the line: doSomething(circle); What’s happening here is that a Circle is passed into a method that expects a Shape. Since a Circle is a Shape it is treated as such by doSomething(). That is, any message that doSomething() can send to a Shape, a Circle can accept. It is a completely safe and logical thing to do. We call this process of treating a derived type as though it were its base type upcasting. The name cast is used in the sense of casting into a mold and the up comes from the way the inheritance diagram is typically arranged, with the base type at the top and the derived classes fanning out downward. Thus, casting to a base type is moving up the inheritance diagram: “upcasting.” An object-oriented program contains upcasting somewhere, because that’s how you decouple yourself from knowing the exact type you’re working with. Look at the code in doSomething(): shape.erase(); //... shape.draw(); Notice it doesn’t say, “If you’re a Circle, do this, if you’re a Square, do that, etc.” If you write that kind of code, which checks for all the possible types a Shape can actually be, it’s messy and you must change it every time you add a new kind of Shape. Here, you just say, “You’re a shape, I know you can erase() and draw() yourself, do it, and take care of the details correctly.” What’s impressive about the code in doSomething() is that, somehow, the right thing happens. Calling draw() for Circle causes different code to be executed than calling draw() for a Square or a Line, but when the draw() message is sent to an anonymous Shape, the correct behavior occurs based on the actual type of the Shape. This is amazing because when the Java compiler is compiling the code for doSomething(), it cannot know exactly what types it is dealing with. Ordinarily, you’d expect it to end up calling the version of erase() and draw() for the base class Shape, and not for the specific Circle, Square, or Line. And yet the right thing happens—that’s polymorphism. The compiler and runtime system handle the details; all you must know is it happens, and more importantly, how to design with it. When you send a message to an object, the object will do the right thing, even when upcasting is involved. The Singly-Rooted Hierarchy An OOP issue that has become especially prominent since the introduction of C++ is whether all classes should by default be inherited from a single base class. In Java (as with virtually all other OOP languages except for C++) the answer is yes, and the name of this ultimate base class is simply Object. There are many benefits to a singly-rooted hierarchy. All objects have a common interface, so they are all ultimately the same fundamental type. The alternative (provided by C++) is that you don’t know that everything is the same basic type. From a backward-compatibility standpoint this fits the model of C better and can be thought of as less restrictive, but for full-on object-oriented programming you must build your own hierarchy to provide the same convenience that’s built into other OOP languages. And in any new class library you acquire, some other incompatible interface is used. It requires effort to work the new interface into your design. Is the extra “flexibility” of C++ worth it? If you need it—if you have a large investment in C—it’s quite valuable. If you’re starting from scratch, alternatives such as Java can be more productive. A singly rooted hierarchy makes it much easier to implement a garbage collector, one of the fundamental improvements of Java over C++. And since information about the type of an object is guaranteed to be in all objects, you’ll never end up with an object whose type you cannot determine. This is especially important with system-level operations, such as exception handling (a language mechanism for reporting errors), and to allow greater flexibility in programming. Collections In general, you don’t know how many objects you need to solve a particular problem, or how long they will last. You also don’t know how to store those objects. How can you know how much space to create if that information isn’t known until run time? The solution to most problems in object-oriented design seems flippant: You create another type of object. The new type of object that solves this particular problem holds references to other objects. You can also do the same thing with an array, available in most languages. But this new object, generally called a collection (also called a container, but the Java libraries use “collection” almost universally), will expand itself whenever necessary to accommodate everything you place inside it. You don’t need to know how many objects you’re going to hold in a collection—just create a collection object and let it take care of the details. Fortunately, a good OOP language comes with a set of collections as part of the package. In C++, it’s part of the Standard C++ Library and is often called the Standard Template Library (STL). SmallTalk has a very complete set of collections. Java also has numerous collections in its standard library. In some libraries, one or two generic collections is considered good enough for all needs, and in others (Java, for example) the library has different types of collections for different needs: several different kinds of List classes (to hold sequences), Maps (also known as associative arrays, to associate objects with other objects), Sets (to hold one of each type of object), and more components such as queues, trees, stacks, etc. From a design standpoint, all you really want is a collection you can manipulate to solve your problem. If a single type of collection satisfied all of your needs, we wouldn’t need different kinds. There are two reasons you need a choice of collections: 1. Collections provide different types of interfaces and external behavior. Stacks and queues are different from sets and lists. One of these might provide a more flexible solution to your problem than another. 2. Different implementations have different efficiencies for certain operations. For example, there are two basic types of List: ArrayList and LinkedList. Both are simple sequences that can have identical interfaces and external behaviors. But some operations have significantly different costs. Randomly accessing elements in an ArrayList is a constant-time operation; it takes the same amount of time regardless of the element you select. However, in a LinkedList it is expensive to move through the list to randomly select an element, and it takes longer to find an element that is farther down the list. On the other hand, to insert an element in the middle of a sequence, it’s cheaper in a LinkedList than in an ArrayList. These and other operations have different efficiencies depending on the underlying structure of the sequence. You might start building your program with a LinkedList and, when tuning for performance, change to an ArrayList. Because of the abstraction via the interface List, you can change from one to the other with minimal impact on your code. Parameterized Types (Generics) Before Java 5, collections held the one universal type in Java: Object. The singly rooted hierarchy means everything is an Object, so a collection that holds Objects can hold anything. 6 This made collections easy to reuse. To use such a collection, you add object references to it and later ask for them back. But, since the collection holds only Objects, when you add an object reference into the collection it is upcast to Object, thus losing its character. When fetching it back, you get an Object reference, and not a reference to the type you put in. How do you turn it back into something with the specific type of the object you put into the collection? Here, the cast is used again, but this time you’re not casting up the inheritance hierarchy to a more general type. Instead, you cast down the hierarchy to a more specific type, so this manner of casting is called downcasting. With upcasting, you know that a Circle is a type of Shape so it’s safe to upcast, but you don’t know that an Object is necessarily a Circle or a Shape so it’s not safe to downcast unless you determine extra type information about that object. It’s not completely dangerous because if you downcast to the wrong type you’ll get a runtime error called an exception, described shortly. When you fetch Object references from a collection, however, you need some way to remember exactly what they are in order to perform a proper downcast. Downcasting and the associated runtime checks require extra time for the running program and extra effort from the programmer. Wouldn’t it make sense to somehow create the collection so it knows the types it holds, eliminating the need for the downcast and a possible mistake? The solution is called a parameterized type mechanism. A parameterized type is a class that the compiler can automatically customize to work with particular types. For example, with a parameterized collection, the compiler can customize that collection so it accepts only Shapes and fetches only Shapes. Java 5 added parameterized types, called generics, which is a major feature. You’ll recognize generics by the angle brackets with types inside; for example, you can create an ArrayList to hold Shape like this: ArrayList shapes = new ArrayList(); There have also been changes to many of the standard library components to take advantage of generics. You will see that generics have an impact on much of the code in this book. Object Creation & Lifetime One critical issue when working with objects is the way they are created and destroyed. Each object requires resources, most notably memory, to exist. When an object is no longer needed it must be cleaned up so these resources are released for reuse. In simple programming situations the question of how an object is cleaned up doesn’t seem too challenging: You create the object, use it for as long as it’s needed, then it should be destroyed. However, it’s not hard to encounter situations that are more complex. Suppose, for example, you are designing a system to manage air traffic for an airport. (The same model might also work for managing crates in a warehouse, or a video rental system, or a kennel for boarding pets.) At first it seems simple: Make a collection to hold airplanes, then create a new airplane and place it in the collection for each airplane that enters the air-traffic-control zone. For cleanup, simply clean up the appropriate airplane object when a plane leaves the zone. But suppose you have some other system to record data about the planes; perhaps data that doesn’t require such immediate attention as the main controller function. Maybe it’s a record of the flight plans of all the small planes that leave the airport. So you have a second collection of small planes, and whenever you create a plane object you also put it in this second collection if it’s a small plane. Then some background process performs operations on the objects in this collection during idle moments. Now the problem is more difficult: How can you possibly know when to destroy the objects? When you’re done with the object, some other part of the system might not be. This same problem can arise in a number of other situations, and in programming systems (such as C++) where you must explicitly delete an object this can become quite complex. Where is the data for an object and how is the lifetime of the object controlled? C++ takes the approach that efficiency is the most important issue, so it gives the programmer a choice. For maximum runtime speed, the storage and lifetime can be determined while the program is written, by placing the objects on the stack (these are sometimes called automatic or scoped variables) or in the static storage area. This places a priority on the speed of storage allocation and release, and this control can be very valuable in certain situations. However, you sacrifice flexibility because you must know the exact quantity, lifetime, and type of objects while you’re writing the program. If you are trying to solve a more general problem such as computer-aided design, warehouse management, or air-traffic control, this is too restrictive. The second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you don’t know until run time how many objects you need, what their lifetime is, or what their exact type is. Those are determined at the spur of the moment while the program is running. If you need a new object, you simply make it on the heap when you need it. Because the storage is managed dynamically, at run time, the amount of time required to allocate storage on the heap can be longer than the time to create storage on the stack (but not necessarily). Creating storage on the stack is often a single assembly instruction to move the stack pointer down and another to move it back up. The time to create heap storage depends on the design of the storage mechanism. The dynamic approach makes the generally logical assumption that objects tend to be complicated, so the extra overhead of finding storage and releasing that storage will not have an important impact on the creation of an object. In addition, the greater flexibility is essential to solve general programming problems. Java uses dynamic memory allocation, exclusively.7 Every time you create an object, you use the new operator to build a dynamic instance of that object. There’s another issue, however, and that’s the lifetime of an object. With languages that allow objects to be created on the stack, the compiler determines how long the object lasts and automatically destroys it. However, if you create it on the heap the compiler has no knowledge of its lifetime. In a language like C++, you must determine programmatically when to destroy the object, which can lead to memory leaks if you don’t do it correctly. Java is built upon a garbage collector which automatically discovers when an object is no longer in use and releases it. A garbage collector is much more convenient because it reduces the number of issues you must track and the code you must write. Thus, the garbage collector provides a much higher level of insurance against the insidious problem of memory leaks, which has brought many a C++ project to its knees. With Java, the garbage collector is designed to take care of the problem of releasing memory (although this doesn’t include other aspects of cleaning up an object). The garbage collector “knows” when an object is no longer in use, and automatically releases the memory for that object. This, combined with the fact that all objects are inherited from the single root class Object and you can create objects only one way—on the heap—makes the process of programming in Java much simpler than programming in C++. You have far fewer decisions to make and hurdles to overcome. Exception Handling: Dealing with Errors Since the beginning of programming languages, error handling has been especially difficult. Because it’s so hard to design a good errorhandling scheme, many languages ignore the issue, passing the problem on to library designers who come up with halfway measures that work in many situations but can easily be circumvented, generally by just ignoring errors. A major problem with most error-handling schemes is that they rely on programmers to follow an agreed-upon convention that is not enforced by the language. If the programmer is not vigilant—often the case if they are in a hurry—these schemes can easily be forgotten. Exception handling wires error handling directly into the programming language and sometimes even the operating system. An exception is an object that is “thrown” from the site of the error and can be “caught” by an appropriate exception handler designed for that particular type of error. It’s as if exception handling is a different, parallel path of execution, taken when things go wrong. Because it uses a separate execution path, it doesn’t interfere with your normally executing code. This can make that code simpler to write because you aren’t constantly forced to check for errors. In addition, a thrown exception is unlike an error value returned from a method or a flag set by a method to indicate an error condition—these can be ignored. An exception cannot be ignored, so it’s guaranteed to be dealt with at some point. Finally, exceptions provide a way to reliably recover from a bad situation. Instead of just exiting the program, you are sometimes able to set things right and restore execution, which produces more robust programs. Java’s exception handling stands out among programming languages, because in Java, exception handling was wired in from the beginning and you’re forced to use it. It is the single acceptable way to report errors. If you don’t write your code to properly handle exceptions, you’ll get a compile-time error message. This guaranteed consistency can sometimes make error handling much easier. It’s worth noting that exception handling isn’t an object-oriented feature, although in object-oriented languages the exception is normally represented by an object. Exception handling existed before object-oriented languages. Summary A procedural program contains data definitions and function calls. To find the meaning of such a program, you must work at it, looking through the function calls and low-level concepts to create a model in your mind. This is the reason we need intermediate representations when designing procedural programs—by themselves, these programs tend to be confusing because the terms of expression are oriented more toward the computer than to the problem you’re solving. Because OOP adds many new concepts on top of what you find in a procedural language, your natural assumption might be that the resulting Java program is far more complicated than the equivalent procedural program. Here, you’ll be pleasantly surprised: A wellwritten Java program is generally simpler and easier to understand than a procedural program. What you’ll see are the definitions of the objects that represent concepts in your problem space (rather than the issues of the computer representation) and messages sent to those objects to indicate activities in that space. One of the delights of object-oriented programming is that, with a well-designed program, it’s easy to understand the code by reading it. Usually, there’s a lot less code as well, because many problems are solved by reusing existing library code. OOP and Java might not be for everyone. It’s important to evaluate your own needs and decide whether Java will optimally satisfy those needs, or if you might be better off with another programming system (perhaps the one you’re currently using). If your needs are very specialized for the foreseeable future and you have specific constraints that might not be satisfied by Java, you owe it to yourself to investigate the alternatives (in particular, I recommend looking at Python). If you still choose Java as your language, you’ll at least understand what the options were and have a clear vision of why you took that direction. 1. This is actually a bit restrictive, since objects can conceivably exist in different machines and address spaces, and they can also be stored on disk. In these cases, the identity of the object must be determined by something other than a memory address. ↩ 2. In some cases we make a distinction, stating that type determines the interface while class is a particular implementation of that interface.↩ 3. I’m indebted to my friend Scott Meyers for this term. ↩ 4. This is enough detail for most diagrams, and you don’t need to get specific about whether you’re using aggregation or composition.↩ 5. Or Liskov Substitution Principle, after Barbara Liskov who first described it↩ 6. They do not hold primitives, but autoboxing simplifies this restriction somewhat. This is discussed in detail later in the book. ↩ 7. Primitive types, which you’ll learn about later, are a special case. ↩ Installing Java and the Book Examples In which we provision ourselves for the journey. Before you can begin learning the language, you must install Java and the book’s source-code examples. Because it is possible for a “dedicated beginner” to learn programming from this book, I explain the process in detail, assuming you haven’t previously used the computer’s command-line shell. If you have, you can skip forward to the installation instructions. If any terminology or processes described here are still not clear to you, you can usually find explanations or answers through Google. For more specific issues or problems, try StackOverflow. Sometimes you can find installation instructions on YouTube. Editors To create and modify Java program files—the code listings shown in this book—you need a program called an editor. You’ll also need an editor to make changes to your system configuration files, which is sometimes required during installation. Programming editors vary from heavyweight Integrated Development Environments (IDEs, like Eclipse, NetBeans and IntelliJ IDEA) to more basic text manipulation applications. If you already have an IDE and are comfortable with it, feel free to use that for this book, but in the interest of keeping things simple, I recommend the Atom editor. Find it at atom.io. Atom is free and open-source, is very simple to install, works on all platforms (Windows, Mac and Linux), and has a built-in Java mode that is automatically invoked when you open a Java file. It isn’t a heavy-duty IDE so it doesn’t get confusing, which is ideal for this book. On the other hand, it has some handy editing features that you’ll probably come to love. More details are on their site. There are many other editors; these are a subculture unto themselves and people even get into heated arguments about their merits. If you find one you like better, it’s not too hard to change. The important thing is to choose one and get comfortable with it. The Shell If you haven’t programmed before, you might be unfamiliar with your operating system shell (also called the command prompt in Windows). The shell harkens back to the early days of computing when everything happened by typing commands and the computer responded by displaying responses; it was all text-based. Although it can seem primitive in the age of graphical user interfaces, a shell provides a surprising number of valuable features. We’ll use the shell regularly in this book, both as part of the installation process and to run Java programs. Starting a Shell Mac: Click on the Spotlight (the magnifying-glass icon in the upperright corner of the screen) and type “terminal.” Click on the application that looks like a little TV screen (you might also be able to hit “Return”). This starts a shell in your home directory. Windows: First, start the Windows Explorer to navigate through your directories: Windows 7: click the “Start” button in the lower left corner of the screen. In the Start Menu search box area type “explorer” then press the “Enter” key. Windows 8: click Windows+Q, type “explorer” then press the “Enter” key. Windows 10: click Windows+E. Once the Windows Explorer is running, move through the folders on your computer by double-clicking on them with the mouse. Navigate to the desired folder. Now click the file tab at the top left of the Explorer window and select “Open command prompt.” This opens a shell in the destination directory. Linux: To open a shell in your home directory: Debian: Press Alt+F2. In the dialog that pops up, type ‘gnometerminal’ Ubuntu: Either right-click on the desktop and select ‘Open Terminal’, or press Ctrl+Alt+T Redhat: Right-click on the desktop and select ‘Open Terminal’ Fedora: Press Alt+F2. In the dialog that pops up, type ‘gnometerminal’ Directories Directories are one of the fundamental elements of a shell. Directories hold files, as well as other directories. Think of a directory as a tree with branches. If books is a directory on your system and it has two other directories as branches, for example math and art, we say you have a directory books with two subdirectories math and art. We refer to them as books/math and books/art since books is their parent directory. Note that Windows uses backslashes rather than forward slashes to separate the parts of a directory. Basic Shell Operations The shell operations I show here are approximately identical across operating systems. For the purposes of this book, here are the essential operations in a shell: Change directory: Use cd followed by the name of the directory where you want to move, or cd.. if you want to move up a directory. If you want to move to a different directory while remembering where you came from, use pushd followed by the different directory name. Then, to return to the previous directory, just say popd. Directory listing: ls (dir in Windows) displays all the files and subdirectory names in the current directory. Use the wildcard * (asterisk) to narrow your search. For example, if you want to list all the files ending in “.java,” you say ls *.java (Windows: dir *.java). If you want to list the files starting with “F” and ending in “.java,” you say ls F*.java (Windows: dir F*.java). Create a directory: use the mkdir (“make directory”) command (Windows: md), followed by the name of the directory you want to create. For example, mkdir books (Windows: md books). Remove a file: Use rm (“remove”) followed by the name of the file you wish to remove (Windows: del). For example, rm somefile.java (Windows: del somefile.java). Remove a directory: use the rm -r command to remove the files in the directory and the directory itself (Windows: deltree). For example, rm -r books (Windows: deltree books). Repeat a command: The “up arrow” on all three operating systems moves through previous commands so you can edit and repeat them. On Mac/Linux, !! repeats the last command and !n repeats the nth command. Command history: Use history in Mac/Linux or press the F7 key in Windows. This gives you a list of all the commands you’ve entered. Mac/Linux provides numbers to refer to when you want to repeat a command. Unpacking a zip archive: A file name ending with.zip is an archive containing other files in a compressed format. Both Linux and Mac have command-line unzip utilities, and you can install a command-line unzip for Windows via the Internet. However, in all three systems the graphical file browser (Windows Explorer, the Mac Finder, or Nautilus or equivalent on Linux) will browse to the directory containing your zip file. Then right-mouse-click on the file and select “Open” on the Mac, “Extract Here” on Linux, or “Extract all …” on Windows. To learn more about your shell, search Wikipedia for Windows Shell or, for Mac/Linux, Bash Shell. Installing Java To compile and run the examples, you must first install the Java development kit. In this book we use JDK8 (Java 1.8). Windows 1. Follow the instructions at this link to Install Chocolatey. 2. At a shell prompt, type: choco install jdk8. This takes some time, but when it’s finished Java is installed and the necessary environment variables are set. Macintosh The Mac comes with a much older version of Java that won’t work for the examples in this book, so you must first update it to Java 8. You will need administration rights to perform these steps. 1. Follow the instructions at this link to Install HomeBrew. Then at a shell prompt, execute brew update to make sure you have the latest changes. 2. At a shell prompt, execute brew cask install java. Once HomeBrew and Java are installed, all other activities described in this book can be accomplished within a guest account, if that suits your needs. Linux Use the standard package installer with the following shell commands: Ubuntu/Debian: 1. sudo apt-get update 2. sudo apt-get install default-jdk Fedora/Redhat: 1. su-c "yum install java-1.8.0-openjdk" Verify Your Installation Open a new shell and type: java -version You should see something like the following (Version numbers and actual text will vary): java version "1.8.0_112" Java(TM) SE Runtime Environment (build 1.8.0_112-b15) Java HotSpot(TM) 64-Bit Server VM (build 25.112-b15, mixed mode) If you see a message that the command is not found or not recognized, review the installation instructions in this chapter. If you still can’t get it to work, check StackOverflow. Installing and Running the Book Examples Once you have Java installed, the process to install and run the book examples is the same for all platforms: 1. Download the book examples from the GitHub Repository. 2. unzip (as described in Basic Shell Operations) the downloaded file into the directory of your choice. 3. Use the Windows Explorer, the Mac Finder, or Nautilus or equivalent on Linux to browse to the directory where you unzipped OnJava8-Examples, and open a shell there. 4. If you’re in the right directory, you should see files named gradlew and gradlew.bat in that directory, along with numerous other files and directories. The directories correspond to the chapters in the book. 5. At the shell prompt, type gradlew run (Windows) or./gradlew run (Mac/Linux). The first time you do this, Gradle will install itself and numerous other packages, so it will take some time. After everything is installed, subsequent builds and runs are faster. Note you must be connected to the Internet the first time you run gradlew so that Gradle can download the necessary packages. Basic Gradle Tasks There are a large number of Gradle tasks automatically available with this book’s build. Gradle uses an approach called convention over configuration which results in the availability of many tasks even if you’re only trying to accomplish something very basic. Some of the tasks that “came along for the ride” with this book are inappropriate or don’t successfully execute. Here is a list of the Gradle tasks you will typically use: gradlew compileJava: Compiles all the Java files in the book that can be compiled (some files don’t compile, to demonstrate incorrect language usage). gradlew run: First compiles, then executes all the Java files in the book that can be executed (some files are library components). gradlew test: Executes all the unit tests (you’ll learn about these in Validating Your Code). gradlew chapter: ExampleName: Compiles and runs a specific example program. For instance, gradlew objects:HelloDate. Objects Everywhere “If we spoke a different language, we would perceive a somewhat different world.”— Ludwig Wittgenstein (18891951) Although it is based on C++, Java is more of a “pure” object-oriented language. Both C++ and Java are hybrid languages, but in Java the designers felt that the hybridization was not as important as it was in C++. A hybrid language allows multiple programming styles; the reason C++ is hybrid is to support backward compatibility with the C language. Because C++ is a superset of the C language, it includes many of that language’s undesirable features, which can make some aspects of C++ overly complicated. The Java language assumes you’re only writing object-oriented programs. Before you can begin you must shift your mindset into an object-oriented world. In this chapter you’ll see the basic components of a Java program and learn that (almost) everything in Java is an object. You Manipulate Objects with References What’s in a name? That which we call a rose, by any other word would smell as sweet. (Shakespeare, Romeo & Juliet) Every programming language manipulates elements in memory. Sometimes the programmer must be constantly aware of that manipulation. Do you manipulate the element directly, or use an indirect representation that requires special syntax (for example, pointers in C or C++)? Java simplifies the issue by considering everything an object, using a single consistent syntax. Although you treat everything as an object, the identifier you manipulate is actually a “reference” to an object.1 You might imagine a television (the object) and a remote control (the reference). As long as you’re holding this reference, you have a connection to the television, but when someone says, “Change the channel” or “Lower the volume,” what you’re manipulating is the reference, which in turn modifies the object. To move around the room and still control the television, you take the remote/reference with you, not the television. Also, the remote control can stand on its own, with no television. That is, just because you have a reference doesn’t mean there’s necessarily an object connected to it. To hold a word or sentence, you create a String reference: String s; But here you’ve created only the reference, not an object. If you now decide to send a message to s, you get an error because s isn’t actually attached to anything (there’s no television). A safer practice is to always initialize a reference when you create it: String s = "asdf"; This uses a special Java feature: Strings can be initialized with quoted text. You must use a more general type of initialization for other types of objects. You Must Create All the Objects The point of a reference is to connect it to an object. You usually create objects with the new operator. The keyword new says, “Make one of these.” So in the preceding example, you can say: String s = new String("asdf"); Not only does this mean “Make a new String,” but it also gives information about how to make the String by supplying an initial group of characters. Java comes with a plethora of ready-made types in addition to String. On top of that, you can create your own types. In fact, creating new types is the fundamental activity in Java programming, and it’s what you’ll be learning about in the rest of this book. Where Storage Lives It’s useful to visualize the way things are laid out while the program is running—in particular, how memory is arranged. There are five different places to store data: 1. Registers. This is the fastest storage because it exists in a place different from that of other storage: inside the central processing unit (CPU)2. However, the number of registers is severely limited, so registers are allocated as they are needed. You don’t have direct control over register allocation, nor do you see any evidence in your programs that