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| Java: A Beginner’s Guide is your gateway to the Herb Schildt series of Java
programming books. Here are some others that you will find of interest: Java: The Complete Reference Herb Schildt’s Java Programming Cookbook The Art of Java Swing: A Beginner’s Guide Introducing JavaFX 8 Programming Chapter 1 Java Fundamentals Key Skills & Concepts ● Know the history and philosophy of Java ● Understand Java’s contribution to the Internet ● Understand the importance of bytecode ● Know the Java buzzwords ● Understand the foundational principles of objectoriented programming y istory opics utorials ffers & Deals ighlights ettings Support Sign Out I ● Create, compile, and run a simple Java program ● Use variables ● Use the if and for control statements ● Create blocks of code ● Understand how statements are positioned, indented, and terminated ● Know the Java keywords ● Understand the rules for Java identifiers n computing, few technologies have had the impact of Java. Its creation in the early days of the Web helped shape the modern form of the Internet, including both the client and server sides. Its innovative features advanced the art and science of programming, setting a new standard in computer language design. The forward thinking culture that grew up around Java ensured it would remain vibrant and alive, adapting to the often rapid and varied changes in the computing landscape. Simply put: not only is Java one of the world’s most important computer languages, it is a force that revolutionized programming and, in the process, changed the world. Although Java is a language often associated with Internet programming, it is by no means limited in that regard. Java is a powerful, fullfeatured, generalpurpose programming language. Thus, if you are new to programming, Java is an excellent language to learn. Moreover, to be a professional programmer today implies the ability to program in Java—it is that important. In the course of this book, you will learn the basic skills that will help you master it. The purpose of this chapter is to introduce you to Java, beginning with its history, its design philosophy, and several of its most important features. By far, the hardest thing about learning a programming language is the fact that no element exists in isolation. Instead, the components of the language work in conjunction with each other. This interrelatedness is especially pronounced in Java. In fact, it is difficult to discuss one aspect of Java without involving others. To help overcome this problem, this chapter provides a brief overview of several Java features, including the general form of a Java program, some basic control structures, and simple operators. It does not go into too many details, but, rather, concentrates on general concepts common to any Java program. THE HISTORY AND PHILOSOPHY OF JAVA THE HISTORY AND PHILOSOPHY OF JAVA Before one can fully appreciate the unique aspects of Java, it is necessary to understand the forces that drove its creation, the programming philosophy that it embodies, and key concepts of its design. As you advance through this book, you will see that many aspects of Java are either a direct or indirect result of the historical forces that shaped the language. Thus, it is fitting that we begin our examination of Java by exploring how Java relates to the larger programming universe. The Origins of Java Java was conceived by James Gosling, Patrick Naughton, Chris Warth, Ed Frank, and Mike Sheridan at Sun Microsystems in 1991. This language was initially called “Oak” but was renamed “Java” in 1995. Somewhat surprisingly, the original impetus for Java was not the Internet! Instead, the primary motivation was the need for a platform independent language that could be used to create software to be embedded in various consumer electronic devices, such as toasters, microwave ovens, and remote controls. As you can probably guess, many different types of CPUs are used as controllers. The trouble was that (at the time) most computer languages were designed to be compiled into machine code that was targeted for a specific type of CPU. For example, consider the C++ language. Although it was possible to compile a C++ program for just about any type of CPU, to do so required a full C++ compiler targeted for that CPU. The problem, however, is that compilers are expensive and time consuming to create. In an attempt to find a better solution, Gosling and the others worked on a portable, crossplatform language that could produce code that would run on a variety of CPUs under differing environments. This effort ultimately led to the creation of Java. About the time that the details of Java were being worked out, a second, and ultimately more important, factor emerged that would play a crucial role in the future of Java. This second force was, of course, the World Wide Web. Had the Web not taken shape at about the same time that Java was being implemented, Java might have remained a useful but obscure language for programming consumer electronics. However, with the emergence of the Web, Java was propelled to the forefront of computer language design, because the Web, too, demanded portable programs. Most programmers learn early in their careers that portable programs are as elusive as they are desirable. While the quest for a way to create efficient, portable (platform independent) programs is nearly as old as the discipline of programming itself, it had taken a back seat to other, more pressing problems. However, with the advent of the Internet and the Web, the old problem of portability returned with a vengeance. After all, the Internet consisted of a diverse, distributed universe populated with many types of computers, operating systems, and CPUs. What was once an irritating but lowpriority problem had become a highprofile necessity. By 1993 it became obvious to members of the Java design team that the problems of portability frequently encountered when creating code for embedded controllers are also found when attempting to create code for the Internet. This realization caused the focus of Java to switch from consumer electronics to Internet programming. So, although it was the desire for an architectureneutral programming language that provided the initial spark, it was the Internet that ultimately led to Java’s largescale success. Java’s Lineage: C and C++ The history of computer languages is not one of isolated events. Rather, it is a continuum in which each new language is influenced in one way or another by what has come before. In this regard, Java is no exception. Before moving on, it is useful to understand where Java fits into the family tree of computer languages. The two languages that form Java’s closest ancestors are C and C++. As you may know, C and C++ are among the most important computer languages ever invented and are still in widespread use today. From C, Java inherits its syntax. Java’s object model is adapted from C++. Java’s relationship to C and C++ is important for a number of reasons. First, at the time of Java’s creation, many programmers were familiar with the C/C++ syntax. Because Java uses a similar syntax, it was relatively easy for a C/C++ programmer to learn Java. This made it possible for Java to be readily utilized by the pool of existing programmers, thus facilitating Java’s acceptance by the programming community. Second, Java’s designers did not “reinvent the wheel.” Instead, they further refined an already highly successful programming paradigm. The modern age of programming began with C. It moved to C++ and then to Java. By inheriting and building on that rich heritage, Java provides a powerful, logically consistent programming environment that takes the best of the past and adds new features related to the online environment and advances in the art of programming. Perhaps most important, because of their similarities, C, C++, and Java define a common, conceptual framework for the professional programmer. Programmers do not face major rifts when switching from one language to another. Java has another attribute in common with C and C++: it was designed, tested, and refined by real working programmers. It is a language grounded in the needs and experiences of the people who devised it. There is no better way to produce a topflight professional programming language. One last point: although C++ and Java are related, especially in their support for objectoriented programming, Java is not simply the “Internet version of C++.” Java has significant practical and philosophical differences from C++. Furthermore, Java is not an enhanced version of C++. For example, it is neither upwardly nor downwardly compatible with C++. Moreover, Java was not designed to replace C++. Java was designed to solve a certain set of problems. C++ was designed to solve a different set of problems. They will coexist for many years to come. How Java Impacted the Internet The Internet helped catapult Java to the forefront of programming, and Java, in turn, had a profound effect on the Internet. First, the creation of Java simplified Internet programming in general, acting as a catalyst that drew legions of programmers to the Web. Second, Java innovated a new type of networked program called the applet that changed the way the online world thought about content. Finally, and perhaps most importantly, Java addressed some of the thorniest issues associated with the Internet: portability and security. Java Simplified Web-Based Programming From the start, Java simplified webbased programming in a number of ways. Arguably the most important is found in its ability to create portable, crossplatform programs. Of nearly equal importance is Java’s support for networking. Its library of readytouse functionality enabled programmers to easily write programs that accessed or made use of the Internet. It also provided mechanisms that enabled programs to be readily delivered over the Internet. Although the details are beyond the scope of this book, it is sufficient to know that Java’s support for networking was a key factor in its rapid rise. Java Applets At the time of Java’s creation, one of its most exciting features was the applet. An applet is a special kind of Java program that is designed to be transmitted over the Internet and automatically executed inside a Javacompatible web browser. If the user clicks a link that contains an applet, the applet will download and run in the browser automatically. Applets were intended to be small programs, typically used to display data provided by the server, handle user input, or provide simple functions, such as a loan calculator. The key feature of applets is that they execute locally, rather than on the server. In essence, the applet allowed some functionality to be moved from the server to the client. The creation of the applet was important because, at the time, it expanded the universe of objects that could move about freely in cyberspace. In general, there are two very broad categories of objects that are transmitted between the server and the client: passive information and dynamic active programs. For example, when you read your e mail, you are viewing passive data. Even when you download a program, the program’s code is still only passive data until you execute it. By contrast, the applet is a dynamic, selfexecuting program. Such a program is an active agent on the client computer, yet it is delivered by the server. In the early days of Java, applets were a crucial part of Java programming. They illustrated the power and benefits of Java, added an exciting dimension to web pages, and enabled programmers to explore the full extent of what was possible with Java. Although it is likely that there are still applets in use today, over time they became less important, and for reasons that will be explained shortly, JDK 9 began their phaseout process. Finally, applet support was removed by JDK 11. Ask the Expert Q : What is C# and how does it relate to Java? A : A few years after the creation of Java, Microsoft developed the C# language. This is important because C# is closely related to Java. In fact, many of C#’s features directly parallel Java. Both Java and C# share the same general C++style syntax, support distributed programming, and utilize a similar object model. There are, of course, differences between Java and C#, but the overall “look and feel” of these languages is very similar. This means that if you already know C#, then learning Java will be especially easy. Conversely, if C# is in your future, then your knowledge of Java will come in handy. Security As desirable as dynamic, networked programs are, they also present serious problems in the areas of security and portability. Obviously, a program that downloads and executes on the client computer must be prevented from doing harm. It must also be able to run in a variety of different environments and under different operating systems. As you will see, Java addressed these problems in an effective and elegant way. Let’s look a bit more closely at each, beginning with security. As you are likely aware, every time that you download a program, you are taking a risk because the code you are downloading might contain a virus, Trojan horse, or other harmful code. At the core of the problem is the fact that malicious code can cause damage because it has gained unauthorized access to system resources. For example, a virus program might gather private information, such as credit card numbers, bank account balances, and passwords, by searching the contents of your computer’s local file system. In order for Java to enable programs to be safely downloaded and executed on the client computer, it was necessary to prevent them from launching such an attack. Java achieved this protection by enabling you to confine an application to the Java execution environment and prevent it from accessing other parts of the computer. (You will see how this is accomplished shortly.) The ability to download an application with a high level of confidence that no harm will be done contributed significantly to Java’s early success. Portability Portability is a major aspect of the Internet because there are many different types of computers and operating systems connected to it. If a Java program were to be run on virtually any computer connected to the Internet, there needed to be some way to enable that program to execute on different types of systems. In other words, a mechanism that allows the same application to be downloaded and executed by a wide variety of CPUs, operating systems, and browsers is required. It is not practical to have different versions of the same application for different computers. The same application code must work in all computers. Therefore, some means of generating portable code was needed. As you will soon see, the same mechanism that helps ensure security also helps create portability. Java’s Magic: The Bytecode The key that allowed Java to address both the security and the portability problems just described is that the output of a Java compiler is not executable code. Rather, it is bytecode. Bytecode is a highly optimized set of instructions designed to be executed by what is called the Java Virtual Machine (JVM), which is part of the Java Runtime Environment (JRE). In essence, the original JVM was designed as an interpreter for Yüklə 83 Mb. Dostları ilə paylaş:
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