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The Java Story

Back in 1990, a gentleman by the name of James Gosling was given the task of creating programs to control consumer electronics.Gosling and his team of people at Sun Microsystems started designing their software using C++, the language that most programmers were praising as the next big thing because of its object-oriented nature. Gosling, however, quickly found that C++ was not suitable for the projects he and his team had in mind. They ran into trouble with complicated aspects of C++ such as multiple inheritance of classes and with program bugs such as memory leaks. Gosling soon decided that he was going to have to come up with his own, simplified computer language that would avoid all the problems he had with C++.

Although Gosling didn't care for the complexity of languages such as C++, he did like the basic syntax and object-oriented features of the language. So when he sat down to design his new language, he used C++ as its model, stripping away all the features of C++ that made that language difficult to use with his consumer-electronics projects. When Gosling completed his language-design project, he had a new programming language that he named Oak. (The story goes that the name Oak came to Gosling as he gazed out his office window at an oak tree.)

Oak was first used in something called the Green project, wherein the development team attempted to design a control system for use in the home. This control system would enable the user to manipulate a list of devices, including TVs, VCRs, lights, and telephones, all from a hand-held computer called *7 (Star Seven). The *7 system featured a touch-sensitive screen that the owner used to select and control the devices supported by the control.

The next step for Oak was the video-on-demand (VOD) project, in which the language was used as the basis for software that controlled an interactive television system. Although neither *7 nor the VOD project led to actual products, they gave Oak a chance to develop and mature. By the time Sun discovered that the name "Oak" was already claimed and they changed the name to Java, they had a powerful, yet simple, language on their hands.

More importantly, Java was a platform-neutral language, which meant that programs developed with Java could run on any computer system with no changes. This platform independence was attained by using a special format for compiled Java programs. This file format, called byte-code, could be read and executed by any computer system that has a Java interpreter. The Java interpreter, of course, must be written specially for the system on which it will run.

In 1993, after the World Wide Web had transformed the text-based Internet into a graphics-rich environment, the Java team realized that the language they had developed would be perfect for Web programming. The team came up with the concept of Web applets, small programs that could be included in Web pages, and even went so far as to create a complete Web browser (now called HotJava) that demonstrated the language's power.

In the second quarter of 1995, Sun Microsystems officially announced Java. The "new" language was quickly embraced as a powerful tool for developing Internet applications. Netscape Communications, the developer of the popular Netscape Navigator Web browser added support for Java to its new Netscape Navigator 2.0. Other Internet software developers are sure to follow suit, including Microsoft, whose Internet Explorer 3 (currently in beta) offers Java support. After more than five years of development, Java has found its home.

Introducing Java

By now, you may be curious why Java is considered such a powerful tool for Internet development projects. You already know that Java is a simplified version of C++. Anyone who has struggled with learning C++ knows that the key word in the previous sentence is "simplified." C++ added so much to the C language that even professional programmers often have difficulty making the transition.

According to Sun Microsystems, Java is "simple, object-oriented, statically typed, compiled, architecture neutral, multi-threaded, garbage collected, robust, secure, and extensible." That's a mouthful, but this description of Java probably doesn't help you understand the language much. The following list of Java's attributes, however, should clear out some of the cobwebs:

• Simple. Java's developers deliberately left out many of the unnecessary features of other high-level programming languages. For example, Java does not support pointer math, implicit type casting, structures or unions, operator overloading, templates, header files, or multiple inheritance.
• Object-oriented. Just like C++, Java uses classes to organize code into logical modules. At runtime, a program creates objects from the classes. Java classes can inherit from other classes, but multiple inheritance, wherein a class inherits methods and fields from more than one class, is not allowed.
• Statically typed. All objects used in a program must be declared before they are used. This enables the Java compiler to locate and report type conflicts.
• Compiled. Before you can run a program written in the Java language, the program must be compiled by the Java compiler. The compilation results in a "byte-code" file that, while similar to a machine-code file, can be executed under any operating system that has a Java interpreter. This interpreter reads in the byte-code file and translates the byte-code commands into machine-language commands that can be directly executed by the machine that's running the Java program. You could say, then, that Java is both a compiled and interpreted language.
• Multi-threaded. Java programs can contain multiple threads of execution, which enables programs to handle several tasks concurrently. For example, a multi-threaded program can render an image on the screen in one thread while continuing to accept keyboard input from the user in the main thread. All applications have at least one thread, which represents the program's main path of execution.
• Garbage collected. Java programs do their own garbage collection, which means that programs are not required to delete objects that they allocate in memory. This relieves programmers of virtually all memory-management problems.
• Robust. Because the Java interpreter checks all system access performed within a program, Java programs cannot crash the system. Instead, when a serious error is discovered, Java programs create an exception. This exception can be captured and managed by the program without any risk of bringing down the system.
• Secure. The Java system not only verifies all memory access but also ensures that no viruses are hitching a ride with a running applet. Because pointers are not supported by the Java language, programs cannot gain access to areas of the system for which they have no authorization.
• Extensible. Java programs support native methods, which are functions written in another language, usually C++. Support for native methods enables programmers to write functions that may execute faster than the equivalent functions written in Java. Native methods are dynamically linked to the Java program; that is, they are associated with the program at runtime. As the Java language is further refined for speed, native methods will probably be unnecessary.
• Well-understood. The Java language is based upon technology that's been developed over many years. For this reason, Java can be quickly and easily understood by anyone with experience with modern programming languages such as C++.

Java Programs

As I mentioned previously. Java can be used to create two types of programs: applets and stand-alone applications. An Applet is simply a part of a Web page, just as an image or a line of text can be. Just as a browser takes care of displaying an image referenced in an HTML document, a Java-enabled browser locates and runs an Applet . When your Java-capable Web browser loads the HTML document, the Java applet is also loaded and executed.

Using applets, you can do everything from adding animated graphics to your Web pages to creating complete games and utilities that can be executed over the Internet. Some applets that have already been created with Java include Bar Chart, which embeds a configurable bar chart in an HTML document; Crossword Puzzle, which enables users to solve a crossword puzzle on the Web; and LED Sign, which presents a scrolling, computerized message to viewers of the Web page within which the applet is embedded.

Applets are small programs that are run from within an HTML document.

Although most Java programmers are excited by the ability to create applets, Java can also be used to create stand-alone applications-that is, applications that don't need to be embedded in an HTML document. The most well-known application is the HotJava Web browser itself. This basic browser is completely written in the Java language, showing how Java handles not only normal programming tasks such as looping and evaluating mathematical expressions, but also how it can handle the complexities of telecommunications programming.

The HotJava Web browser is written entirely in the Java programming language.

The Java Developer's Kit

Java is actually more than a computer language; it's also a programming environment that includes a complete set of programming tools. These tools include a compiler, an interpreter, a debugger, a disassembler, a profiler, and more. To create a Java program, you first use a text editor to create the source-code file. You write the source code, of course, in the Java language. After completing the source code, which is always saved with a .java file extension, you compile the program into its byte-code format, the file for which has the .class file extension. It is the .class file that the interpreter loads and executes. Because the byte-code files are fully portable between operating systems, they can be executed on any system that has a Java interpreter.

After compiling and running a Java program, you may discover that the source code needs modification. The Java debugger can help you find your errors, whereas the Java profiler provides handy information about your program. If you run into a compiled Java program that you'd like to see in source-code form, the Java disassembler will do the translation for you. Java also includes a program that creates the files you need to take advantage of native methods (functions written in another language, such as C++). There's even a program that can create HTML documents from Java source-code files. Although all the development tools are DOS applications-that is, they don't run under Windows-they provide a complete environment for creating and managing Java projects.
Why Is Java Interesting?
When it was introduced in late 1995, Java took the Internet by storm.Java 1.1, released in early 1997, nearly doubles the speed of the Java interpreter and includes many important new features. With the addition of APIs to support database access, remote objects, an object component model, internationalization, printing, encryption, digital signatures, and many other technologies, Java is now poised to take the rest of the programming world by storm.Despite all the hype surrounding Java and the new features of Java 1.1, it's important to remember that at its core, Java is just a programming language, like many others, and its APIs are just class libraries, like those of other languages. What is interesting about Java, and thus the source of much of the hype, is that it has a number of important features that make it ideally suited for programming in the heavily networked, heterogenous world of the late 1990s.
Why Is Java Interesting?
In one of their early papers about the language, Sun described Java as follows:

Java: A simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language.

Sun acknowledges that this is quite a string of buzzwords, but the fact is that, for the most part, they aptly describe the language. In order to understand why Java is so interesting, let's take a look at the language features behind the buzzwords. Object-Oriented

Java is an object-oriented programming language. As a programmer, this means that you focus on the data in your application and methods that manipulate that data, rather than thinking strictly in terms of procedures. If you're accustomed to procedure-based programming in C, you may find that you need to change how you design your programs when you use Java. Once you see how powerful this new paradigm is, however, you'll quickly adjust to it.

In an object-oriented system, a class is a collection of data and methods that operate on that data. Taken together, the data and methods describe the state and behavior of an object. Classes are arranged in a hierarchy, so that a subclass can inherit behavior from its superclass. A class hierarchy always has a root class; this is a class with very general behavior.

Java comes with an extensive set of classes, arranged in packages, that you can use in your programs. For example, Java provides classes that create graphical user interface components (the java.awt package), classes that handle input and output (the java.io package), and classes that support networking functionality (the java.net package). The Object class (in the java.lang package) serves as the root of the Java class hierarchy.

Unlike C++, Java was designed to be object-oriented from the ground up. Most things in Java are objects; the primitive numeric, character, and boolean types are the only exceptions. Strings are represented by objects in Java, as are other important language constructs like threads. A class is the basic unit of compilation and of execution in Java; all Java programs are classes.

While Java is designed to look like C++, you'll find that Java removes many of the complexities of that language. If you are a C++ programmer, you'll want to study the object-oriented constructs in Java carefully. Although the syntax is often similar to C++, the behavior is not nearly so analogous. For a complete description of the object-oriented features of Java, see Interpreted

Java is an an interpreted language: the Java compiler generates byte-codes for the Java Virtual Machine (JVM), rather than native machine code. To actually run a Java program, you use the Java interpreter to execute the compiled byte-codes. Because Java byte-codes are platform-independent, Java programs can run on any platform that the JVM (the interpreter and run-time system) has been ported to.

In an interpreted environment, the standard "link" phase of program development pretty much vanishes. If Java has a link phase at all, it is only the process of loading new classes into the environment, which is an incremental, lightweight process that occurs at run-time. This is in contrast with the slower and more cumbersome compile-link-run cycle of languages like C and C++.