1.1. What Is Java?

In discussing Java, it is important to distinguish between the Java programming language, the Java Virtual Machine, and the Java platform. The Java programming language is the language in which Java applications, applets, servlets, and components are written. When a Java program is compiled, it is converted to byte codes that are the portable machine language of a CPU architecture known as the Java Virtual Machine (also called the Java VM or JVM). The JVM can be implemented directly in hardware, but it is usually implemented in the form of a software program that interprets and executes byte codes.

The Java platform is distinct from both the Java language and Java VM. The Java platform is the predefined set of Java classes that exist on every Java installation; these classes are available for use by all Java programs. The Java platform is also sometimes referred to as the Java runtime environment or the core Java APIs (application programming interfaces). The Java platform can be extended with optional packages (formerly called standard extensions). These APIs exist in some Java installations but are not guaranteed to exist in all installations.

1.1.1. The Java Programming Language

The Java programming language is a state-of-the-art, object-oriented language that has a syntax similar to that of C. The language designers strove to make the Java language powerful, but, at the same time, they tried to avoid the overly complex features that have bogged down other object-oriented languages like C++. By keeping the language simple, the designers also made it easier for programmers to write robust, bug-free code. As a result of its elegant design and next-generation features, the Java language has proved popular with programmers, who typically find it a pleasure to work with Java after struggling with more difficult, less powerful languages.

Java 5.0, the latest version of the Java language,^[1] includes a number of new language features, most notably generic types, which increase both the complexity and the power of the language. Most experienced Java programmers have welcomed the new features, despite the added complexity they bring.

^[1] Java 5.0 represents a significant change in version numbering for Sun. The previous version of Java is Java 1.4 so you may sometimes hear Java 5.0 informally referred to as Java 1.5.

1.1.2. The Java Virtual Machine

The Java Virtual Machine, or Java interpreter, is the crucial piece of every Java installation. By design, Java programs are portable, but they are only portable to platforms to which a Java interpreter has been ported. Sun ships VM implementations for its own Solaris operating system and for Microsoft Windows and Linux platforms. Many other vendors, including Apple and various commercial Unix vendors, provide Java interpreters for their platforms. The Java VM is not only for desktop systems, however. It has been ported to set-top boxes and handheld devices that run Windows CE and PalmOS.

Although interpreters are not typically considered high-performance systems, Java VM performance has improved dramatically since the first versions of the language. The latest releases of Java run remarkably fast. Of particular note is a VM technology called just-in-time (JIT) compilation whereby Java byte codes are converted on the fly into native platform machine language, boosting execution speed for code that is run repeatedly.

1.1.3. The Java Platform

The Java platform is just as important as the Java programming language and the Java Virtual Machine. All programs written in the Java language rely on the set of predefined classes^[2] that comprise the Java platform. Java classes are organized into related groups known as packages. The Java platform defines packages for functionality such as input/output, networking, graphics, user-interface creation, security, and much more.

^[2] A class is a module of Java code that defines a data structure and a set of methods (also called procedures, functions, or subroutines) that operate on that data.

It is important to understand what is meant by the term platform. To a computer programmer, a platform is defined by the APIs he can rely on when writing programs. These APIs are usually defined by the operating system of the target computer. Thus, a programmer writing a program to run under Microsoft Windows must use a different set of APIs than a programmer writing the same program for a Unix-based system. In this respect, Windows and Unix are distinct platforms.

Java is not an operating system. Nevertheless, the Java platform provides APIs with a comparable breadth and depth to those defined by an operating system. With the Java platform, you can write applications in Java without sacrificing the advanced features available to programmers writing native applications targeted at a particular underlying operating system. An application written on the Java platform runs on any operating system that supports the Java platform. This means you do not have to create distinct Windows, Macintosh, and Unix versions of your programs, for example. A single Java program runs on all these operating systems, which explains why "Write once, run anywhere" is Sun's motto for Java.

The Java platform is not an operating system, but for programmers, it is an alternative development target and a very popular one at that. The Java platform reduces programmers' reliance on the underlying operating system, and, by allowing programs to run on top of any operating system, it increases end users' freedom to choose an operating system.

1.1.4. Versions of Java

As of this writing, there have been six major versions of Java. They are:

Java 1.0

This was the first public version of Java. It contained 212 classes organized in 8 packages. It was simple and elegant but is now completely outdated.

Java 1.1

This release of Java more than doubled the size of the Java platform to 504 classes in 23 packages. It introduced nested types (or "inner classes"), an important change to the Java language itself, and included significant performance improvements in the Java VM. This version is outdated.

Java 1.2

This was a very significant release of Java; it tripled the size of the Java platform to 1,520 classes in 59 packages. Important additions included the Collections API for working with sets, lists, and maps of objects and the Swing API for creating graphical user interfaces. Because of the many new features included in the 1.2 release, the platform was rebranded as "the Java 2 Platform." The term "Java 2" was simply a trademark, however, and not an actual version number for the release.

Java 1.3

This was primarily a maintenance release, focused on bug fixes, stability, and performance improvements (including the high-performance "HotSpot" virtual machine). Additions to the platform included the Java Naming and Directory Interface (JNDI) and the Java Sound APIs, which were previously available as extensions to the platform. The most interesting classes in this release were probably java.util.Timer and java.lang.reflect.Proxy. In total, Java 1.3 contains 1,842 classes in 76 packages.

Java 1.4

This was another big release, adding important new functionality and increasing the size of the platform by 62% to 2,991 classes and interfaces in 135 packages. New features included a high-performance, low-level I/O API; support for pattern matching with regular expressions; a logging API; a user preferences API; new Collections classes; an XML-based persistence mechanism for JavaBeans; support for XML parsing using both the DOM and SAX APIs; user authentication with the Java Authentication and Authorization Service (JAAS) API; support for secure network connections using the SSL protocol; support for cryptography; a new API for reading and writing image files; an API for network printing; a handful of new GUI components in the Swing API; and a simplified drag-and-drop architecture for Swing. In addition to these platform changes, the 1.4 release introduced an assert statement to the Java language.

Java 5.0

The most recent release of Java introduces a number of changes to the core language itself including generic types, enumerated types, annotations, varargs methods, autoboxing, and a new for/in statement. Because of the major language changes, the version number was incremented. This release would logically be known as "Java 2.0" if Sun had not already used the term "Java 2" for marketing Java 1.2.

In addition to the language changes, Java 5.0 includes a number of additions to the Java platform as well. This release includes 3562 classes and interfaces in 166 packages. Notable additions include utilities for concurrent programming, a remote management framework, and classes for the remote management and instrumentation of the Java VM itself.

See the Preface for a list of changes in this edition of the book, including pointers to coverage of the new language and platform features.

To write programs in Java, you must obtain the Java Development Kit ( JDK). Sun releases a new version of the JDK for each new version of Java. Don't confuse the JDK with the Java Runtime Environment ( JRE). The JRE contains everything you need to run Java programs, but it does not contain the tools you need to develop Java programs (primarily the compiler).

In addition to the Standard Edition of Java used by most Java developers and documented in this book, Sun has also released the Java 2 Platform, Enterprise Edition (or J2EE) for enterprise developers and the Java 2 Platform, Micro Edition (J2ME) for consumer electronic systems, such as handheld PDAs and cellular telephones. See Java Enterprise in a Nutshell and Java Micro Edition in a Nutshell (both by O'Reilly) for more information on these other editions.