Adapted from the Java Tutorial ...

What Is Java?

Java is two things: a programming language and a platform.

The Java Programming Language

Each of the preceding buzzwords is explained in The Java Language Environment white paper, by James Gosling and Henry McGilton.

Java is also unusual in that each Java program is both compiled and interpreted. With a compiler, you translate a Java program into an intermediate language called Java bytecodes--the platform-independent codes interpreted by the Java interpreter.

With an interpreter, each Java bytecode instruction is parsed and run on the computer. Compilation happens just once; interpretation occurs each time the program is executed. This figure illustrates how this works.

You can think of Java bytecodes as the machine code instructions for the Java Virtual Machine (JVM or Java VM).

Every Java interpreter, whether it's a Java development tool or a Web browser that can run Java applets, is an implementation of the JVM. The JVM can also be implemented in hardware.

Java bytecodes help make "write once, run anywhere" possible. You can compile your Java program into bytecodes on any platform that has a Java compiler. The bytecodes can then be run on any implementation of the JVM.

The Java Platform

The Java platform has two components:

• The Java Virtual Machine (JVM)
• The Java Application Programming Interface (Java API)

The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets. The Java API is grouped into libraries (packages) of related components.

The following figure depicts a Java program, such as an application or applet, that's running on the Java platform. As the figure shows, the Java API and Virtual Machine insulates the Java program from hardware dependencies.

As a platform-independent environment, Java can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring Java's performance close to that of native code without threatening portability.

Here is a what the whole process of Java development and execution looks like:

What Can Java Do?

Probably the most well-known Java programs are Java applets. An applet is a Java program that adheres to certain conventions that allow it to run within a Java-enabled browser.

However, Java is not just for writing cute, entertaining applets for the World Wide Web ("Web"). Java is a general-purpose, high-level programming language and a powerful software platform. Using the generous Java API, you can write many types of programs.

A standalone program that runs directly on the Java platform is called a Java application. A special kind of application known as a server serves and supports clients on a network. Another specialized program is a servlet. Servlets are similar to applets in that they are runtime extensions of applications. Instead of working in browsers, though, servlets run within Java servers, configuring or tailoring the server.

How does the Java API support all of these kinds of programs? With packages of software components that provide a wide range of functionality. The core API is the API included in every full implementation of the Java platform. The core API gives you the following features:

• The Essentials: Objects, strings, threads, numbers, input and output, data structures, system properties, date and time, and so on.
• Applets: The set of conventions used by Java applets.
• Networking: URLs, TCP and UDP sockets, and IP addresses.
• Internationalization: Help for writing programs that can be localized for users worldwide. Programs can automatically adapt to specific locales and be displayed in the appropriate language.
• Security: Both low-level and high-level, including electronic signatures, public/private key management, access control, and certificates.
• Software components: Known as JavaBeans, can plug into existing component architectures such as Microsoft's OLE/COM/Active-X architecture, OpenDoc, and Netscape's Live Connect.
• Object serialization: Allows lightweight persistence and communication via Remote Method Invocation (RMI).
• Java Database Connectivity (JDBC): Provides uniform access to a wide range of relational databases.

How Will Java Change My Life?

We can't promise you fame, fortune, or even a job if you learn Java. Still, Java is likely to make your programs better and requires less effort than other languages. We believe that Java will help you do the following:

• Get started quickly: Although Java is a powerful object-oriented language, it's easy to learn, especially for programmers already familiar with C or C++.
• Write less code: Comparisons of program metrics (class counts, method counts, and so on) suggest that a program written in Java can be four times smaller than the same program in C++.
• Write better code: The Java language encourages good coding practices, and its garbage collection helps you avoid memory leaks. Java's object orientation, its JavaBeans component architecture, and its wide-ranging, easily extendible API let you reuse other people's tested code and introduce fewer bugs.
• Develop programs faster: Your development time may be as much as twice as fast versus writing the same program in C++. Why? You write fewer lines of code with Java and Java is a simpler programming language than C++.
• Avoid platform dependencies with 100% Pure Java: You can keep your program portable by following the purity tips mentioned throughout this book and avoiding the use of libraries written in other languages.
• Write once, run anywhere: Because 100% Pure Java programs are compiled into machine-independent bytecodes, they run consistently on any Java platform.
• Distribute software more easily: You can upgrade applets easily from a central server. Applets take advantage of the Java feature of allowing new classes to be loaded "on the fly," without recompiling the entire program.