On the Road to Understanding Java
This chapter gives a quick introduction to the Javatm technology. First, we explain what the Java platform is and what it can do. Next are step-by-step instructions on how to compile and run two simple programs on the Win32, the UNIX/Linux or the MacOS platforms.1 After that, we take a look at the code for the two programs, so you can see how they work. The chapter ends with questions and exercises to test and expand your knowledge, followed by a table of download instructions for the code used in this chapter.
The software development kits (SDKs) that Sun Microsystems provides include a minimal set of tools to let you run and compile your programs. Serious developers are advised to use a professional Integrated Development Environment (IDE).2 See Integrated Development Environments (page 540) for a list of IDEs.
About the Java Technology
Talk about Java technology seems to be everywhere, but what exactly is it? The next two sections explain how it is both a programming language and a platform.
The Java Programming Language
The Java programming language is a high-level language that can be characterized by all the following buzzwords:3
- Object oriented
- Architecture neutral
- High performance
With most programming languages, you either compile or interpret a program so that you can run it on your computer. The Java programming language is unusual in that a program is both compiled and interpreted. With the compiler, first you translate a program into an intermediate language called Java bytecodesthe platform-independent codes interpreted by the interpreter on the Java platform. The interpreter parses and runs each Java bytecode instruction on the computer. Compilation happens just once; interpretation occurs each time the program is executed. Figure 1 illustrates how this works.
Figure 1 Programs written in the Java programming language are first compiled and then interpreted.
You can think of Java bytecodes as the machine code instructions for the Java Virtual Machine (Java VM). Every Java interpreter, whether it's a development tool or a Web browser that can run applets, is an implementation of the Java VM.
Java bytecodes help make "write once, run anywhere" possible. You can compile your program into bytecodes on any platform that has a Java compiler. The bytecodes can then be run on any implementation of the Java VM. That means that as long as a computer has a Java VM, the same program written in the Java programming language can run on Windows 2000, a Solaris workstation, or on an iMac, as shown in Figure 2.
Figure 2 Programs can be written once and run on almost any platform.
The Java Platform
A platform is the hardware or software environment in which a program runs. We've already mentioned some of the most popular platforms, such as Windows 2000, Linux, Solaris, and MacOS. Most platforms can be described as a combination of the operating system and hardware. The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other, hardware-based platforms.
The Java platform has two components:
The Java Virtual Machine (Java VM)
The Java Application Programming Interface (Java API)
You've already been introduced to the Java VM. It's the base for the Java platform and is ported onto various hardware-based platforms.
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 of related classes and interfaces; these libraries are known as packages. The next section highlights what functionality some of the packages in the Java API provide.
Figure 3 depicts a program that's running on the Java platform. As the figure shows, the Java API and the virtual machine insulate the program from the hardware.
Figure 3 The Java API and the Java VM insulate the program from hardware dependencies.
Native code is code that, after you compile it, runs on a specific hardware platform. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.
What Can Java Technology Do?
The most common types of programs written in the Java programming language are applets and applications. If you've surfed the Web, you're probably already familiar with applets. An applet is a program that adheres to certain conventions that allow it to run within a Java-enabled browser. To see a running applet, go to this page in the online version of this tutorial:
There you can see an animation of the Java platform's mascot, Duke, waving at you:
However, the Java programming language is not just for writing cute, entertaining applets for the Web. The general-purpose, high-level Java programming language is also a powerful software platform. Using the generous API, you can write many types of programs.
An application is a standalone program that runs directly on the Java platform. A special kind of application known as a server serves and supports clients on a network. Examples of servers are Web servers, mail servers, and print servers.
Another specialized program is a servlet. A servlet can almost be thought of as an applet that runs on the server side. Java servlets are a popular choice for building interactive Web applications, replacing the use of CGI scripts. Servlets are similar to applets in that they are run-time extensions of applications. Instead of working in browsers, though, servlets run within Java Web servers, configuring or tailoring the server.
How does the API support all these kinds of programs? It does so with packages of software components that provide a wide range of functionality. Every full implementation of the Java platform 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 (Transmission Control Protocol), UDP (User Datagram Protocol) sockets, and IP (Internet Protocol) 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 and priveate key management, access control, and certificates.
Software components: Known as JavaBeanstm, can plug into existing component architectures.
Object serialization: Allows lightweight persistence and communication via RMI (Remote Method Invocation).
Java Database Connectivity (JDBCtm): Provides uniform access to a wide range of relational databases.
The Java platform also has APIs for 2D and 3D graphics, accessibility, servers, collaboration, telephony, speech, animation, and more. Figure 4 depicts what is included in the Java 2 SDK.
Figure 4 The Java 2 SDK, Standard Edition v. 1.3. The Java 2 Runtime Environment (JRE) consists of the Java VM, the Java platform core classes, and supporting files. The Java 2 SDK includes the JRE and development tools, such as compilers and debuggers.
This book covers the Java programming language and parts of the core API that beginning- to intermediate-level programmers will use most frequently. If you need additional information not found in this book, you can explore the other two books in The Java Tutorial series: The JFC Swing Tutorial and The Java Tutorial Continued. The contents of both books are included on the CD that accompanies this book and can be found in the online tutorial: