All this may not sound too exciting to you. There are lots of object-oriented programming languages, after all, and with Mozilla plug-ins you can download executable programs from web servers and execute them on your local machine.

But Java is more than just an object-oriented programming language. One of its most exciting aspects is platform independence . That means you can write and compile your Java program and then deploy it on almost every machine, whether it is a lowly ’386 running Linux, a powerful Pentium IV running the latest bloatware from Microsoft, or an IBM mainframe. Sun Microsystems calls this “Write Once, Run Anywhere.” Unfortunately, real life is not as simple as design goals. There are tiny but frustrating differences that make a program work on one platform and fail on another. With the advent of the GUI library Swing, a large step was made toward remedying this problem.

The neat feature of compiling code once and then being able to run it on another machine is made possible by the Java Virtual Machine (JVM ), the piece of software the interprets the byte code generated by the Java compiler: The Java compiler does not generate object code for a particular CPU and operating system like gcc does — it generates code for the JVM. This “machine” does not exist anywhere in hardware (yet), but is instead a specification. This specification says which so-called opcodes the machine understands and what the machine does when it encounters them in the object file. The program is distributed in binary form containing so-called byte codes that follow the JVM specification.

Now all you need is a program that implements the JVM on your particular computer and operating system. These are available nowadays for just about any platform—no vendor can dare not provide a JVM for its hardware or operating system. Such programs are also called Java interpreters because they interpret the opcodes compiled for the JVM and translate them into code for the native machine.

This distinction, which makes Java both a compiled and an interpreted language, makes it possible for you to write and compile your Java program and distribute it to someone else, and no matter what hardware and operating system she has, she will be able to run your program as long as a Java interpreter is available for it.

Alas, Java ’s platform independence comes at a steep price. Because the object code is not object code of any currently existing hardware, it must pass through an extra layer of processing, meaning that programs written in Java typically run 10 to 20 times slower than comparable programs written in, for example, C. Although this does not matter for some cases, in other cases it is simply unacceptable. So-called just-in-time compilers are available that first translate the object code for the JVM into native object code and then run this object code. When the same object code is run a second time, the precompiled native code can be used without any interpretation and thus runs faster. But the speed that can be achieved with this method is still inferior to that of C programs. Newer compilers use a technology called just-in-time compilation, but the promise of an execution speed “comparable to C programs” has not been met yet, and it is doubtful whether it ever will.

Java also distinguishes between applications and applets. Applications are standalone programs that are run from the command line or your local desktop and behave like ordinary programs. Applets, on the other hand, are programs (usually smaller) that run inside your web browser. (To run these programs, the browser needs a Java interpreter inside.) When you browse a web site that contains a Java applet, the web server sends you the object code of the applet, and your browser executes it for you. You can use this for anything from simple animations to complete online banking systems.^[*]

When reading about the Java applets, you might have thought, “And what happens if the applet contains mischievous code that spies on my hard disk or even maybe deletes or corrupts files?” Of course, this would be possible if the Java designers had not designed a multistep countermeasure against such attacks: all Java applets run in a so-called sandbox, which allows them access only to certain resources. For example, Java applets can output text on your monitor, but they can’t read data from your local filesystem or even write to it unless you explicitly allow them. Although this sandbox paradigm reduces the usefulness of applets, it increases the security of your data. With recent Java releases, you can determine how much security you need and thus have additional flexibility. It should be mentioned that there have been reports of serious security breaches in the use of Java in browsers, although at least all known ones are found and fixed in most current web browsers.

If you decide that Java is something for you, we recommend that you get a copy of Thinking in Java (Prentice Hall). It covers most of the things you need to know in the Java world and also teaches you general programming principles. Other Java titles that are well worth looking into include Learning Java (O’Reilly) and Core Java (Prentice Hall).

Fortunately, there is a Linux port of the so-called JDK, the Java Developers Kit provided by Sun Microsystems for Solaris and Windows that serves as a reference implementation of Java. In the past, there was usually a gap between the appearance of a new JDK version for Solaris and Windows and the availability of the JDK for Linux. Luckily, this is no longer the case.

The “official” Java implementation JDK contains a compiler, an interpreter, and several related tools. Other kits are also available for Linux, often in the form of open source software. We cover the JDK here, though, because that’s the standard. There are other Linux implementations, including a very good one from IBM, as well; you might even have them on your distribution CDs.

One more note: most distributions already contain the JDK for Linux, so it might be easier for you to simply install a prepackaged one. However, the JDK is moving fast, and you might want to install a newer version than the one your distribution contains.

Your one-stop shop for Java software (including environments for Linux) is http://java.sun.com. Here, you will find documentation and news, and of course you can download a copy of the JDK for your machine.

After unpacking and installing the JDK according to the instructions, you have several new programs at your disposal. javac is the Java compiler, java is the interpreter, and appletviewer is a small GUI program that lets you run applets without using a full-blown web browser.