Installing the JDK

You can obtain detailed instructions on how to install the JDK for your particular operating system from the Sun web site, so I won’t go into all the variations for different systems here. However, you should watch out for a few things that may not leap out from the pages of the installation documentation.

First of all, the JDK and the documentation are separate, and you install them separately. The JDK for Windows is available in two versions - as a web install where components are downloaded incrementally, and as a full download of an .exe file that you just execute to start installation. The documentation for the JDK consists of a large number of HTML files structured in a hierarchy that are distributed in a ZIP archive. You will find it easier to install the JDK first, followed by the documentation. If you install the JDK to drive C: under Windows, the jdk1.5.0 directory sometimes referred to as the root directory for Java.

In some contexts it is also referred to as the Java home directory. The actual root directory name may have the release version number appended, in which case the actual directory name will be of the form jdk1.5.0_n where n is a release number, so in the first maintenance release, it will be jdk1.5.0_01, for example.

The sample directory contains sample applications that use JNLP, which is the Java Network Launching Protocol that is used for executing applications or applets from a network server without the need for a browser or the need to download and install the code. You don’t need to worry about the contents of most of these directories, at least not when you get started, but you should add the path for the jdk1.5.0\bin directory to the paths defined in your PATH environment variable. That way you will be able to run the compiler and the interpreter from anywhere without having to specify the path to it. If you installed the JDK to C:, then you need to add the path C:\jdk1.5.0\bin. A word of warning - if you have previously installed a commercial Java development product, check that it has not modified your PATH environment variable to include the path to its own Java executables.

If it has, when you try to run the Java compiler or interpreter, you are likely to get the versions supplied with the commercial product rather that those that came with the JDK. One way to fix this is to remove the path or paths that cause the problem. If you don’t want to remove the paths that were inserted for the commercial product, you will have to use the full path specification when you want to run the compiler or interpreter from the JDK. The jre directory contains the Java Runtime facilities that are used when you execute a Java program. The classes in the Java libraries are stored in the jre\lib directory. They don’t appear individually though. They are all packaged up in the archive, rt.jar. Leave this alone. The Java Runtime takes care of retrieving what it needs from the archive when your program executes.

The CLASSPATH environment variable is a frequent source of problems and confusion to newcomers to Java. The current JDK does NOT require CLASSPATH to be defined, and if it has been defined by some other Java version or system, it is likely to cause problems. Commercial Java development systems and versions of the Java Development Kit prior to 1.2 may well define the CLASSPATH environment variable, so check to see whether CLASSPATH has been defined on your system. If it has and you no longer have whatever defined it installed, you should delete it. If you have to keep the CLASSPATH environment variable - maybe because you want to keep the system that defined it or you share the machine with someone who needs it - you will have to use a command-line option to define CLASSPATH temporarily whenever you compile or execute your Java code. This corresponds to C:\jdk1.5.0 if you installed the JDK to your C: drive. This will create a new subdirectory, docs, to the root directory, and install the documentation files in that. To look at the documentation, you just open the index.html file that is in the docs sub-directory.