This information about obtaining a Java runtime is primarily for users with Mathematica 4.0 and 4.1. Beginning with version 4.2, the Mathematica distribution includes its own Java runtime for use by J/Link. An exception to this is the Macintosh, which already has a Java runtime suitable for J/Link.
Here are some useful links to help you obtain a suitable Java runtime or full JDK (Java Development Kit) for your platform. Many systems already have a recent Java runtime installed. Try the following command line to see if you have Java installed and, if so, what version it is:
It is strongly recommended that you get the latest version of Java available for your platform. In particular, the jump from the 1.1 to the 1.2 version of Java was very significant. You should be using a 1.3 or later version if at all possible.
You will sometimes see the abbreviations JRE and JDK when dealing with Java distributions. JRE stands for Java Runtime Environment, and JDK stands for Java Development Kit. A JRE has just the tools needed to run existing Java programs, not compile new ones. If you want to write your own Java language programs, you will need the full JDK.
Java Development Kits are available for free on all platforms that Mathematica supports.
Sun and IBM both have excellent Java distributions. Before you download one, try the java -version command; you may already have at least a JRE installed on your machine somewhere. J/Link has been tested most extensively on Sun's Java, which is currently at version 1.4 and can be obtained from http://java.sun.com/j2se.
J/Link is available in a native version for Mac OS X. The current version of J/Link is not supported on System 8 or 9, although older versions of J/Link work. All OS X users already have an appropriate Java installation to run J/Link.
Java distributions are available from Blackdown, IBM, and Sun. Some very recent Linux distributions include Java, but many users will have to download it. Sun's 1.4 version for Linux is available at http://java.sun.com/j2se. IBM has an excellent Java for Linux, available at http://www.ibm.com/developerworks/java/jdk/index.html. The Blackdown version is also popular, and can be obtained from http://www.blackdown.org.
Java can be obtained from http://www.sun.com/software/solaris/java. The 1.4 version is available now.
Java can be obtained from http://www.ibm.com/developerworks/java/jdk/aix. Java versions 1.2.2 and later are available for AIX 4.3.3 and later.
Java can be obtained from http://www.hp.com/products1/unix/java.
HP Tru64 Unix
Java can be obtained from http://www.hp.com/java.