To do this, you should be using KDE 3.1, but you can probably apply this guide to gnome.
Download the Java® installer (In auto-extracting format, not RPM), and copy it into /usr/local.
Open the /usr/bin directory as root from KDE,  right click the installer and click on permissions. Now make it executable and click on OK. Right click in a blank point oh your window manager, and select "Open a terminal here", then type
ls   (Remember to hit the enter key after every command you type)
You will get a list of the file in your home, including the java installer, now type
./put the name of the Java® installer here.
Read and accept the license, the installer will create a directory, move it to /usr/local and rename it java.
Open a new window in /usr
Now with one window open the java directory, the with the other one open /usr/bin, in the first window open the bin directory, select all the files, and drag them over to the second window (The one you used to open /usr/bin), you will get a little menu, click on link here and you've finished. You obviously have to do this as root. Java application will run for all the users just by clicking on them.
To enable the java plugin for Netscape®/Firefox®, open the java directory, open the plugin/i386/ns610-gcc32/ directory and copy libjavaplugin_oji.so to the Netscape/plugins or Firefox/plugins directory. and restart Netscape®/Firefox®