Java 1.1 is supported by Netscape 4.5 and MSIE 4 (and higher).
Java 1.2 and Java 1.5 require the Java Plugin.
On Windows or Linux you can download the Java plugin here.
Look for the "Java SE Runtime Environment (JRE)".
I'm told that you must use the "Software Update"
feature to update your Java version (on OSX 10.5 Leopard, anyway).
There may be some other Software Update items that have to be completed
first before you can even see the option of updating your Java version.