Java for i386
Add the hoary-extras repository (AddingRepositoriesHowto)
Search for sun-j2re1.5 in synaptic and install it.
This will configure the Firefox Java plugin. Alternatively, you can build your own Java package by following the instructions on JavaPackageBuildNewVersion.
Java on other Architechtures
Ubuntu PPC, please see: ["JavaPPC"]. Ubuntu AMD64, please see: ["JavaAMD64"].
Installing the JDK
If you want to install the Java compiler as well as the runtime environment, the easiest thing to do is to [https://java.sun.com/j2se/1.5.0/download.jsp get it directly from Sun]. Take care to use the "Linux self-extracting file" version. Converting the RPM with alien will not work.
Then follow the instructions as set forth on JavaPackageBuildNewVersions to create a .deb file you can install using dpkg.