Java-Sandbox is a project mainly written in Java, it's free.
Test space for testing and teaching Java
Ryan is nuts. Really he is...
I've been hacking around with Java for 10+ years. I say "hacking" because its been more like bootstrapping code together rather than establishing any kind of mastery over the development process.
This Java Sandbox is my attempt to take a very simple code base and make sure I understand all of the key principles of the language and the tools involved in development.