I personally use this event as an opportunity to stretch my Java skills outside of their comfort zone by doing simple tasks with more advanced or complex tools than necessary. I make small projects which quickly get me into new territory.
A couple other people use this time to ask questions or follow along with the wacky project of the hour(such as Hwacha Invaders or Kitten Pong).
Some still use the opportunity to just sit down and work on their code(Java and otherwise) in an environment of other programmers.
Currently, I'm using this time for a more practical pursuit: I am making a program to help sell our surplus equipment as well as inventory and map our non-surplus equipment. I intend to make this as a java program which will be available as Open Source and Free to all Hackerspaces.
Come in find your chair and find a programming friend.