You will need a java implementation - this package has been well tested with openjdk-6, but is also likely to work equally as well with other implementations (oraclejdk has also had minimal testing).
If you would like to build the rosjava_core documentation, you will also need Pygments 1.5+ and Sphinx 1.1.3+. If you don’t have native binaries, then
sudo pip install --upgrade sphinx Pygments
This java package no longer requires a ros environment to be installed. In this case, you simply need to clone the github repository
git clone https://github.com/rosjava/rosjava_core cd rosjava_core git checkout -b hydro origin/hydro
and proceed immediately to the section on Building.