Recursively add files by pattern

Sergio Acosta's answer is probably your best bet if some of the files to be added may not already be tracked. If you want to limit yourself to files git already knows about, you could combine git-ls-files with a filter:

git ls-files [path] | grep '\.java$' | xargs git add

Git doesn't provide any fancy mechanisms for doing this itself, as it's basically a shell problem: how do you get a list of files to provide as arguments to a given command.