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.

You can use git add [path]/\*.java to add java files from subdirectories,
e.g. git add ./\*.java for current directory.

From git add documentation:

Adds content from all *.txt files under Documentation directory and its subdirectories:

$ git add Documentation/\*.txt

Note that the asterisk * is quoted from the shell in this example; this lets the command include the files from subdirectories of Documentation/ directory.