I need a directory to store application data.
of course this will be different on Windows, Mac, or Linux.

i think these are the best directorys for this

for windows i need

[current user directory]/AppData/Local

on OS X i need

[current user directory]/Library/Application Support

detect the platform!!

System.getProperty("os.name")

--- Update ---

and you need the username too
String username = System.getProperty("user.name");