You can chose highlight mode for the actual file from "Edit -> Highlight mode", but that only works until the file is closed.
A permanet solution is to add *.java to the filemask used for c/c++-syntax highlighting: Click and chose "Settings -> Editor.. -> Syntax highlighting -> Syntax highlighting for: C/C++ -> Filemasks..." and add "*.java" .
EDIT:
:cry:
Not fast enough.