playing any java game on linux that uses opengl is problematic after java7 got released as you will have to add the java library folder to LD_LIBRARY_PATH before launching the game.
the official runescape wiki have a few methods listed for running the game, everything from running the game client through wine, to unofficial client ports or java-wrappers
easiest would be to install the RuneScape Unix Client port that is under active development by me as it contains several optimization and tweaking options to improve the clients performance
sudo apt-add-repository ppa:hikariknight/unix-runescape-client
sudo apt-get update
sudo apt-get install unix-runescape-client