Nice to meet you. We always welcome help with fixings bugs and implementing new features - previous experience of Java isn't even always necessary, but if you are studying it already that is great.
What you've already done is pretty much right - looking for a bug you like the look of and then submitting a patch. Please feel free to come to talk to the developers in #arianne on irc.freenode.net to discuss which may be the best bugs or features to work on so that you're assured your efforts are heading in the right direction. If it happens that you're around when most of us our sleeping, we can still reply and you can read the channel logs later (check the channel topic for details.)
Another idea, since you have played the game, is to create a new quest. It's a good introduction to some of the core Stendhal code. We have a quest contribution guide - from the ideas stage through discussion with the team, coding and testing at http://stendhalgame.org/wiki/Stendhal_Quest_Contribution