Hi, i'm also developing a platforms style game in my spare time, and i've alredy developed that part of the game that you need, the Phisics Engine (that allows your "mario" to jump, and also slide) and a collision engine (that will allow "mario" to know where he's hitting) if you're interested i can send you the source code, it's not very well commented (because i didn't have time to do it) but it's simple enough for you to understand (if you come across a big problem i can help you out).
While browsing java-gaming.org i've came across a game that might interest you, it's a commander keen clone:
http://www.java-gaming.org/cgi-bin/JGOForums/YaBB.cgi?board=Announcements;action=display;num=1038469923I didn't have time to look into the game's source code but it's a pretty nice beggining of a game.
