This project (GemaGame) is basically a Java Applet Game that I am making to farther my programming skills. It will change rapidly, probably.
You can use w/s/a/d to move, and up/left/right/down to shoot.
All Sides Collision with Platforms. You can add a platform with one function, and it is auto-displayed and collided with.
You can try out the Applet HERE: http://dr.newlanders.me/play-gema-game.html
I try to keep that web page updated with the most recent version, but no guarantees. Usually has the most recent version *that works*.

You can create levels as well, and save them as long as you are using the downloadable version.