If your ability in Java is very good and you're willing to throw ethics out of the window, then doing assignments for other students can easily bring in $30 (~£25) per easy assignment, but as a student yourself with assignments to do, this probably wouldn't work out anyway, and I wouldn't condone it myself.
In terms of making applications that other people would want to pay for, then all you need to do is think of a problem you and others face daily which could be made so much easier by an automated application. However, this is easier said than done. You might find many little things you can program to make your life easier, but these utility programs would rarely be sought after by other people, but are amazing for improving your Java skills.
That's all I've got for you I'm afraid, best of luck making your millions