Or you could try to do what you are most probably being asked and draw it yourself, instead of having a program generate it for. I can't imagine your project would be to auto generate a class diagram from your code....
There are a few tools that can help you out.
Netbeans 6.1 -
http://www.netbeans.org
This is a full IDE from sun and has capability for doing class diagrams, it might even auto generate them for you. It's free so nothing to lose if you don't like it.
Visual Paradigm -
http://www.visual-paradigm.com/
The sole purpose of this software is UML modeling and imo it does a pretty darn good job of it. You can download and use a free community edition. This does have diagram generation from code but you need to get one of pricey versions.
Those are 2 that I have used and found to be fairly decent. Netbeans was ok because the modeling was built into the IDE i use everyday anyway. But I found Visual Paradigm much much better, even the free version. It was easier to link up classes, rearrange things, adding methods and the diagrams were more compact and looked nice when exported and embedded into docs. The interface can be a bit intimidating to start with.
good luck!