ObjectAID is the first UML tool you may actually use; I know I should talk about its features or something - but the real reason you may actually use this application is that it is Fast.
1. Run the wizard to create a new diagram
2. Drag and Drop your classes into the diagram
3. I have found the "auto image" setting helpful so it writes out an image (PNG,GIF,etc..) every time you make a change.
Here is one of the many diagrams pulled together for the GeoTools User Guide I have been rewriting.
You can install ObjectAID from the eclipse marketplace (in the help menu).