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.

GeoTools FeatureSource

You can install ObjectAID from the eclipse marketplace (in the help menu).