I've used this handy Eclipse plugin for years. But very few of my colleagues were aware of it. This fact surprises me and therefore I would like to highlight it. It is very handy for generation of UML class diagrams from your code. It is also invaluable for analyzing of existing design.
One of the top UML tools is Enterprise Architect. But I was never happy with Enterprise Architect usability. I find it much easier to sketch class structure in Java and generate UML class diagrams by ObjectAid. Advantage of this approach is that you have basic skeleton of the module. You can check-it in, so that team can start working on implementation.
Have to mention that it’s commercial product, but Class Diagram version is free. For my UML class diagrams needs (design analyze, generating UML diagrams for documentation, modules design sketching) it was enough so far.
They provide also paid Sequence diagram and Diagram Add-On versions. To be honest I have never tried these. For sequence diagrams I was using only Enterprise Architect so far. I would be definitely pushing towards buying ObjectAid licenses if we wouldn’t have Enterprise Architect licenses already.
There’s no point for me to provide detailed description of it, because ObjectAid site contains short and explanatory overview. I suggest to start with One-Minute Introduction.
Everyone understand how inaccurate can UML class diagrams become. Keeping them up to date manually doesn’t make sense at all. ObjectAid fills this gap, because you can maintain your class diagrams up to date. But I had problems in the past when diagrams couldn’t handle renaming or moving of Java classes. Not sure if this is still problem of recent versions, because my diagrams are short lived. I can easily generate new diagram by just dragging appropriate classes into Class diagram editor. This is beauty of ObjectAid plugin.