One of our KdG students pointed us to the SequenceDiagram plugin for the IntelliJ IDE. The plugin reverse engineers a method into a UML sequence diagram. You click on a method and get a graphical view of its inner workings and interaction with other objects.
It also has a nice solution for representing lambda's, something the UML standard does not have specific support for.
That's not all, from each method in the sequence diagram you can navigate back to your code.
Only remark I have, is that return arrows are not entirely compliant with the standard notation.
But this is a nice effort, and way better respecting UML conventions than IntelliJ's own class diagram generator.