BasicTreeUI has behavior which is common to both the focusLost and focusGained methods. Instead of placing this code in a separate method it was instead placed in focusGained and focusLost calls focusGained. This causes a problem with subclassing. In my UI subclass if I register my own focus listener but still want to use the superclasses implementation I can't. This is because if I call super.focusLost() it ends up calling my focusGained().

This common code in BasicTreeUI should be put in a separate method and the handlers should call that.