Extend JMenu to check the box?

I plan to combine JMenu and JCheckBoxMenuItem so that I can: (1) a popup menu containing instances of this new component. (2) this new component would have a flag on the left, and could also expand to the right and show other submenus (for example, the usual JMenu).

I had a few questions:

First, am I reinventing the wheel? / Was this done before?

Secondly, I am trying to understand how swing knows how to get the user interface class for the extended Component class? (I see that JMenu, for example, has a String uiClassID member that was somehow used for this, but that is not the exact name of the class ... I debugged it to search for HashTable in UIDefaults)