If you use the NetBeans
IDE and the GUI builder that NetBeans has, and you use the wizard to create a new JFrame form, it will generate a class for you that extends JFrame (and probably the GUI builder even expects it to be that way).
Since your class that extends JFrame really represents the window you're creating, I don't see any problem in this case with using inheritance. If you'd use inheritance only because you need functionality from the superclass (and there is no clear "is a" relationship between the superclass and the subclass) then I'd object (
) to it.