Abstract classes and interfaces in Java serve fundamentally different purposes. Learn the differences between these Java language elements and how to use them in your programs.

I've got a class in java for a swing application that extends JFrame. This class is pretty much reponsible for everything to do with a certain window in the program.

Java is not the language it used to be, and that is mostly a good thing. ... Sealed classes, for example, allow the class creator to specify exactly which classes can extend it.