Is jframe an abstract class?

Answer 1

I don't believe so. You do not have to implement any of its methods.

Answer 2

Math is just a final class. Not abstract.
Not sure why you'd think those other ones are. Remember an abstract class cannot be instantiated (Which those can).
You probably create strings all the time.

@Josh
EventListener is an interface.

Answer 3

I do not believe any of them are.
The eventListener class however might be.