Which Classes and Interfaces to study?

[Randall Twede]

On many of the mock tests I have seen questions about details of a Class such as "what is the return type of method a of Class A?"
or "What method of Class A does this?". These are hard for me as I normally just look up stuff like that.
There are hundreds and hundreds of core Classes as you know. I would like a list of Classes and Interfaces that will most likely have this type of question about them.
I'll start the list.
String
Integer
Math

[madhumathi k]

Continued ...
java.util.EventObject getSource()
java.awt.AWTEvent getID()
Runnable's run()
EventListeners' methods