Picture of the Relations

Perhaps a picture is needed. Clouds represent class definitions, solid rectangles represent objects, and dotted rectangles represent the static part of a class.

Look back at the example program as you study the diagram.

• ButtonDemo2 extends the class JFrame.
• The main() method is static, so is not part of any object. It contains two reference variables:
  • frm, which refers to a buttonDemo object, and
  • wquit, which refers to a WindowQuitter object
• There is a single ButtonDemo2 frame, which has a graphic representation on the screen.
• Events from the button are sent to the ButtonDemo2 frame.
  • The ButtonDemo2 frame is registered as a listener for Action Events.
  • The ButtonDemo2 class implements ActionListener.
• Events from the frame are sent to the WindowQuitter object.
  • The WindowQuitter object was registered as a listener for window events.
  • The WindowQuitter class extends WindowAdapter.

Don't be discouraged if this is less than perfectly clear! It usually takes some time to get all of these relationships straight. A bit more practice would not hurt, either.