Thanks,you are so kind.
Actually I have read the page you mentioned in RHE book,but I still confuesd.
Let me simplify my question.Why the answer of following question is "b" ??how about "a"?
In order to cause the paint(Graphics) method to execute, which of the following is the most appropriate method to call:
a.paint()
b.repaint()
c.paint(Graphics)
d.update(Graphics)
e.None – you should never cause paint(Graphics) to execute
Select the most appropriate answer.