Thank you for your help, Tami and Loreta! I checked my file name over and over again, but it was not the problem... I still have this problem. I use TextPad to write the code at home, and then I have this problem. so I tried jGRASP on my school computer. Then, all of my programs did work!! Meanwhile, the text says "The keyboard class is not part of the Java standard class library. ..... You may have to configure your environment so that it knows where to find the Keyboard class." (P99-100). I think my application needs some work to overcome this problem. How do you think? Anyway, thanks a lot!
-Hiroe