Exception in thread "main" java.lang.NoClassDefFoundError

Hi,
I am a newbie to Java. I just installed Java on my laptop and ran some
example programs. And I got the message that says Exception in thread
"main" java.lang.NoClassDefFoundError even though this example program
is very simple and it's from a book. I would appreciate it if you give
me some hints why this happens.

Thank you very much.

.