为什么编译通过了,模拟器跳出来一下就没了()

为什么编译通过了,模拟器跳出来一下就没了(在线等)
本人一新手,刚编了第一个MIDlet文件,出了这样状况.郁闷
哪位高手能教下该怎么办?
是配置问题吗?下面是本人console的消息:
Running   with   storage   root   DefaultGrayPhone
java.lang.ClassNotFoundException:   MainMidlet
at   com.sun.midp.midlet.MIDletState.createMIDlet(+14)
at   com.sun.midp.midlet.Scheduler.schedule(+52)
at   com.sun.midp.main.Main.runLocalClass(+28)
at   com.sun.midp.main.Main.main(+116)
Execution   completed.
694875   bytecodes   executed
5   thread   switches
740   classes   in   the   system   (including   system   classes)
3528   dynamic   objects   allocated   (95956   bytes)
1   garbage   collections   (0   bytes   collected)

------解决方案--------------------
java.lang.ClassNotFoundException: MainMidlet

这是错误啊