遇到了问题大家帮忙看看！

Microsoft Windows [版本 6.1.7600]
版权所有 (c) 2009 Microsoft Corporation。保留所有权利。

C:\Users\sl>javac HelloWorld.java

C:\Users\sl>java HelloWorld.java
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld/java
Caused by: java.lang.ClassNotFoundException: HelloWorld.java
      at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
      at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: HelloWorld.java.  Program will exit.
这是什么情况？？我头大了