Java Hello world, First day

Exception in thread "main" java.lang.NoClassDefFoundError: hello/class

java_home=E:\jdk
classpath=.;E:\jdk\lib\tools.jar;E:\jdk\lib
path=.;E:\jdk

I searched this error in google. I think my environment is right, Why?
I use "java hello.class"
Be supposed to use "java hello", not have extension.