no class error

 Can any body help me with an error i am trying to fix.
i have sucessfully compiled visad with the help from curtis, but the thing
is that if i try to run any programm after sucessfully compiling the
program, i get an error that saids:

C:\brobbs>java Rivers
Exception in thread "main" java.lang.NoClassDefFoundError: Rivers

i get this error for all the programs that i have compiled and tried to run.
it may sound dumb, but i need help on this.

thanks,

Isaac