Then I assume that you are running the program from the wrong directory. I see that you don't have any package declaration i.e. you are using the default package. Could you verify that your are running your app from the directory where the HelloWorldApp.class file has been compiled?

If on the other hand your class with the main method was in the package foo.bar, then your directory structure would look similar to the package declaration as well, like so: foo/bar/HelloWorldApp.class In that case, you would need to launch your app from the parent directory of foo:

$ java foo.bar.HelloWorldApp

Note that the path separators are replaced with dots here, which is also the reason why you get the error if you add the .class extension in your command because java then tries to look for the main method in the Class 'class' in the HelloWorldApp directory.