[link|http://www.google.com/search?q=%22java+HelloWorldApp.class%22|"java HelloWorldApp.class"]...

Make a habit of googling for your exceptions, too: [link|http://www.google.com/search?q=%22java.lang.NoClassDefFoundError%3a+HelloWorldApp/class%22|"java.lang.NoClassDefFoundError: HelloWorldApp/class"] (this works because it's a common tutorial name, apparently)

With respect to your "every tutorial" comment, go to the horse's mouth:
[link|http://java.sun.com/docs/books/tutorial/index.html|http://java.sun.com/...torial/index.html]
Specifically: [link|http://java.sun.com/docs/books/tutorial/getStarted/cupojava/unix.html|http://java.sun.com/...upojava/unix.html]

"Don't believe everything you read on the Internet." ;-)

Alternatively, don't read everything you believe on the Internet either, I suppose.