Nope, the applet cannot be compared directly to running the jar. The problem is that applets often require signing for what they can and cannot do. Here's a link for what they can and cannot do: http://docs.oracle.com/javase/tutori.../security.html
Here's the instructions for jar signing: http://docs.oracle.com/javase/1.5.0/...a_signing.html
. It is a big process yes, but if your application does anything listed in the first links section for what the unsigned applets "cannot" do, than you must sign the applet.
Let me take a step back here though. I didn't see your error the first time as being cannot find a class. Its been many years since I've written an applet, but like java.exe I'd expect that the .class on the codebase is not what you want. Remove the .class from the applet code attribute, and try again.