The *only* method called when you double-click your JAR file is main, so if you leave it empty, as you did, nothing will happen :).

You probably want to create a window, add an instance of your applet to that window, init() your applet and start() it. There are already several classes that do this which people have written - try http://www.acme.com/java/software/Acme.MainFrame.html (click "fetch the software", and put the .java file in a folder called Acme right inside your JAR). Then your main() should contain something like new Acme.MainFrame(new MainApp(), args, 400, 400);.