In general, it's not easy to get a list of the classes which could be used by a Java program, whether they're in named packages or (as in Greenfoot) in the un-named package. The approaches I've seen rely on being able to look for all the .class files in the project folder(s), or in the Jar file used for a stand-alone application. I don't imagine that the Java Security Manager would let you scan a Jar file from a remote URL, so this technique might not work at all running an Applet from a web server (e.g. the Gallery).