This Stack Overflow page has some suggestions for tools to do this. If you just want something basic then you can use our BlueJ tool to show a diagram. Make a new BlueJ project somewhere, close BlueJ then copy in all your Greenfoot classes and load BlueJ again. They won't compile because it won't find the Greenfoot classes, but it should at least show all the links between the classes in terms of inheritance and which class uses which.