Save the world is a funktion of Greenfoot to save every actor that is in the world at the moment. All actors and there current location will be writen down in a method in the world subclass that is written by Greenfoot itselves. So you don't have to write anything and Greenfoot will do all the work for you. And then if you compile your scenario and create a new world the next time the actory you have saven will be at the position you saved them.