Make the Levels class the initial start-up world and have three constructor methods for it. The first with no parameters to zero the level int field and create and set the first level world active, and the other two to override the constructors of the World class (to pass on construction of the subclasses). Then you can make the level int field a static one, so it remains unaffected during level changes. If you are not quite sure about what I am trying to say to do, refer to my Level class, in this scenario.