First, before creating the world of Physics, it's necessary to create a new object from the Window class as in the exemplo:

Window win = new Window(800,600);

After that, we are able to create the world of Physics as in the example:

Physics p = new Physics();

Now with the object from the Physics class created, we only need to use the function createWorld, using as parameters the dimension of the screen.

p.createWorld(800,600);