I think your main problem is the order in which you execute the parts of the construction of the background. Obviously, you should set the background image last. And maybe you should fill the image with green and then draw the black frame around it (before setting the image to the background). Unless you are going to be creating Walls of different sizes other than the cell-size of the world, it is not necessary to overload the constructor of the Wall class. In fact, the parameter can be removed and you can use 'OppositeWorld.CELL_SIZE' to get that value (because the 'CELL_SIZE' field is 'public static', you can just use the class name followed by the field name, as shown, to get that value from anywhere in your project).

I tried doing what you said, but now the game is just a green background with a black square in it, and there are no grids. Also, I still do not know how to fit the black square within a square in the grid. Here is the code for the World class: import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) import java.awt.Color; public class OppositeWorld extends World { public final static int CELL_SIZE = 40; public OppositeWorld() { super(30, 30, 20, false); GreenfootImage img = new GreenfootImage(CELL_SIZE, CELL_SIZE); img.setColor(Color.GREEN); img.fill(); img.drawRect(0, 0, CELL_SIZE-1, CELL_SIZE-1); setBackground(img); addObject(new Wall(), 3, 3); } } Here is the code for the Wall class: import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) public class Wall extends Actor { public Wall() { this(40);//call the other constructor } /** * Overload the constructor */ public Wall(int size) { GreenfootImage img = new GreenfootImage(size,size); img.fill(); setImage(img); } }

For one thing, I am not sure if you intended this: but, your world cell-size is 20 and yet you are using 40 for a grid size. Maybe you meant to set CELL_SIZE to 20 (but I could be mistaken). Then, your wall would also need to be sized at 20.