public void draw(Graphics g) {
        
        g.setColor(Color.black);
        for (Wall wall : walls)
            g.drawRect(wall.getX()-1, wall.getY()-1, wall.getWidth()+1, wall.getHeight()+1);      // Shape
        
        g.setColor(Color.GRAY);
        for (Wall wall : walls)
            g.fillRect(wall.getX(), wall.getY(), wall.getWidth(), wall.getHeight());
    }