public Space()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(800, 600, 1); 
        addObject (gameover, 400, 300);
        addObject (lifeboard, 500, 10);
        addObject (new Player(), 400, 550);
        addObject (new Invaders(), 0, 0);
        addObject (new Barriers(), 0, 0);
        addObject (scoreboard, 100, 10);
        setPaintOrder(GameOver.class,Player.class,Scoreboard.class);
    }

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Barrier here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Barrier extends Actor
{
    /**
     * Act - do whatever the Barrier wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */    
    private GreenfootImage base1;
    private GreenfootImage base2;
    private GreenfootImage base3;
    private GreenfootImage base4;
    
    public void act() 
    {
        Actor invadermissile = getOneIntersectingObject(InvaderMissile.class);
        if(invadermissile != null)
        {
            if (getImage().equals(base1)) 
            {
                setImage(base2);
            }
            else 
            if (getImage().equals(base2)) 
            {
                setImage(base3);
            }
            else 
            if (getImage().equals(base3)) 
            {
                setImage(base4);
            }
            else 
            if (getImage().equals(base4)) 
            {
                getWorld().removeObject(this);
            }
        }
        
        Actor playermissile = getOneIntersectingObject(PlayerMissile.class);        
        if(playermissile != null)
        {
            if (getImage().equals(base1)) 
            {
                setImage(base2);
            }
            else 
            if (getImage().equals(base2)) 
            {
                setImage(base3);
            }
            else 
            if (getImage().equals(base3)) 
            {
                setImage(base4);
            }
            else 
            if (getImage().equals(base4)) 
            {
                getWorld().removeObject(this);
            }
        }
    }    

    public void addedToWorld(World world)
    {
        base1 = new GreenfootImage("base1.png");
        base2 = new GreenfootImage("base2.png");
        base3 = new GreenfootImage("base3.png");
        base4 = new GreenfootImage("base4.png");
    }
}

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Barriers here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Barriers extends Actor
{
    /**
     * Act - do whatever the Barriers wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    
    public void act() 
    {
        if(getWorld() .getObjects(Barrier.class) .size() == 0)
        {
            for(int x = 450; x < 451; x++)
            {
                for(int y = 300; y < 301; y++)
                {
                    getWorld() .addObject(new Barrier(), x * 1000 + 30, y * 100+ 30);
                }
            }
        }
    }    
}