protected void addedToWorld(World world)  
{  
    //my Counter   
}

protected void addedToWorld(World world)
{
    if (actCounter == null) actCounter = new Counter("Act Cycles: ");
    world.addObject(actCounter, 140, 50);
}

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

/**
 * Write a description of class Rocket here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Rocket extends Animal
{
    /**
     * Act - do whatever the Rocket wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act()
    {
        move(7);
        die();
        System.out.println(("Points:") +money ); //to check if my counter is displaying the correct things 
        if (getWorld() != null) 
        {
            kill(); 
        }
    }

    protected void addedToWorld(World world)
    {
        if (moneyCounter == null) 
        {moneyCounter = new Counter("Money: ");
        world.addObject(moneyCounter, 60, 40);
        }
        moneyCounter.setValue(moneyCounter.getValue() + money);
    }

    static int money;
    public Counter moneyCounter;

    public void die()
    {
        if(isTouching(Bullet.class))
        {

            money++;
            getWorld().removeObject(this);

        }
    }

    public void kill()
    {
        if(isTouching(Robot.class))
        {
            getWorld().removeObject(this);
            //Greenfoot.stop();
        }
    }
}

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

/**
 * Write a description of class MyWorld here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class MyWorld extends World
{

    /**
     * Constructor for objects of class MyWorld.
     * 
     */
    public MyWorld()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(900, 900, 1); 
        prepare();
        rocketSpawn(1);
    }
    
    public void act()
    {
        regulateRockets();
    }
    
    public void regulateRockets()
    {
        if (getObjects(Rocket.class).size() < 1)
        {
           rocketSpawn(1);
        }
    }

        public void prepare()
    {
        Robot robot = new Robot();
        addObject(robot , 450 ,450 );
    }
    
    public void rocketSpawn (int howMany)
    {
        for(int i=0; i<howMany; i++) {
            int border = Greenfoot.getRandomNumber(4);
            int position = Greenfoot.getRandomNumber(900);
            Rocket r = new Rocket();
            if(border == 1)
            {
                addObject(r ,position ,0);
            }else if(border == 2)
            {
                addObject(r ,0 ,position);
            }else if(border == 3)
            {
                addObject(r ,position ,900);
            }else if(border == 4)
            {
                addObject(r ,900 ,position);
            }
            r.turnTowards(450, 450);
        }
    }
}