Space Code:

import greenfoot.*;  // (World, Actor, GreenfootImage, and Greenfoot)
import java.util.List;
/**
 * Space. Something for rockets to fly in.
 * 
 * @author Michael Kolling
 * @version 1.0
 */
public class Space extends World
{
    private Counter scoreCounter;
    private Counter levelCounter;
    private int startAsteroids = 2;
    public int asteroidNumber = 2;
    public int round = 1;
    private int startStars = 2500;
    private boolean explosionFound;
    HealthBar healthbar = new HealthBar();
    public Space() 
    {
        super(600, 400, 1);
        GreenfootImage background = getBackground();//use this to add text, the first line before the equal sign
        background.setColor(Color.BLACK);
        background.fill();

        Rocket rocket = new Rocket();
        addObject(rocket, getWidth()/2 + 100, getHeight()/2);

        addAsteroids(startAsteroids);
        createStars(startStars);

        //addAsteroids2(level2Asteroids);
        scoreCounter = new Counter("Score: ");
        addObject(scoreCounter, 60, 380);

        // levelUpper = new Levelupp("Level: ");
        // addObject (levelUpper, 580, 380);
        levelCounter = new Counter("Level: ");
        addObject(levelCounter, 540, 380);
        levelCounter.add(1);

        Explosion.initializeImages();
        ProtonWave.initializeImages();

        addObject(healthbar, 518, 30);
    }

    /**
     * Add a given number of stars to our world..
     */
    private void createStars(int number) 
    {
        GreenfootImage background = getBackground();
        for(int i = 0; i < number; i++) 
        {
            int x = Greenfoot.getRandomNumber(getWidth());
            int y = Greenfoot.getRandomNumber(getHeight());
            int r = Greenfoot.getRandomNumber(256);
            background.setColor(new Color(r,r,r));
            //int g = Greenfoot.getRandomNumber(256);
            //int b = Greenfoot.getRandomNumber(256);
            //background.setColor(new Color(r,g,b));
            background.fillOval(x, y, 4 , 4); //the two two controls the size of my stars!
            //we add the same r value to make it gray if we would have many then we would get rainbow!
        }
    }

    /**
     * Add a given number of asteroids to our world. Asteroids are only added into
     * the left half of the world.
     */
    private void addAsteroids(int count) 
    {
        for(int i = 0; i < count; i++) 
        {
            int x = Greenfoot.getRandomNumber(getWidth()/2);
            int y = Greenfoot.getRandomNumber(getHeight()/2);
            addObject(new Asteroid(), x, y);
        }

    }

    /**
     * 
     */
    public void countScore(int add)
    {
        scoreCounter.add(100);
        // score += add;
    }
    /**
     * to increase the # of asteroids after increase in levels
     */
    public void nextRound(int add)
    {
        asteroidNumber += add;
        if(asteroidNumber == 0)
        {
            round++;
            levelCounter.add(1);
            addAsteroids(round + 1);
            asteroidNumber = round + 1;
            List<Rocket> rockets = getObjects(Rocket.class);
        for ( Rocket rocket : rockets )
        {
            rocket.next();
        }
        }
    }
    /**
     * this is to get our healthbar info back!
     */
    public HealthBar getHealthBar()
    {
        return healthbar;
    }
    /**
     * if Explosion occurs then game over, if it doesn't, continue
     */
    public void act()
    {

        if (!explosionFound && !getObjects(Explosion.class).isEmpty()) explosionFound = true;
        if (explosionFound && getObjects(Explosion.class).isEmpty()) gameOver();

    }
    /**
     * This method is called when the game is over to display the final score.
     */
    public void gameOver() 
    {
        addObject(new ScoreBoard(scoreCounter.getValue()), getWidth()/2, getHeight()/2); // the coordinates div
        // TODO: show the score board here. Currently missing.
    }

}

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

/**
 * A rocket that can be c   ontrolled by the arrowkeys: up, left, right.
 * The gun is fired by hitting the 'space' key. 'z' releases a proton wave.
 * 
 * @author Poul Henriksen
 * @author Michael Kolling
 * 
 * @version 1.0
 */
public class Rocket extends SmoothMover
{
    private static final int gunReloadTime = 5;         // The minimum delay between firing the gun.
    private static final int protonWaveCalm = 250;      //minimum delay between activating prton wave
    private int reloadDelayCount;                       // How long ago we fired the gun the last time.
    private int reloadPowerCount;                       // How long ago we activated the prton wave
    private GreenfootImage rocket = new GreenfootImage("rocket.png");    
    private GreenfootImage rocketWithThrust = new GreenfootImage("rocketWithThrust.png");
    private boolean yes = false;
    
    /**
     * Initilise this rocket.
     */
    public Rocket()
    {
        reloadDelayCount = 5;
        reloadPowerCount = 50;
        addForce(new Vector(getRotation(), 0.3));
    }

    /**
     * Do what a rocket's gotta do. (Which is: mostly flying about, and turning,
     * accelerating and shooting when the right keys are pressed.)
     */
    public void act()
    {
        checkKeys();
        reloadDelayCount++; //increases our counter for bullets
        reloadPowerCount++; //increases our counter for proton waves
        ignite (Greenfoot.isKeyDown("up"));
        move();
        checkCollision();//leave last in our act mehthod (order matters (if rocket is gone, rocket can't check keys)
    }

    /**
     * Check whether there are any key pressed and react to them.
     */
    private void checkKeys() 
    {
        if (Greenfoot.isKeyDown("space")) 
        {
            fire();
        }
        if (Greenfoot.isKeyDown("left")) 
        {
            //get my rotatio, and then based off of that, set my rotation
            //to change by 5 degrees
            setRotation(getRotation()-5);
        }
        if (Greenfoot.isKeyDown("right")) 
        {
            //get my rotatio, and then based off of that, set my rotation
            //to change by 5 degrees
            setRotation(getRotation()+5);
        }
        ignite(Greenfoot.isKeyDown("up")); //isKeyDown provides the
        if (Greenfoot.isKeyDown("z"))
        {
            //to add a prton wave into our world when this key is pressed
            StartProton();

        }
    }

    /**
     * changes between two rocket images, depending on if the up key is pressed 
     * adds force in the direction we are facing if up is pressed
     */
    private void ignite (boolean boosterOn)
    {
        if (boosterOn)
        {
            setImage ("rocketWithThrust.png");
            addForce(new Vector(getRotation(), 0.3));//we are asking for direction get the rotation we are already pointing in
        }
        else
        {
            setImage ("rocket.png");
        }
    }

    /**
     * Fire a bullet if the gun is ready.
     */
    private void fire() 
    {
        if (reloadDelayCount >= gunReloadTime) 
        {
            Bullet bullet = new Bullet (getMovement().copy(), getRotation());
            getWorld().addObject (bullet, getX(), getY());
            bullet.move ();
            reloadDelayCount = 0;
        }
    }

    /**
     * method checks to see if we have collided with an asteroid, if we have remove rocket and add explosion BOOM!
     */
    private void checkCollision()
    {
        //Actor a = getOneIntersectingObject(Asteroid.class);
        //if (a != null)                       // a way to simplify (a is storing the intersecting object line)
        //{ World world = getWorld(); and then what follows which are the two line in my if statement below
        if (getOneIntersectingObject(Asteroid.class) != null && yes == false)// as long as it doesn't equal nothing
        //instead of going to world now it goes to space but space is a world so inherited methods still not affected for other objects
        { //this is casting , the use of this in the below using space instead of world
            Space space = (Space)getWorld(); //go to our world and run this method      
            HealthBar healthbar = space.getHealthBar();
            healthbar.health--;
            healthbar.adjustValue(-1);
            if (healthbar.health == 0)//ERROR HERE!!!!!!
            {
               space.addObject(new Explosion(),getX(),getY()); // short answer question of statements make difference remember why did it not work when this line of 
               space.removeObject(this); // refers to the rocket running the code  
               yes = true;
                //space.gameOver(); //exam question!!! Why do we get an error here? //another question! us doing casting
            }
        }
            else if (getOneIntersectingObject(Asteroid.class) != null)
            {
                yes = false;
            }
            //check if colliding with asteroid
            //if we are
            //remove rocket
            //add explosion
            //game over
            //another place to see casting is in Newton's project in the smoothmover method With the double values into 
        }
        
        // Space space = (Space)getWorld(); //go to our world and run this method      
        // space.addObject(new Explosion(),getX(),getY()); // short answer question of statements make difference remember why did it not work when this line of 
        // space.removeObject(this); // refers to the rocket running the code  
      
        /**
         * Fire up a proton if the count is ready and it has already calmed down.
         */
        private void StartProton() 
        {
            if (reloadPowerCount >= protonWaveCalm) 
            {
                ProtonWave protonWave = new ProtonWave ();
                getWorld().addObject (protonWave, getX(), getY());
                reloadPowerCount = 0;
            }
        }
        /**
         * 
         */
        public void next()
        {
            getWorld().removeObject(this);
            int x = Greenfoot.getRandomNumber(getWidth());
            int y = Greenfoot.getRandomNumber(getHeight());
            addObject(new Rocket(), x, y);
        }
    }