HealthBar
import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
//import java.awt.Color;
/**
 * Write a description of class HealthBar here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class HealthBar extends Actor
{
    int health = 20;
    int healthBarWidth = 80;
    int healthBarHeight = 10;
    int pixelsPerHealthPoint = healthBarWidth/health;
    /**
     * Act - do whatever the HealthBar wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public HealthBar()
    {
        update();
        //death();
    }
    public void act() 
    {
        update();
        death();
    }    
    public void update()
    {
        setImage(new GreenfootImage(healthBarWidth + 2, healthBarHeight + 2));
        getImage().setColor(Color.WHITE);
        getImage().drawRect(0, 0,healthBarWidth + 1, healthBarHeight + 1);
        getImage().setColor(Color.RED);
        getImage().fillRect(1, 1, health * pixelsPerHealthPoint, healthBarHeight);
    }
    public void death()
    {
        if(health == 0)
        {
           Space space = (Space)getWorld(); //go to our world and run this method      
           space.addObject(new Explosion(),getX(),getY()); // rocket actor coordinates needed here!
           space.removeObject(this);// refers to the rocket (but how can I refer the rocket!) running the code  
           space.gameOver();//exam question!!
        }
    }
}

Space (my world)
import greenfoot.*;  // (World, Actor, GreenfootImage, and Greenfoot)


/**
 * Space. Something for rockets to fly in.
 * 
 * @author Michael Kolling
 * @version 1.0
 */
public class Space extends World
{
    private Counter scoreCounter;
    private int startAsteroids = 3;
    private int startStars = 2500;
    HealthBar healthbar = new HealthBar();
    public Space() 
    {
        super(600, 400, 1);
        GreenfootImage background = getBackground();
        background.setColor(Color.BLACK);
        background.fill();
        
        Rocket rocket = new Rocket();
        addObject(rocket, getWidth()/2 + 100, getHeight()/2);
        
        addAsteroids(startAsteroids);
        createStars(startStars);
        
        scoreCounter = new Counter("Score: ");
        addObject(scoreCounter, 60, 380);

        Explosion.initializeImages();
        ProtonWave.initializeImages();
        
        addObject(healthbar, 424, 34);
    }
    /**
     * 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()
    {
        scoreCounter.add(100);
    }
    /**
     * 
     */
    public HealthBar getHealthBar()
    {
        return healthbar;
    }
    /**
     * 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
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--;
            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 
    }
    // public void done()
    // {
           // 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  
           // space.gameOver();//exam question!!
    // }
    /**
     * 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;
        }
    }

}