public class Explosion extends Actor
{
    /** How many images should be used in the animation of the explostion */
    private final static int IMAGE_COUNT= 8;

    /** 
     * The images in the explosion. This is static so the images are not
     * recreated for every object.
     */
    private static GreenfootImage[] images;

    /** Current size of the explosion */
    private int size=0;

    /** How much do we increment the index in the explosion animation. */
    private int increment=1; 

    public Explosion() {

        initialiseImages();
        setImage(images[0]);        
        Greenfoot.playSound("explosion.wav");

    }

    /** 
     * Create the images for explosion.
     */
    public synchronized static void initialiseImages() {
        if(images == null) {
            GreenfootImage baseImage = new GreenfootImage("explosion.png");
            int maxSize = baseImage.getWidth()*8;
            int delta = maxSize / (IMAGE_COUNT+1);
            int size = 0;
            images = new GreenfootImage[IMAGE_COUNT];
            for(int i=0; i < IMAGE_COUNT; i++) {
                size = size + delta;
                images[i] = new GreenfootImage(baseImage);
                images[i].scale(size, size);
            }
        }
    }

    /**
     * EXPLODE!
     */
    public void act() 
    {
        setImage(images[size]);

        size += increment;
        if(size>=IMAGE_COUNT) {
            increment = -increment;
            size += increment;
        }
        
        explodeOthers();
        if(size <= 0) {
            getWorld().removeObject(this);
        }
    }

    /**
     * Explodes all intersecting objects.
     */
    private void explodeOthers() 
    {        
        List explodeEm = getIntersectingObjects(null);        
        Iterator i = explodeEm.iterator();
        
        }
    }

public class Hero extends Actor
{
    /**
     * Act - do whatever the Hero wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        checkKeys();
    }    

    /**
     *Check wich key is being pressed
     */   

    public void checkKeys()
    {
        if(Greenfoot.isKeyDown("up"))
        {
            setLocation(getX(), getY()-4);
        }
        if(Greenfoot.isKeyDown("down"))
        {
            setLocation(getX(), getY()+4);
        }

    }      
}

public class Missile extends Actor
{
    /**
     * Act - do whatever the Missile wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        move(4);
        if(getOneIntersectingObject(Hero.class) != null) {
            explode();
        } 
        
    }

    /**
     * Make an explosion.
     */
    private void explode() {
        Space w;
        List hero = getWorld().getObjects(Hero.class);

        getWorld().addObject(new Explosion(), getX(), getY());
        getWorld().removeObject(this);
    }

    
}