import greenfoot.*;  // (World, Actor, GreenfootImage, and Greenfoot)
import java.util.List;

/**
 * A rocket that can be controlled by the arrowkeys: up, left, right.
 * The gun is fired by hitting the 'space' key.
 * 
 * @author Poul Henriksen
 * @author Michael Kölling
 * 
 * @version 2.0
 */
public class Rocket extends Mover
{
    private int gunReloadTime;              // The minimum delay between firing the gun.
    private int reloadDelayCount;           // How long ago we fired the gun the last time.
    private Vector acceleration;            // A vector used to accelerate when using booster.
    private int shotsFired;                 // Number of shots fired.

    private GreenfootImage rocket = new GreenfootImage("rocket.png");
    private GreenfootImage rocketWithThrust = new GreenfootImage("rocketWithThrust.png");

    /**
     * Initialise this rocket.
     */
    public Rocket()
    {
        gunReloadTime = 20;
        reloadDelayCount = 0;
        acceleration = new Vector(0, 0.3);    // used to accelerate when thrust is on
        increaseSpeed(new Vector(13, 0.3));   // initially slowly drifting
        shotsFired = 0;
    }

    /**
     * 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()
    {
        move();
        checkKeys();
        checkCollision();
        reloadDelayCount++;
    }

    /**
     * Return the number of shots fired from this rocket.
     */
    public int getShotsFired()
    {
        return shotsFired;
    }

    /**
     * Set the time needed for re-loading the rocket's gun. The shorter this time is,
     * the faster the rocket can fire. The (initial) standard time is 20.
     */
    public void setGunReloadTime(int reloadTime)
    {
        gunReloadTime = reloadTime;
    }

    /**
     * Check whether we are colliding with an asteroid.
     */
    private void checkCollision() 
    {

        Asteroid a = (Asteroid) getOneIntersectingObject(Asteroid.class);
        if (a != null) 
        {
            getWorld().addObject(new Explosion(), getX(), getY());
            getWorld().removeObject(this);
            getWorld().removeObjects(getWorld().getObjects(Asteroid.class));
        }
    }

    /**
     * Check whether there are any key pressed and react to them.
     */
    private void checkKeys() 
    {
        ignite(Greenfoot.isKeyDown("up"));

        if(Greenfoot.isKeyDown("left")) {
            turn(-5);
        }        
        if(Greenfoot.isKeyDown("right")) {
            turn(5);
        }
        if(Greenfoot.isKeyDown("space")) {
            fire();
        }        
    }

    /**
     * Should the rocket be ignited?
     */
    private void ignite(boolean boosterOn) 
    {
        if (boosterOn) {
            setImage(rocketWithThrust);
            acceleration.setDirection(getRotation());
            increaseSpeed(acceleration);
        }
        else {
            setImage(rocket);        
        }
    }

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

I am attempting to remove all of the Asteroids from the world once the rocket has been removed. But I keep getting a run time error on my code: getWorld().removeObjects(getWorld().getObjects(Asteroid.class));

In addition, I want an image to appear after the World is clear.