I need help ;-;

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

/**
 * Write a description of class Play here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Play extends World
{
    private static int aliencount=1;
    /**
     * Constructor for objects of class Play.
     * 
     */
    public Play()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(800, 550, 1); 
        
        prepare();
    }
    
    public void act()
    {
        spawn();
    }
    
    /**
     * Prepare the world for the start of the program. That is: create the initial
     * objects and add them to the world.
     */
    private void prepare()
    {
        
        Rocket rocket = new Rocket();
        addObject(rocket,130, 286);
        
    }
    
    public void spawn()//spawn for aliens
    {
        if (getObjects(Alien.class).size() < alienCount)
        {
            addObject(new Alien(), (800), Greenfoot.getRandomNumber(550));
        }
    }
}