if(getX() < 10 || getX() > getWorld().getWidth() + 10) 

    if(getX() < 10 || getX() > getWorld().getWidth() - 10)   

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

/**
 * Write a description of class DarkPower here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class DarkPower extends Actor
{
    public DarkPower(int rotation)
    {
        setRotation(rotation);
    }

    /**
     * Act - do whatever the DarkPower wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        move(2);
        tryToEat(); 
    }

    public void tryToEat()
    {
        if (canSee(Starfighter.class) )
        {
            eat(Starfighter.class);
            gameOver();
        }
        if (canSee(Starfighter2.class) )
        {
            eat(Starfighter2.class);
            gameOver();
            Label1 label1 = new Label1();
            getWorld().addObject(label1, 450, 350);
        } 
        if (canSee(Starfighter3.class) )
        {
            eat(Starfighter3.class);
            gameOver();
            Label2 label2 = new Label2();
            getWorld().addObject(label2, 450, 350);
        } else {
            if(atWorldEdge())
            {
                getWorld().removeObject(this);
            }   
        }
    }

    public void gameOver()
    {
        Greenfoot.stop();
        GameOver gameover = new GameOver();
        getWorld().addObject(gameover, getWorld().getWidth()/2, getWorld().getHeight()/2);
        Greenfoot.playSound("gameover.wav");
        Greenfoot.stop();
        GameOver2 gameover2 = new GameOver2();
        getWorld().addObject(gameover2, getWorld().getWidth()/2, getWorld().getHeight()/2);
        Greenfoot.playSound("gameover.wav");
    }

    public boolean atWorldEdge()
    {
        if(getX() < 10 || getX() > getWorld().getWidth() + 10) 
            return true;
        if(getY() < 10 || getY() > getWorld().getHeight() + 10)
            return true;
        else
            return false;
    }

    public boolean canSee(Class clss)
    {
        Actor actor = getOneObjectAtOffset(0, 0, clss);
        return actor != null;        
    }

    public void eat(Class clss)
    {
        Actor actor = getOneObjectAtOffset(0, 0, clss);
        if(actor != null) {
            getWorld().removeObject(actor);
        }
    }
}    

if(getX() < 10 || getX() > getWorld().getWidth() - 10)   
            return true;  
        if(getY() < 10 || getY() > getWorld().getHeight() - 10)  
            return true;