From where are you calling the destroyHinder method?

import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) /** * Write a description of class Missile here. * * @author (your name) * @version (a version number or a date) */ public class Missile extends Actor { /**int h = 0;**/ public void act() { setLocation(getX() + speed, getY()); checkBoundaries(); if (getWorld() != null) destroyHelicopter(); if (getWorld() != null) destroyHinder(); } //we add a method "checkBoundaries()" that destroys bullets that are off screen. public void checkBoundaries() { if (isAtEdge()) getWorld().removeObject(this); } //"destroyEnemies()" destroys enemies. public void destroyHelicopter() { Actor helicopter = getOneIntersectingObject(Helicopter.class); if(helicopter != null) { getWorld().removeObject(helicopter); Myworld myworld = (Myworld)getWorld(); myworld.addScore(20); } } public void destroyHinder() { Actor hinder = getOneIntersectingObject(Hinder.class); if (this != null) { getWorld().removeObject(this); health--; if (health == 0) getWorld().removeObject(Hinder); } } private int speed = 10; private int health = 3; }

Apparently, you have modified the destroyHinder method to the following: Line 4 is peculiar as this is never null. Plus, it does not make sense to remove a Hinder object when the health of a missile goes to zero (line 8). Anyway, getWorld on line 8 will fail to a NullPointerException as the missile will not be in a world to get at that point (because of line 6).

public void destroyHinder() { Actor Missile = getOneIntersectingObject(Missile.class); if (Missile != null) { getWorld().removeObject(Missile); health--; if (health == 0) getWorld().removeObject(this); } } private int health = 3; I changed the code and put it in my Hinder class instead

Then I called the method in my public void act! if(getWorld() != null)destroyHinder();

That should work.