Just quickly (you've probably worked this out) but moving the call to updateImage() from the constructor to a method named public void addedToWorld(World w) should fix the problem.

Hi, i still don't understand how to fix the NullPointerException. If someone could give me the right piece of code i would be very pleased. import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) import java.awt.Color; public class GameOver extends Actor { String msgTxt = "GAME OVER"; public GameOver() { updateImage(); } public GameOver(String txt) { msgTxt = txt; updateImage(); } private void updateImage() { GreenfootImage image = new GreenfootImage(getWorld().getWidth(), getWorld().getHeight()); // or whatever size you want the object image.setColor(Color.cyan); // color of your choice image.fill(); GreenfootImage txtImg = new GreenfootImage(msgTxt, 36, Color.black, new Color(0, 0, 0, 0)); // colors and font size of your choice image.drawImage(txtImg, (image.getWidth() - txtImg.getWidth()) / 2, (image.getHeight() - txtImg.getHeight() / 2)); // whatever else you might want to do for the image setImage(image); } }

The problem is that you are calling 'updateImage', which uses 'getWorld', from the constructor. The object is not yet in a world when being constructed, hence your NullPointerException ('getWorld' returns 'null'). To avoid this, you can update the image from the 'addedToWorld' method which is automatically called when the object is placed in a world.