The error appears to be occurring in the Rocket class (although, with this type of error, the problem may be anywhere). First, however, post your entire Rocket class (including imports).

import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) /** * Write a description of class Rocket here. * * @author (your name) * @version (a version number or a date) */ public class Rocket extends Actor { /** * Act - do whatever the Rocket wants to do. This method is called whenever * the 'Act' or 'Run' button gets pressed in the environment. */ MouseInfo mouse = Greenfoot.getMouseInfo(); public void act() { Controls(); } private void Controls() { Thread ctl = new Thread() { public void run() { while(true) { if(Greenfoot.mouseClicked(getWorld())) //null = any click, not only on this object { if(Greenfoot.mouseDragged(getWorld())) { MouseInfo mouse = Greenfoot.getMouseInfo(); setLocation(mouse.getX(), getY()); //set y to mouse y, not x } } } } }; ctl.start(); } } but now i have an other problem it says in the error log that: JDWP exit error AGENT_ERROR_INVALID_EVENT_TYPE(204): ExceptionOccurred

You should avoid using the Thread class in greenfoot. Also, greenfoot does the initial looping for you that creates the running of the simulation. So, a 'run' method is not needed. As such, you do not program for the life of the actor, but for each instance the actor exists. What this means is that you can remove the 'while(true)' condition. The 'Controls' method would then be: The comment at the end of the 'setLocation' line says to keep the x and change the y to mouse y, so I took the liberty to adjust the code.