And here is the full error message: java.lang.NullPointerException at Player.move(Player.java:171) at Player.act(Player.java:33) at greenfoot.core.Simulation.actActor(Simulation.java:567) at greenfoot.core.Simulation.runOneLoop(Simulation.java:530) at greenfoot.core.Simulation.runContent(Simulation.java:193) at greenfoot.core.Simulation.run(Simulation.java:183)

It's a bit tricky without knowing the exact line numbers but the only things that I can see that can be null are Waffe and this.getImage(). (I think in more recent Java/Greenfoot, it should tell you what exactly was null, incidentally.) If Waffe can be null, you might prefer to write either "Blue".equals(Waffe) or Objects.equals(Waffe, "Blue"), both of which handle the possibility of Waffe being null without throwing an exception. If it's not Waffe, it's because your image is null.

Ok so now i dont get an error message, but i still cant move the player. The String Waffe is never null because its assigned by starting the game and changed inside the game. here is the code maybe you can see the problem: In this world is the play button: public class Start extends World { protected GreenfootSound music = new GreenfootSound("01 Title Theme.mp3"); private String Waffe; public Start() { super(1000, 700, 1); GreenfootImage bg = this.getBackground(); bg.scale(1000, 700); this.setBackground(bg); this.Waffe = "Pfeil"; prepare(); } private void prepare() { StartButton startButton = new StartButton(Waffe); addObject(startButton,242,221); startButton.setLocation(this.getWidth()/2,this.getHeight()/2); } This is the start button: public StartButton(String Waffen) { this.getImage().scale(this.getImage().getWidth()*3, this.getImage().getHeight()*3); this.Waffe = Waffen; } public void act() { this.setLocation(this.getWorld().getWidth()/2, this.getWorld().getHeight()/2); pPlayerX = 472; pPlayerY = 646; this.button(inventar, pPlayerX, pPlayerY, Waffe); } public void button(ArrayList inventar, int pPlayerX, int pPlayerY, String Waffe) { if(Greenfoot.mouseClicked(this)) //soll nur funktionieren wenn es den button klickt { this.Level = 0; Greenfoot.setWorld( new Lobby(inventar, pPlayerX, pPlayerY, rotationStart, Level, Waffe)); } } and this is the lobby in wich the player gets assigned the attributes: public Lobby(ArrayList inventar, int pPlayerX, int pPlayerY, int rotation, int Livel, String Waffen) { super(1000, 700, 1); this.getBackground().scale(this.getWidth()*2, this.getHeight()*2); this.pInventar = inventar; this.coordinatesX = pPlayerX; this.coordinatesY = pPlayerY; this.pRotation = rotation; this.Level = Livel; prepare(pInventar, pPlayerX, pPlayerY, pRotation, Waffe); this.Waffe = Waffen; } private void prepare(ArrayList pInventar, int pPlayerX, int pPlayerY, int pRotation, String Waffe) { Player player = new Player(Level, Waffe); addObject(player , coordinatesX , coordinatesY); player.setRotation(pRotation); } this is the code where the player gets assigned the attributes: public Player(int Livel, String Waffen) { this.getImage().scale(this.getImage().getWidth()*2, this.getImage().getHeight()*2); this.Level = Livel; this.Waffe = Waffen; } please look into it again

You show this code: The prepare method is being called before the Waffe variable is being assigned a value. No wonder its value is null. Switch the last two lines around.