1 2 3 if (getWorld() instanceof Level1) // or if (Level1.class.isInstance(getWorld()))

Of all the lines given, line 4 is the only one that I recognize as being a valid one. It does not check if a world is active, however. It checks the world that the actor is in for it to be of a specific type, regardless of the active state of that world. I am not aware of any method that tells you if a world is active or not by returning a boolean (true/false) value. Just the fact that its act method is automatically executed is enough to say it is the active world. Maybe you are confused about when a change in worlds takes place. Using setWorld does not instantly set the new world active. The current act call must still complete its course before the new world is actually set active.

oh now I get it that's why It doesnt work..

I mean data not date

Like I said, you need to break that one line up so you create the new world on one line and set that world active on another: The first line assigns the new world to a local variable, so that you can reference that world and add the actor to it.

public class Karakter1 extends Actor { public void act() { if(Greenfoot.mouseClicked(this)){addedToWorld();} } public void addedToWorld() {World mainLoc = new MainLoc(); Greenfoot.setWorld(mainLoc); getWorld().addObject(new Vera(),100,450); } }