private boolean clickable;//check whether the button can be clicked;

public Button() {//the constructor of your button class;
    setImage(new GreenfootImage(1, 1));
    clickable = false;
}

public void act() {
    if (clickable && Greenfoot.mouseClicked(this)) {
        //button clicked -> next level;
    }
}

public void activate() {
    clickable = true;
    setImage("theImagesFileName.jpg");
}

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Weiter1 here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Weiter1 extends Objekte
{
    private boolean clickable;    //Überprüfen ob der Button anklickbar ist  
    private Counter counter;
         
    public Weiter1() 
    {  
        setImage(new GreenfootImage(69,377));  
        clickable = false;  
    }  
      
    public void act()
    {  
        if (clickable && Greenfoot.mouseClicked(this))
          {  
            Greenfoot.setWorld(new Level2());   // Erstellt das neue Level 
         }  
    }  
      
    public void activate()
    {  
        if (counter.getValue()==3)
        {
        clickable = true;  
        setImage("weiter.jpg");
        }
    }    
 
  
}