Actors not showing?

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

/**
 * Write a description of class MyWorld here.
 * 
 * @]author (your name) 
 * @version (a version number or a date)
 */
public class MainScreen extends World
{

    /**
     * Constructor for objects of class MyWorld.
     * 
     */
    public MainScreen()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(1000, 1000, 1); 
		String s = Greenfoot.ask("What group do you want to access?");
		if(s=="21")
		{
			Greenfoot.setWorld(new Group21());
    	}
		if(s=="22")
		{
			Greenfoot.setWorld(new Group22());
    	}
		if(s=="23")
		{
			Greenfoot.setWorld(new Group23());
    	}
		if(s=="24")
		{
			Greenfoot.setWorld(new Group24());
    	}
		if(s=="25")
		{
			Greenfoot.setWorld(new Group25());
    	}
    }
}

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

/**
 * Write a description of class Group21 here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Group21 extends World
{
    /**
     * Constructor for objects of class Group21.
     * 
     */
    public Group21()
    {    
        super(1000,1000,1);
        addObject(new Consecrate(),500,500);
    }
    public void act()
    {
        
    }
}