Removing an actor

public void act() 
{
    if (Greenfoot.mouseClicked(this)) addObjects();
}

public void addobjects()
{
    World world = getWorld();
    world.removeObjects(world.getObjects(null));
    world.setBackground(new GreenfootImage("modification background.fw.png"));
    world.addObject(new skisbutton(), 3 , 2);
    world.addObject(new tracksbutton(), 5 , 2);
    world.addObject(new wheelsbutton(), 7 , 2);
    world.addObject(new largebaybutton(), 7 , 3);
    world.addObject(new mediumbaybutton(), 5 , 3);
    world.addObject(new smallbaybutton(), 3 , 3);
    world.addObject(new backbutton(), 8 , 7);
}

public void act()
{
    if (Greenfoot.mouseClicked(this)) Greenfoot.setWorld(new Menu(getWorld());
}
// with the new world Menu subclass:
import greenfoot.*;

public class Menu extends World
{
    private World main;

    public Menu(World world)
    {
        super(>width<, >height<, >cellsize<);
        main = world;
        setBackground(new GreenfootImage("modification background.fw.png"));
        addObject(new skisbutton(), 3 , 2);
        addObject(new tracksbutton(), 5 , 2);
        addObject(new wheelsbutton(), 7 , 2);
        addObject(new largebaybutton(), 7 , 3);
        addObject(new mediumbaybutton(), 5 , 3);
        addObject(new smallbaybutton(), 3 , 3);
        addObject(new backbutton(), 8 , 7);
    }

    public void act()
    {
        // code for button click detection here
        // examples
        if (>back button clicked<) back();
        if (>wheels button clicked<) applyWheels();
    }

    // button action methods here
    // examples
    private void back()
    {
        Greenfoot.setWorld(main);
    }

    private void applyWheels()
    {
        Player player = (Player) main.getObjects(Player.class).get(0);
        player.applyWheels(); // calls method in Player class to put wheels on
    }
}

getWorld().removeObjects(getWorld().getObjects(playButton.class));