Changing actors

Greenfoot.setWorld(new GameWorld(1));

public GameWorld(int shipType)
{
    super(100, 100, 1, false);
    addObject(new Ship(shipType), 50, 50);
}

private int type = 0;
public Ship(int type)
{
    this.type = type;
    updateImage();
}

private void updateImage()
{
    //update image based on ship type
}

public class Ship extends Actor {}
public class BattleShip extends Ship {}
public class ResearchShip extends Ship {}

public class Hero extends Actor
{
    private Ship ship = new BattleShip();
    
    public void act()
    {
        if (Greenfoot.isKeyDown("w")) {
            setLocation(getX(), getY() - 1);
        }
    }
    
    public void setShip(final Ship newShip) {
        getWorld().removeObject(ship);
        ship = newShip;
        getWorld().addObject(ship, getX(), getY());
    }
    
    @Override
    public void addedToWorld(final World newWorld) {
        setShip(ship);
    }
    
    @Override
    public void setLocation(int x, int y) {
        super.setLocation(x, y);
        ship.setLocation(x, y);
    }
    
}

public class ShipButton extends Actor
{
    private final Hero hero;
    
    private final Ship ship;
    
    public ShipButton(final Hero theHero, final Ship shipToChoose) {
        hero = theHero;
        ship = shipToChoose;
        setImage(ship.getImage());
    }
    
    public void act()
    {
        if (Greenfoot.mouseClicked(this)) {
            hero.setShip(ship);
        }
    }
}

import java.util.ArrayList;
import java.util.List;

public class ShipChooserMenu extends Actor
{
    
    final List<ShipButton> buttons = new ArrayList<>();
    
    public ShipChooserMenu(final Hero hero) {
        buttons.add(new ShipButton(hero, new BattleShip()));
        buttons.add(new ShipButton(hero, new ResearchShip()));
    }
    
    @Override 
    public void addedToWorld(final World world) {
        for (int i = 0; i < buttons.size(); ++i) {
            world.addObject(buttons.get(i), getX() + 150 * i, getY());
        }
    }
    
}

public class MyWorld extends World
{

    public MyWorld()
    {
        super(600, 400, 1); 
        prepare();
    }
    
    public void prepare() {
        final Hero hero = new Hero();
        addObject(hero, 100, 300);
        final ShipChooserMenu choose = new ShipChooserMenu(hero);
        addObject(choose, 50, 50);
    }
}