Cannot find symbol - method scroll()

Question

I am working on a simple Mario side-scroller, and When I try to call the Scrolling objects' scroll() method, I get the Cannot find symbol - method scroll() error thrown. Please help!
Here is the Subworld file

import greenfoot.*;
import java.util.ArrayList;
/**
 * Write a description of class MarioWorld here.
 * 
 * @author (your name) 
 * @version (a version number or a 


public class MarioWorld extends World
{
    ArrayList<MovingActor> moving = new ArrayList<MovingActor>();
    ArrayList<Actor> things = new ArrayList<Actor>();
    Message messagebox = new Message("");
    /**
     * Constructor for objects of class MarioWorld.
     * 
     */
    public MarioWorld()
    {
        super(800, 600, 1);
        for(int i = 0; i < 40; i++)
            {
                things.add(new GreenApple());
                things.add(new Shamrock());

            }
            for(int i = 0; i<8;i++)
                things.add(new Brick());
            moving.add(new Mario());
            for(int r = 0;r < things.size();r++)
            {
                addObject(things.get(r),Greenfoot.getRandomNumber(800),Greenfoot.getRandomNumber(600));
            }
    }
    public void act()
    {

       for(int i = 0;i < things.size();i++)
            {
                things.get(i).scroll();
            }
            addObject(moving.get(0),15,300);
            moving.get(0).worldact();
    }


}
`

Here is one of the 3 scrolling classes(they are identical except for Name/picture

 import greenfoot.*;
public class Shamrock extends Actor implements Scrollable
{
    /**
     * Act - do whatever the Shamrock wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */

    public Actor worldact()
    {
        scroll();
        return this;
    }
    public void scroll()
    {
        int x = getX();
        int y = getY(); 
        if(x <= 0)
            x = 800;
        x -= 6;
        setLocation(x,y);

    }
}

Here is the 'Scrollable' interface

import greenfoot.*;

public interface Scrollable  
{
 public void scroll();
}

Answer 1

Here moving is a object of ArrayList not Shamrock so you can call only arraylist Class method with moving Object like add() remove().

Here ArrayListis generic type Safedarraylist. So, you need to remove theobjectfrom the arraylist then only try to apply methodScroll()`

Use this code

((Shamrock)things.get(i)).scroll(); 

instead of things.get(i).scroll();