Counter Class Error

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.awt.Color;
import java.util.List;

/**
 * Chocolate Dispenser Mchine
 * 
 * 
 * 
 * @version 1.0
 */
public class Counter extends Actor
{
    private int credit;
    private boolean moneyFound  = false;

    /**
     * Act - do whatever the Counter wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act()
    {
        coinFound();
        noteFound();
        getCredit();
        update();
    }

    /**
     * Counting all Coins in Money class, which are inserted into the Coin Insertion.
     */
    public void coinFound()
    {
        if (!moneyFound && !getWorld().getObjectsAt(517, 314, Money.class).isEmpty())
        {
            moneyFound = true; 
            List<Money> money2 = getWorld().getObjectsAt(517, 314, Money.class);
            Money money = money2.get(0);
            credit += money.getValue();
        }

        if (moneyFound && getWorld().getObjectsAt(517, 314, Money.class).isEmpty())
        {
            moneyFound = false;
        }
    }

    /**
     * Counting all Notes in Money class, which are inserted into the Note Insertion.
     */
    public void noteFound()
    {
        if (!moneyFound && !getWorld().getObjectsAt(664, 367, Money.class).isEmpty())
        {
            moneyFound = true;
            List<Money> money2 = getWorld().getObjectsAt(664, 367, Money.class);
            Money money = money2.get(0);
            credit += money.getValue();
        }

        if (moneyFound && getWorld().getObjectsAt(664, 367, Money.class).isEmpty())
        {
            moneyFound = false;
        }
    }

    /**
     * 
     */
    public int getCredit()
    {
        return credit;
    }

    /**
     * 
     */
    public void update()
    {
        ((Screen) getWorld().getObjects(Screen.class).get(0)).setText("Credit: " + credit);
    }
}