i need a help

import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
/**
 * Sets up the SortGrid world. This includes creating all nodes
 * and numbers, connecting the nodes up and placing them in the world.
 *
 * @author Joseph Lenton
 */
public class SortGrid extends World
{
    private static final int WIDTH = 400;
    private static final int HEIGHT = 600;
    private static final int NUMBER_Y = 50;
    private static final int NUMBER_PADDING = 100;
    private static final int NUMBER_X = 150;
    private static final int NUM_POSITIONS = 2;
    
    /**
     * Creates and sets up the SortGrid world ready to be run.
     */
    public SortGrid()
    {
        super( WIDTH, HEIGHT, 1 );
        // create the network node
        Node first = new Node();final int leftX = WIDTH/2;
        addObject( first , leftX, 300 );
        // end nodes
        End[] ends = new End[ NUM_POSITIONS ];
        for ( int i = 0; i < ends.length; i++ ) {
            End end = new End();
            ends[i] = end;
            addObject( end, NUMBER_X + NUMBER_PADDING*i, HEIGHT-NUMBER_Y );
        }
        // create the connections
        first.setDestinations( ends[0], ends[1] );
        final int[] randomNums = {1, 2};
        final Number[] numbers = new Number[ NUM_POSITIONS ];
        // make the numbers
        for ( int i = 0; i < NUM_POSITIONS; i++ ) {
            final Number number = new Number( randomNums[i] );
            final int x = NUMBER_X + NUMBER_PADDING*i;
            final int y = NUMBER_Y;
            numbers[i] = number;
            addObject( number, x, y );
            addObject( new Start(), x, y );
        }
        // set the number destinations
        numbers[0].setDestination( first );
        numbers[1].setDestination( first );
        // paint ordering
        setPaintOrder(
            Number.class,
            Node.class,
            Position.class,
            Start.class
        );
      
    }
}