import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
Here is my Tank and World code

/**
 * Write a description of class Tank here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Tank extends Animal
{
    /**
     * Act - do whatever the Tank wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        navigate();
    } 
    public void navigate()
    {
        if (Greenfoot.isKeyDown("w"))
        {
            move(1);
        }
        if (Greenfoot.isKeyDown("s"))
        {
            move(-1);
        }
        if (Greenfoot.isKeyDown("a"))
        {
            turn(-1);
        }
        if (Greenfoot.isKeyDown("d"))
        {
            turn(1);
        }
    }
}

import greenfoot.*;  // (Actor, World, Greenfoot, GreenfootImage)

public class TankWorld extends World
{
    private GreenfootSound sound = new GreenfootSound("Background.wav");
    /**
     * Create the crab world (the beach). Our world has a size 
     * of 560x560 cells, where every cell is just 1 pixel.
     */
    public TankWorld() 
    {
        super(1000, 560, 1);
        setUp();
    }
    public void act()
    {
        
    }
    public void setUp()
    {
        Tank tank = new Tank();
        addObject(tank, 280, 280);
        TankTurret tankTurret = new TankTurret();
        addObject(tankTurret, 280, 280);
        Tracker track = new Tracker();
        addObject(track, 280, 280);
    }
}