Pong Continued: Moving Ball and Paddles

To begin with, here is the "Board" class code (my world):

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

/**
 * The Board class is my world.
 * @Robert Griffith
 * @11/05/2013
 */
public class Board extends World
{
    public Board()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(640, 480, 1); 
        prepare();
    }
    private void prepare()
    {
        //Counter counter = new Counter();
        //addObject(counter, 48, 19);
        Ball ball = new Ball();
        addObject(ball, 320, 240);
        Paddle1 paddle1 = new Paddle1();
        addObject(paddle1, 10, 240);
        Paddle2 paddle2 = new Paddle2();
        addObject(paddle2, 630, 240);
    }
}

Here is the moving Ball class code -- no bouncing your interaction yet:

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

/**
 * This is the Ball for our Pong game.
 * @Robert Griffith 
 * @11/05/2013
 */
public class Ball extends Actor
{
    int x = 320;
    int y = 240;
    int yDirection;
    int ballSpeed;
    int start = 0;

    /**
     * Runs the main program.
     */
    public void act() 
    {
        if (start == 0){ startup(); }
        x = x + ballSpeed;
        y = y + yDirection;
        setLocation(x, y);
    }    
    
        /**
        * Set the direction and speed of the Ball
        */
        public void startup(){  
        start++;
        if ( Greenfoot.getRandomNumber(100) < 50 ) 
        { yDirection = -3; } 
        else { yDirection = 3; }

        if ( Greenfoot.getRandomNumber(100) < 50 ) 
        { ballSpeed = -3; } 
        else { ballSpeed = 3; }
    }
}

Next we have the paddles. Here is Paddle1:

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

/**
 * The left paddle (Paddle1) class.
 * @Robert Griffith
 * @11/05/213
 */
public class Paddle1 extends Actor
{
    int y = 240;
    int paddleSpeed = 10;

    /**
     * Run the program.
     */
    public void act() 
    {
        checkKeys();
    }    

    /**
     *  Check keys and move the paddle
     */
    public void checkKeys()
    {
        if ( Greenfoot.isKeyDown("a"))
        {
            y=y-paddleSpeed;
            if (y < 48) {y = 48; } // y = half my paddle height
            setLocation(10, y); // Set the x coordinate at 10
        }
        if ( Greenfoot.isKeyDown("z"))
        {
            y=y+paddleSpeed;
            if (y > 432) {y = 432; } // y = world width - half my paddle height
            setLocation(10, y); // Set the x coordinate at 10
        }
    }
}

Note that Paddle2 is nearly identical to Paddle1 except for being further over on the x axis.