مساعدة في فهم كود

بسم الله الرحمن الرحيم

السلام عليكم

أنا حالياً أدرس هذا الكود و هناك شيء لا أفهمه به : الكود هو :

package dk.koderko.games.pong;

import java.io.IOException;
import javax.microedition.lcdui.game.GameCanvas;
import javax.microedition.lcdui.game.Sprite;
import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.Image;

public class PongCanvas extends GameCanvas implements Runnable {

	public PongCanvas() {
		super(false);
	}

	public void run() {

		while(true) {

			updateScreen(getGraphics());

			try {
				Thread.sleep(sleepTime);
			} catch (Exception e) {

			}
		}

	}

	public void start() {

		try {
			ballImg = Image.createImage("/ball.png");
		} catch (IOException ioex) {
			System.out.println(ioex);
		}

		ballSprite = new Sprite(ballImg, 3, 3);
		ballSprite.defineReferencePixel(2, 2);
		ballSprite.setRefPixelPosition(ballX, ballY);

		Thread runner = new Thread(this);
		runner.start();
	}

	private void createBackground(Graphics g) {
		g.setColor(0x000000);
		g.fillRect(0, 0, getWidth(), getHeight());
	}

	private void updateScreen(Graphics g) {

		createBackground(g);

		moveBall();

		ballSprite.setRefPixelPosition(ballX, ballY);
		ballSprite.paint(g);

		flushGraphics();

	}

	private void moveBall() {
									// NOTE: I have deleted the two lines previously present here

		if (ballDirection == 0) {				// Added: All of the lines below
			ballX -= ballXVel;
			ballY -= ballYVel;
		} else if (ballDirection == 1) {
			ballX += ballXVel;
			ballY -= ballYVel;
		} else if (ballDirection == 2) {
			ballX += ballXVel;
			ballY += ballYVel;
		} else if (ballDirection == 3) {
			ballX -= ballXVel;
			ballY += ballYVel;
		}

		if (ballDirection == 0 && ballX < 0) {
			ballDirection = 1;
		} else if (ballDirection == 0 && ballY < 0) {
			ballDirection = 3;
		} else if (ballDirection == 1 && ballY < 0) {
			ballDirection = 2;
		} else if (ballDirection == 1 && ballX > getWidth()) {
			ballDirection = 0;
			if (sleepTime > 5) sleepTime--;
		} else if (ballDirection == 2 && ballY > getHeight()) {
			ballDirection = 1;
		} else if (ballDirection == 2 && ballX > getWidth()) {
			ballDirection = 3;
			if (sleepTime > 5) sleepTime--;
		} else if (ballDirection == 3 && ballY > getHeight()) {
			ballDirection = 0;
		} else if (ballDirection == 3 && ballX < 0) {
			ballDirection = 2;
		}							// Added: All of the lines above
	}

	private int sleepTime = 30;

	private Image ballImg;
	private Sprite ballSprite;
	private int ballX = getWidth() / 2;
	private int ballY = getHeight() / 2;
	private final static int ballXVel = 3;
	private final static int ballYVel = 1;
	private int ballDirection = 1;					// Added

}

و ما لا أفهمه بالكود هو :

if (ballDirection == 0) {
			ballX -= ballXVel;
			ballY -= ballYVel;
		} else if (ballDirection == 1) {
			ballX += ballXVel;
			ballY -= ballYVel;
		} else if (ballDirection == 2) {
			ballX += ballXVel;
			ballY += ballYVel;
		} else if (ballDirection == 3) {
			ballX -= ballXVel;
			ballY += ballYVel;
		}

		if (ballDirection == 0 && ballX < 0) {
			ballDirection = 1;
		} else if (ballDirection == 0 && ballY < 0) {
			ballDirection = 3;
		} else if (ballDirection == 1 && ballY < 0) {
			ballDirection = 2;
		} else if (ballDirection == 1 && ballX > getWidth()) {
			ballDirection = 0;
			if (sleepTime > 5) sleepTime--;
		} else if (ballDirection == 2 && ballY > getHeight()) {
			ballDirection = 1;
		} else if (ballDirection == 2 && ballX > getWidth()) {
			ballDirection = 3;
			if (sleepTime > 5) sleepTime--;
		} else if (ballDirection == 3 && ballY > getHeight()) {
			ballDirection = 0;
		} else if (ballDirection == 3 && ballX < 0) {
			ballDirection = 2;
		}

أرجو المساعدة ......:( و هذه الصورة توضح الاتجاهات :