Stumped in a space invader tutorial: why are these code for?

Question

EDIT: Hi thanks to those who viewed my post and attempted to help. I found the answer, it's called double buffering.
Anyway if you guys want to make additional infos or add something feel free to do so. Because I don't fully understand what's going on. (like what kind of graphic does getGraphics() return?)
Thanks again!
Hi, I'm following a tutorial about creating a space invaders game and it doesn't really explain much the objects and methods.
Here's what I've done so far:
import java.awt.Canvas;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import java.util.HashMap;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Invaders extends Canvas{
     static final int WIDTH = 800;
     static final int HEIGHT = 600;
     static final int SPEED = 10;
     HashMap sprites;
     int posX, posY, vX;
     BufferedImage buffer;
     Invaders(){
          sprites = new HashMap();
          posX = WIDTH/2;
          posY = HEIGHT/2;
          vX = 2;
          buffer = new BufferedImage(WIDTH,HEIGHT,BufferedImage.TYPE_INT_RGB);
          JFrame ventana = new JFrame("Space Invaders");
          JPanel panel = (JPanel)ventana.getContentPane();
          setBounds(0, 0, WIDTH, HEIGHT);
          panel.setPreferredSize(new Dimension(WIDTH,HEIGHT));
          panel.setLayout(null);
          panel.add(this);
          ventana.setBounds(0, 0, WIDTH, HEIGHT);
          ventana.setVisible(true);
          ventana.addWindowListener(new WindowAdapter(){@Override
          public void windowClosing(WindowEvent e) {
               // TODO Auto-generated method stub
               super.windowClosing(e);
          ventana.setResizable(false);
          ventana.setIgnoreRepaint(true);
     public BufferedImage loadImage(String name) {
          URL url = null;
          try {
               url = getClass().getClassLoader().getResource(name);
               return ImageIO.read(url);
          } catch (IOException e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
               return null;
     BufferedImage getSprite(String name){
          BufferedImage img = (BufferedImage)sprites.get(name);
          if (img == null){
               img = loadImage("res/"+name);
               sprites.put(name, img);
          return img;
     public void paintWorld(){
          Graphics g = buffer.getGraphics();
          g.setColor(getBackground());
          g.fillRect(0, 0, getWidth(), getHeight());
          g.drawImage(getSprite("bicho.gif"), posX, posY, this);
          getGraphics().drawImage(buffer, 0, 0, this);
     public void paint(Graphics g){}
     public void update(Graphics g) {}
     public void updateWorld() {
          posX += vX;
          if (posX < 0 || posX > WIDTH) vX = -vX;
     public void game(){
          while (isVisible()){
               updateWorld();
               paintWorld();
               paint(getGraphics());
               try {
                    Thread.sleep(SPEED);
               } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
      * @param args
     public static void main(String[] args) {
          // TODO Auto-generated method stub
          Invaders inv = new Invaders();
          inv.game();
}I tried reading the Java API but I don't get what it's saying for these:
buffer = new BufferedImage(WIDTH,HEIGHT,BufferedImage.TYPE_INT_RGB); //what is the meaning of TYPE_INT_RGB?
ventana.setIgnoreRepaint(true); //why ignore repaint?
public void paintWorld() {
Graphics g = buffer.getGraphics();
g.setColor(getBackground());
g.fillRect(0,0,getWidth(),getHeight());
g.drawImage(getSprite("bicho.gif"), posX, posY,this);
getGraphics().drawImage(buffer,0,0,this); //why add this line?
}and lastly:
      public void game() {
        while (isVisible()) {
         updateWorld();
         paintWorld();
         paint(getGraphics()); //why call paint here?
         try {
            Thread.sleep(SPEED);
         } catch (InterruptedException e) {}
     }I understand the rest of the code, it's just the parts that I've mentoined that I can't really understand.
I'd appreciate any help. Thanks! :)
Edited by: ajushi on Dec 16, 2007 2:38 PM
Edited by: ajushi on Dec 16, 2007 2:43 PM
Edited by: ajushi on Dec 16, 2007 2:52 PM

Answer 1

I got the sucker to draw but now it won't stop! You have to kill it's JVM with taskManager... and I haven't got a clue how to fix it.
package forums;
import java.util.Map;
import java.util.HashMap;
import java.awt.Canvas;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Container;
import java.awt.image.BufferedImage;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.io.IOException;
public class SpaceInvaders extends JFrame
  private static final long serialVersionUID = 0L;
  private static final int WIDTH = 800;
  private static final int HEIGHT = 600;
  private static final int SPEED = 100;
  private int posX = WIDTH/2;
  private int posY = HEIGHT/2;
  private int vX = 5;
  private BufferedImage buffer = null;
  private Container pane = null;
  private BufferedImage image = null;
  SpaceInvaders() {
    super("Space SpaceInvaders");
    super.setBounds(0, 0, WIDTH, HEIGHT);
    super.setResizable(false);
    super.setIgnoreRepaint(true);
    super.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE);
    super.setLayout(null);
    this.addWindowListener(
      new WindowAdapter() {
        @Override
        public void windowClosing(WindowEvent e) {
          super.windowClosing(e);
    pane = super.getContentPane();
    pane.setPreferredSize(new Dimension(WIDTH,HEIGHT));
    pane.setLayout(null);
    try {
      image = ImageIO.read(new URL("file:///c:/java/home/src/krc/games/bomb.jpg"));
    } catch (Exception e) {
      e.printStackTrace();
      throw new RuntimeException("failed to load image: "+e.toString());
    buffer = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
    super.setVisible(true);
  public void paintWorld() {
    Graphics g = buffer.getGraphics();
    g.setColor(getBackground());
    g.fillRect(0, 0, getWidth(), getHeight());
    g.drawImage(image, posX, posY, this);
    this.getGraphics().drawImage(buffer, 0, 0, this);
  public void paint(Graphics g){}
  public void update(Graphics g){}
  public void updateWorld() {
    posX += vX;
    if (posX < 0 || posX > WIDTH) {
      vX = -vX;
  public void play() {
    while (super.isVisible()){
      updateWorld();
      paintWorld();
      Thread.currentThread().yield();
      try{Thread.sleep(SPEED);}catch(InterruptedException eaten){}
  public static void main(String args[]) {
    try {
      java.awt.EventQueue.invokeLater(
        new Runnable() {
          public void run() {
            SpaceInvaders game = new SpaceInvaders();
            game.play();
    } catch (Exception e) {
      e.printStackTrace();
}