Animation in applets

Question

This is a prototype of a game that I am trying to create. It is supposed to create a large rectangle then with the user pressing the space bar, shoot a little square across the screen. But it doesnt't paint the square as its going across the screen. Here is the source:
import java.awt.event.*;
import java.awt.*;
import java.applet.*;
public class ShootBall extends Applet implements KeyListener
//dimensions of the rectangles
public int rect_x = -10;
public int rect_y = -10;
public int r_x = 0;
public int r_y = 0;
public int r_width = 20;
public int r_height = 10;
public int rect_width = 5;
public int rect_height = 5;
//moveable black rectangle
public Rectangle r;
static final int MIN_DELAY = 50;
public Rectangle rect;
// declare two instance variables at the head of the program
public Image dbImage;
public Graphics dbg;
// Instance variable AudioClip gun
public AudioClip gun;
//the threads that will run the different parts of the animations
public Thread th;
public int x_speed = 1;
//current background color for the applet
public Color backColor;
//this method overrides the init method from the Applet class
public void init()
// Load an audio file which is in the same directory as the class files of //the applet
gun = getAudioClip(getCodeBase(), "gun.au");
r = new Rectangle(r_x, r_y, r_width, r_height);
rect = new Rectangle(rect_x , rect_y, rect_width, rect_height);
backColor = Color.white;
addKeyListener(this);
th = new Thread();
/** Update - Method, implements double buffering */
public void update(Graphics g)
// initialize buffer
if (dbImage == null)
dbImage = createImage (this.getSize().width, this.getSize().height);
dbg = dbImage.getGraphics ();
// clear screen in background
dbg.setColor (getBackground ());
dbg.fillRect (0, 0, this.getSize().width, this.getSize().height);
// draw elements in background
dbg.setColor (getForeground());
paint (dbg);
// draw image on the screen
g.drawImage (dbImage, 0, 0, this);
//paints the rectangle at the updated position
public void paint(Graphics g)
g.setColor(Color.red);
setBackground(backColor);
g.fillRect(r_x, r_y, r_width, r_height);
g.drawRect(rect_x, rect_y, rect_width, rect_height);
//g.fillRect(rect.x, rect.y, rect.width, rect.height);
public void keyPressed(KeyEvent e)
//for this method, use the key code to generate movement
int keyCode = e.getKeyCode();
//move the rectangle
if(keyCode == KeyEvent.VK_LEFT)
r_x -= 5;
if(r_x < 0) r_x = 0;
repaint();
else if(keyCode == KeyEvent.VK_RIGHT)
r_x += 5;
if(r_x > getSize().width-r_width)
r_x = getSize().width-r_width;
repaint();
else if(keyCode == KeyEvent.VK_UP)
r_y -= 5;
if(r_y < 0) r_y = 0;
repaint();
else if(keyCode == KeyEvent.VK_DOWN)
r_y += 5;
if(r_y > getSize().height-r.height)
r_y = getSize().height-r_height;
repaint();
public void keyReleased(KeyEvent e)
Thread th = Thread.currentThread();
int key = e.getKeyCode();
// user presses left space bar
if (key == 32)
gun.play();
rect_x = r_x +20;
rect_y = r_y;
while(rect_x < 300 && Thread.currentThread() == th)
rect_x += 10;
repaint();
//update();
try
th.sleep(MIN_DELAY);
catch(InterruptedException ie) {}
public void keyTyped(KeyEvent e)
If you can find the problem please tell me,
thanks

Answer 1

First of all, try to make your event handlers as short as possible.
The reason why it doesn't draw anything is as follows:
- The Event thread calls your event handler
- Your event handler requests a repaint
- Your event handler doesn't return
Now you also need to know that the paint will be done only if you return from your handler :)
So you have two options:
A) use
g = getGraphics();
while () {
    paint(g); 
dispose(g);to force a repaint.
This will work, but is more or less a hack.
b) Create 2nd thread, and inside this thread do all your game stuff:
while (!gameOver) {
    if (isLeft) {
    } else if (isRight) {
    repaint();
    Thread.yield(); // or sleep
}And inside your event handler only set the isLeft, isRight variables.
For your bullets, create a small class like this:
public class Bullet {
    public Point pos;
    public Point speed;
    public Bullet(int x, int y) {
         pos = new Point(x,y);
         speed = new Point(0, -2);
    public void move() {
        pos.translate(speed.x, speed.y);
}If you want to fire a bullet, simply create a new one, and then move() all bullets in the above mentioned main loop.
Hope that helps..