Double Buffering help!

[Ryan Crichton]

Hi, I'm new here and I need some help with an Applet that I'm writing. Its a very basic applet that I'm writing just to help myself learn more java. I'm having a problem with implementing Double Buffering. The applet just displays nothing at all, just a white screen, when I run it. Could some one plz have a look at the code and help me out. I'm sure there is something silly that I'm missing. Here's the source:

/**
 * 
 */
package topscroller;
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
 
/**
 * @author Ryan
 *
 */
public class RacerApp extends Applet implements Runnable, KeyListener{

Thread t;

Graphics gBuffer;
Image offscreen;

GameObject[] gameObjects;

boolean leftPressed,rightPressed = false;

public void init()
{
offscreen = createImage(size().width,size().height);
gBuffer = offscreen.getGraphics();

this.resize(150,200);
t = new Thread(this);
t.start();
gameObjects = new GameObject[10];
gameObjects[0] = new Car(75,180);
gameObjects[1] = new Obstacle(50,0,0,3);
addKeyListener(this);
}

public void run()
{
while(true)
{
if (leftPressed)
{
((Car)gameObjects[0]).moveLeft(3);
if (gameObjects[0].getX() > size().width-5)
gameObjects[0].setX(size().width);
if (gameObjects[0].getX() < 5)
gameObjects[0].setX(5);
}
if (rightPressed)
{
((Car)gameObjects[0]).moveRight(3);
if (gameObjects[0].getX() > size().width-5)
gameObjects[0].setX(size().width-5);
if (gameObjects[0].getX() < 5)
gameObjects[0].setX(5);
}

repaint();

try {
Thread.sleep(1000/30);
} catch (InterruptedException e) 
{}
}
}

public void paint(Graphics g)
{
gBuffer.clearRect(0,0,size().width,size().height);

drawThings();//shouldn't have to take in a garphics object - draw to buffer
 
g.drawImage(offscreen,0,0,this);
}

public void update(Graphics g)
{
paint(g);
}

public void drawThings()
{
gBuffer.setColor(Color.gray);//use buffer
gBuffer.fillRect(0,0,size().width,size().height);//use buffer

for (int j = 0 ; j < gameObjects.length ; j++)
{
gameObjects[j].redraw(gBuffer);//use buffer
}
}

public void keyPressed(KeyEvent e)
{
if (e.getKeyCode() == KeyEvent.VK_LEFT)
{
leftPressed = true;
}
if (e.getKeyCode() == KeyEvent.VK_RIGHT)
{
rightPressed = true;
}
}
 
public void keyReleased(KeyEvent e)
{
if (e.getKeyCode() == KeyEvent.VK_LEFT)
{
leftPressed = false;
}
if (e.getKeyCode() == KeyEvent.VK_RIGHT)
{
rightPressed = false;
}
}
 
public void keyTyped(KeyEvent arg0) {
// TODO Auto-generated method stub

}
}

Also any general help will be appreciated as I'm sure my programming style needs work :roll: Also I haven't included any of the classes that this applet uses as I don't think that they are needed (If they are I'l post them)
Thanx in advance

[Craig Wood]

One possible problem is initializing offscreen in the init method. There may not be any size info or graphics context available unitl the Applet has been realized, ie, made visible. The size method is deprecated, see the method detail in the Component api.
Here are some suggestions/possibilities in uncompiled pseudo–code:

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
 
/**
* @author Ryan
*
*/
public class RacerApplet extends Applet implements Runnable, KeyListener{
 
    Thread t;
 
    Graphics gBuffer;
    Image offscreen;
 
    GameObject[] gameObjects;
 
    boolean leftPressed,rightPressed = false;
 
    public void init()
    {
        this.resize(150,200);
        gameObjects = new GameObject[10];
        gameObjects[0] = new Car(75,180);
        gameObjects[1] = new Obstacle(50,0,0,3);
        addKeyListener(this);
    }
 
    /** applet method */
    public void start()
    {
        t = new Thread(this);
        t.start();
    }
 
    /** applet method */
    public void stop()
    {
        t.interrupt();
        t = null;
    }
 
    public void run()
    {
        while(true)
        {
            if (leftPressed)
            {
                ((Car)gameObjects[0]).moveLeft(3);
                if (gameObjects[0].getX() > size().width-5)
                    gameObjects[0].setX(size().width);
                if (gameObjects[0].getX() < 5)
                    gameObjects[0].setX(5);
            }
            if (rightPressed)
            {
                ((Car)gameObjects[0]).moveRight(3);
                if (gameObjects[0].getX() > size().width-5)
                    gameObjects[0].setX(size().width-5);
                if (gameObjects[0].getX() < 5)
                    gameObjects[0].setX(5);
            }
 
            drawThings();  // draw to buffer
            repaint();
 
            try {
                Thread.sleep(1000/30);
            } catch (InterruptedException e) {
                System.out.println("interrupted " + e);
            }
        }
    }
 
    public void paint(Graphics g)
    {
        // this is one way to make sure that you can/do
        // get valid size info and a graphics context
        if(offscreen == null)
        {
            int w = getWidth();
            int h = getHeght();
            offscreen = createImage(w, h);
            gBuffer = offscreen.getGraphics();
        }
        g.drawImage(offscreen,0,0,this);
    }
 
    public void update(Graphics g)
    {
        paint(g);
    }
 
    public void drawThings()
    {
        int w = getWidth();
        int h = getHeght();
        //works with a call to setBackground
        //gBuffer.clearRect(0,0,size().width,size().height);
        gBuffer.setColor(Color.gray);        //use buffer
        gBuffer.fillRect(0,0,w,h);           //use buffer
 
        for (int j = 0 ; j < gameObjects.length ; j++)
        {
            gameObjects[j].redraw(gBuffer);  //use buffer
        }
    }
 
    public void keyPressed(KeyEvent e)
    {
        if (e.getKeyCode() == KeyEvent.VK_LEFT)
        {
            leftPressed = true;
        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT)
        {
            rightPressed = true;
        }
    }
 
    public void keyReleased(KeyEvent e)
    {
        if (e.getKeyCode() == KeyEvent.VK_LEFT)
        {
            leftPressed = false;
        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT)
        {
            rightPressed = false;
        }
    }
 
    public void keyTyped(KeyEvent arg0) {
        // TODO Auto-generated method stub
    }
}

[Ryan Crichton]

Thanx man, I'l try your suggestions as soon as I get Home (or get to a computer with Eclipse ) I'l let you know how it went as soon as I get to test it out.

Thanx for the quick responce.

[Keith Lynn]

I normally put the initialization of the off screen image and off screen graphics in the init method, but you can use the getWidth() and getHeight() methods.

public void init()
{
offscreen = createImage(getWidth(),getHeight());
gBuffer = offscreen.getGraphics();

this.resize(150,200);
t = new Thread(this);
t.start();
gameObjects = new GameObject[10];
gameObjects[0] = new Car(75,180);
gameObjects[1] = new Obstacle(50,0,0,3);
addKeyListener(this);
}

[Ryan Crichton]

Thanx guys I got the buffer working now. Your help was much appreciated