Killer Game Programming in Java

The AWT Imaging Model

JDK 1.0 introduced the AWT imaging model for downloading and drawing images. Back then, it was thought that the most common use of imaging would involve applets pulling graphics from the Web. A standard '90s example (with the exception of using JApplet) is shown in Example 5-1.

Example 5-1. ShowImage applet (Version 1) using Image

import javax.swing.*;
import java.awt.*;

public class ShowImage extends JApplet
{
  private Image im;

  public void init()
  {  im = getImage( getDocumentBase(), "ball.gif");  }

  public void paint(Graphics g)
  {  g.drawImage(im, 0, 0, this);  }
}

The getDocumentBase() method returns the URL of the directory holding the original web document, and this is prepended to the image's filename to get a URL suitable for getImage().

The central problem with networked image retrieval is speed. Consequently, the Java designers considered it a bad idea to have an applet stop while an image crawled over from the server side. As a result, we have confusing behavior for getImage() and drawImage(). Neither of these do what their name implies. The getImage() method is poorly named since it doesn't get (or download) the image at all; instead it prepares an empty Image object (im) for holding the image, returning immediately after that. The downloading is triggered by drawImage() in paint(), which is called as the applet is loaded into the browser after init() has finished.

The fourth argument supplied to drawImage() is an ImageObserver (usually the applet or JFrame ...