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An Example of the Factory Method (p 107)

Intent

Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

An example from Sun's Java Development Kit (JDK beta)

The general purpose abstract class, Component, provides a good deal of functionality including the ability to repaint a component (an Applet is a component), set or get Font styles, post or handle events, show or hide the component, etc. The Component class defines a Factory Method called createImage which creates new objects of the class Image. Applets (like the example below) often use this method to implement double-buffering.

Source code - Using the Component's createImage factory method

import java.awt.Graphics;
import java.awt.*;

public class Animator2 extends java.applet.Applet implements Runnable
{
  final int MAX_IMAGES = 9;
  Thread animThread = null;
  Image frame[] = new Image[MAX_IMAGES];
  Image offScrImg;
  Graphics offScrGraphics;
  boolean imagesLoaded = false;
  final int PauseSeconds = 380;
  int frame_number;

  public String getAppletInfo()
  { return "Animator2 by Joshua Kerievsky"; }

  public void init()
  {
    try
    {
      //////////// call to Factory Method //////////////
      /////////////////  createImage ///////////////////
      offScrImg = createImage(size().width, size().height);
      offScrGraphics = offScrImg.getGraphics();
    } catch (Exception e)
       offScrGraphics = null;
  }

  public void start()
  {
    if (animThread == null)
    {
      animThread = new Thread(this);
      animThread.start();
    }
  }

  public void stop()
  {
    if (animThread != null && animThread.isAlive())
      animThread.stop();
    animThread = null;
  }

  void pause(int time)
  {
    try { Thread.sleep(time); }
    catch (InterruptedException e) {}
  }

  private void loadImages()
  {
    getAppletContext().showStatus("Loading up animation images...");
    // load up all images (but last) using a MediaTracker
    MediaTracker trk = new MediaTracker(this);
    for (int i=0; i < MAX_IMAGES-1; i++)
    {
      frame[i] = getImage(getCodeBase(), i+".gif");
      trk.addImage(frame[i], i+1);
      try { trk.waitForID(i+1); } // wait for complete image to be loaded
      catch (Exception e)
        frame[i] = null;
    }
    imagesLoaded = true;
    getAppletContext().showStatus("All images loaded.");
  }

  public void run()
  {
    frame_number = MAX_IMAGES - 1;  // start with last
    while (true)
    {
      if ( !imagesLoaded )
       loadImages();
      repaint();
      pause(PauseSeconds);
      frame_number++;
      if (frame_number == MAX_IMAGES) frame_number = 0;
    }
  }

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

  public void paint(Graphics g)
  {
    if (offScrGraphics != null)
    {
      offScrGraphics.drawImage(frame[frame_number], 0, 0, this);
      g.drawImage(offScrImg,0,0,this);
    }
    else
      g.drawImage(frame[frame_number],0,0,this);
  }
}

Output using Example Program

This Java applet implements double-buffering by making a call to a Factory Method which returns an Image object.

An Example of the Factory Method (p 107)

Intent

An example from Sun's Java Development Kit (JDK beta)

Source code - Using the Component's createImage factory method

Output using Example Program

Commentary on the Factory Method