• The Need:
  • Control access to an object
• Examples:
  • The object may be expensive to use (e.g., the manipulation of an image) and it may not be necessary to do so (e.g., because the image is not visible or because you are using a thumbnail)
  • The object being accessed may reside on a "remote" address space/machine and special "communications" may be required
  • The object being accessed may be "privileged" (e.g., for security reasons) in some applications and not in others

javaexamples/proxy/IconProxy.java

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

/**
 * A proxy for an Icon (an example of the Proxy pattern)
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class IconProxy implements Icon
{
    private boolean           isLoaded, shouldLoad;
    private Image             image;
    private ImageIcon         realIcon;
    private int               height, width;
    private Toolkit           toolkit;
    private URL               url;

    private static final int  IMAGE_ID = 1;
    
    /**
     * Explicit Value Constructor
     *
     * @param file    The name of the file containing the image
     * @param width   The default width of the image
     * @param height  The default height of the image
     */
    public IconProxy(String file, int width, int height)
    {
       this.width  = width;
       this.height = height;

       image       = null;
       realIcon    = null;
       isLoaded    = false;

       toolkit  = Toolkit.getDefaultToolkit();


       try 
       {
          url = new URL(file);
          shouldLoad = true;
       } 
       catch (MalformedURLException mue) 
       {
          shouldLoad = false;
       }
    }
    
    
    /**
     * Get the height in pixels (required by Icon)
     *
     * @return   The height (in pixels)
     */
    public int getIconHeight() 
    {
       if (isLoaded) return realIcon.getIconHeight();
       else          return height;
    }



    /**
     * Get the width in pixels (required by Icon)
     *
     * @return   The width (in pixels)
     */
    public int getIconWidth() 
    {
       if (isLoaded) return realIcon.getIconWidth();
       else          return width;
    }


    
    /**
     * Draw the icon at the specified location (required by Icon)
     *
     * @param c   The Component the Icon is being drawn on
     * @param g   The Graphics context
     * @param x   The horizontal position of the upper-left corner
     * @param y   The vertical position of the upper-left corner
     */
    public void paintIcon(Component c, Graphics g, int x, int y) 
    {
       // Get the Image if necessary
       if (shouldLoad && image == null) image = toolkit.createImage(url);


       // If the URL is ok, check the status of the Image
       if (shouldLoad) 
       {
          if (image.getWidth(c) >= 0) isLoaded = true;
       }


       if(isLoaded) // Delegate to the real Icon
       {
          if (realIcon == null) realIcon = new ImageIcon(image);
          realIcon.paintIcon(c, g, x, y);
       } 
       else        // Draw a border
       {
          g.setColor(Color.black);
          g.drawString("Image", x+width/2, y+height/2);
       }
    }
}

        

javaexamples/proxy/IconFrame.java

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

/**
 * A frame that contains an Icon
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class IconFrame extends JFrame
{
    private Icon       icon;


    /**
     * Explicit Value Constructor
     *
     * @param icon   The Icon
     */
    public IconFrame(Icon icon)
    {
        this.icon = icon;
    }



    /**
     * Paint this Component
     *
     * @param g    The Graphics context
     */
    public void paint(Graphics g)
    {
        super.paint(g);
        

        icon.paintIcon(this, g, 0, 0);
    }

}
        

javaexamples/proxy/Driver.java

import javax.swing.*;

/**
 * An example that uses IconProxy (an example of the Proxy pattern)
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class Driver
{
    /**
     * The entry point
     *
     * @param args   The command-line arguments
     */
    public static void main(String[] args)
    {
        Icon         icon;
        IconFrame    frame;



        // For example: http://www.lib.jmu.edu/special/SCPix/cav85+/cav85+2.jpg
        icon = new IconProxy(args[0],200,200);
        
        frame = new IconFrame(icon);
        frame.setSize(400,400);
        frame.setVisible(true);
    }


}