Cannot get KeyListener to work with JAI ScrollImagePanel

Question

I am trying to display a TIF image and use the page up and page down keys to show page 1 and page 2 of the image. I started with an example for the JAI documentation.
The problem is that I can't get the key event listener to "see" keystokes.
I think ScrollImagePanel() is doing its own event listening and I don't know how to get at it.
Here is the code:
// http://java.sun.com/products/java-media/jai/forDevelopers/samples/MultiPageRead.java
A single page of a multi-page TIFF file may loaded most easily by using the page
parameter with the "TIFF" operator which is documented in the class comments of
javax.media.jai.operator.TIFFDescriptor. This example shows a
means of loading a single page of a multi-page TIFF file using the ancillary codec
classes directly.
import java.awt.*;
import java.awt.event.*;
import java.io.File;
import java.io.IOException;
import java.awt.image.RenderedImage;
import java.awt.image.renderable.ParameterBlock;
import javax.media.jai.*;
import javax.media.jai.widget.*;
import com.sun.media.jai.codec.*;
public class MultiPageRead4 extends Frame
            implements KeyListener {                              //1
    ScrollingImagePanel panel;
    File file = null;
    ImageDecoder dec = null;
    RenderedImage op = null;
    // side to load, 0=front and 1=back
    static int imageToLoad = 0;
    /** The key code to flip between back and front images */
    private static final int FLIP_IMAGE_KEY1 = KeyEvent.VK_PAGE_UP;
    private static final int FLIP_IMAGE_KEY2 = KeyEvent.VK_PAGE_DOWN;
    /** Short cut key to exit the application */
    private static final int EXIT_KEY = KeyEvent.VK_ESCAPE;
    public MultiPageRead4(String filename) throws IOException {  //2
        super("Multi page TIFF Reader");
        // won't work here either
        // addKeyListener(this);
        addWindowListener(new WindowAdapter() {                  //3
           public void windowClosing(WindowEvent we) {           //4
           System.exit(0);
           }                                                     //4
        });                                                      //3
        if(file == null) {                                       //3
            file = new File(filename);
            SeekableStream s = new FileSeekableStream(file);
            TIFFDecodeParam param = null;
            dec = ImageCodec.createImageDecoder("tiff", s, param);
            System.out.println("Number of images in this TIFF: " +
                               dec.getNumPages());
            // Which of the multiple images in the TIFF file do we want to load
            // 0 refers to the first, 1 to the second and so on.
            // int imageToLoad = 1;
            }                                                    //3
        // display side of tiff indicated by imageToLoad which is initialized to 0
        // and set by page up / page down
        displaySide();
        }                                                         //2
    // Methods required by the KeyListener interface.
     * Process a key being pressed. Does nothing as we wait for the
     * release before moving.
     * @param evt The event that caused this method to be called
    public void keyPressed(KeyEvent evt)
         System.out.println("Key pressed.");   // debug -- see if it works
        // do nothing
     * Process a key being type (press and release). Does nothing as this is
     * always dodgy about when we get the event. Better to look for the
     * release.
     * @param evt The event that caused this method to be called
    public void keyTyped(KeyEvent evt)
         System.out.println("Key typed.");    // debug -- see if it works
        // do nothing
     * Process a key being pressed. Does nothing as we wait for the
     * release before moving.
     * @param evt The event that caused this method to be called
    public void keyReleased(KeyEvent evt)
    {                                                              //2
        System.out.println("Key released.");  // debug -- see if it works
        switch(evt.getKeyCode())
        {                                                         //3
            case EXIT_KEY:
                System.exit(0);
                break;
            case FLIP_IMAGE_KEY1:
               if(imageToLoad == 1) {                             //4
                  imageToLoad = 0;
                  displaySide();
                  break;
               }                                                  //4
            case FLIP_IMAGE_KEY2:
               if(imageToLoad == 0) {                             //4
                  imageToLoad = 1;
                  displaySide();
                  break;
               }                                                  //4
        }                                                         //3
    }                                                             //2
    public void displaySide() {                                   //2
        try {
            op = new NullOpImage(dec.decodeAsRenderedImage(imageToLoad),
                            null,
                            OpImage.OP_IO_BOUND,
                            null);
            }  catch (java.io.IOException ioe) {
                  System.out.println(ioe);
    * Create a standard bilinear interpolation object to be
    * used with the �scale� operator.
       Interpolation interp = Interpolation.getInstance(
             Interpolation.INTERP_BILINEAR);
    * Stores the required input source and parameters in a
    * ParameterBlock to be sent to the operation registry,
    * and eventually to the �scale� operator.
       ParameterBlock params = new ParameterBlock();
    // params.addSource(image1);
       params.addSource(op);
       params.add(0.70F); // x scale factor
       params.add(0.70F); // y scale factor
       params.add(0.0F); // x translate
       params.add(0.0F); // y translate
       params.add(interp); // interpolation method
    /* Create an operator to scale image1. */
       RenderedOp image2 = JAI.create("scale", params);
    /* Get the width and height of image2 + 3% */
       int width = (int)(image2.getWidth() * 1.03);
       int height = (int)(image2.getHeight() * 1.03);
    /* Attach image2 to a scrolling panel to be displayed. */
       panel = new ScrollingImagePanel(
          image2, width, height);
       panel.addKeyListener(this);
       // Display the original in a 800x800 scrolling window
       // panel = new ScrollingImagePanel(op, 1000, 600);
       add(panel);
       pack();
       show();
                                                                  //2
    public static void main(String [] args) {
        String filename = "034363331.TIF";
        try {
            MultiPageRead4 window = new MultiPageRead4(filename);
            }  catch (java.io.IOException ioe) {
                  System.out.println(ioe);
        System.out.println("finished.");
// http://java.sun.com/products/java-media/jai/forDevelopers/samples/MultiPageRead.javaI can't get the KeyListener to act (or send the debug messages to System.out.println()).
If "add( panel );" is removed and the comment before " keyListener(this); " in the constructor the key listening logic works -- of course no image is displayed.
Thanks for any help.
Bill Blalock

Answer 1

Thanks for exanding your explanation. I am not yet skilled enough in Java to follow you completely but I am getting the idea.
I would appreciate a code snippet using my example so I can better understand. In the mean time I found one way to make it work.
In the example I eventually found that this change ..
       show();
       panel.requestFocus();enabled the key listener. Bringing the window, even clicking on the panel, didn't give it focus. I understand that the window has to have the focus in order for the key listener to function (different from the mouse listender). I don't understand why the application did not have "focus" even though it was on top and active.
Oh well, lots to learn.
Thanks for your suggestion!