JPanel communication: What is a good design?

Question

I've been struggling with the proper design for an application I'm helping develop... Here are the pertinent details:
1. The application extends JFrame and contains two JPanels.
2. The first JPanel, call it "datePane", contains a label displaying the current date along with two buttons for incrementing or decrementing the date by one day.
3. The second JPanel, call it "activityPane", shows a list of activities that occurred on the date shown in datePane. The panel contains a table, some buttons, and some labels that describe and manipulate the display.
My question is what is the best way for the datePane to notify the activityPane when the date changes? Since the buttons on the datePane are not members of the main frame or activityPane, I see no way to register a listener that would tell the activityPane to change it's content.
I can post some code if it would make this scenario clearer. Thanks for any help!

Answer 1

Any thread worth its silk should have some working code! Here's a quick example using EventListenerList. The object duke has the property "waving" and 3 frames keep in synch with a shared duke. Save this in file App.java and let it rip!
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
import java.io.IOException;
import javax.swing.*;
import javax.swing.event.*;
class App {
     public static void main(String[] args) throws IOException {
          Duke duke = new Duke();
          DukeView view1 = new ImageView(duke);
          DukeView view2 = new ImageView(duke);
          DukeView view3 = new WaveController(duke);
          view1.showGUI(0,0);
          view2.showGUI(200,0);
          view3.showGUI(400,0);
class Duke {
     //modified code from javax.swing.event.EventListenerList's javadoc
      private EventListenerList listenerList = new EventListenerList();
      private ChangeEvent event = new ChangeEvent(this); //immutable => reusable
      public void addChangeListener(ChangeListener l) {
          listenerList.add(ChangeListener.class, l);
      public void removeChangeListener(ChangeListener l) {
          listenerList.remove(ChangeListener.class, l);
      protected void fireChange() {
          Object[] listeners = listenerList.getListenerList();
          for (int i = listeners.length-2; i>=0; i-=2)
              if (listeners==ChangeListener.class)
     ((ChangeListener)listeners[i+1]).stateChanged(event);
     //end of code from javadoc
     private boolean waving;
     public boolean isWaving() {
          return waving;
     public void setWaving(boolean waving) {
          if (this.waving != waving) {
               this.waving = waving;
               fireChange();
     public boolean hasNoListeners() {
          return 0 == listenerList.getListenerCount();
abstract class DukeView {
     private Duke duke;
     private JFrame frame;
     private ChangeListener listener;
     protected DukeView(Duke duke) {
          if(duke == null)
               throw new NullPointerException();
          this.duke = duke;
          this.duke.addChangeListener(listener = new ChangeListener(){
               public void stateChanged(ChangeEvent evt) {
                    getInSynchWithDuke();
     public void showGUI(int x, int y) {
          if (frame == null) {
               frame = new JFrame("duke");
               buildGUI(frame);
               frame.addWindowListener(new WindowAdapter(){
                    public void windowClosing(WindowEvent evt) {
                         duke.removeChangeListener(listener);
                         if (duke.hasNoListeners())
                              System.exit(0);
               frame.pack();
          frame.setLocation(x,y);
          frame.show();
     protected Duke getDuke() {
          return duke;
     protected abstract void getInSynchWithDuke();
     protected abstract void buildGUI(JFrame frame);
class ImageView extends DukeView {
     private static final int FRAME_COUNT = 9;
     private JLabel imageLabel = new JLabel("", JLabel.CENTER);
     private static Image[] images = new Image[FRAME_COUNT];
     private static IOException exception;
     static { //read all images
          try {
               String prefix = "http://java.sun.com/people/jag/green/duke/T";
               GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
               GraphicsDevice gd = ge.getDefaultScreenDevice();
               GraphicsConfiguration gc = gd.getDefaultConfiguration();
               for(int i=1; i<=FRAME_COUNT; ++i)
                    images[i-1] = readCompatibleImage(new java.net.URL(prefix + i + ".gif"), gc);
          } catch(IOException e) {
               exception = e; //signal to constructor
     private int imageOffset;
     private Timer timer = new Timer(100, new ActionListener(){
          public void actionPerformed(ActionEvent evt) {
               if (getDuke().isWaving()) {
                    imageOffset = (imageOffset+1)%FRAME_COUNT;
                    imageLabel.setIcon(new ImageIcon(images[imageOffset]));
               } else
                    timer.stop();
     public ImageView(Duke duke) throws IOException {
          super(duke);
          if (exception != null)
               throw exception;
     protected void getInSynchWithDuke() {
          if (getDuke().isWaving())
               timer.start();
          else
               timer.stop();
     protected void buildGUI(JFrame frame) {
          imageLabel.setIcon(new ImageIcon(images[0]));
          frame.getContentPane().add(imageLabel);
          if (getDuke().isWaving())
                    timer.start();
     //turn image into one that is perhaps more compatible with screen
     private static BufferedImage readCompatibleImage(java.net.URL url, GraphicsConfiguration gc) throws IOException {
          BufferedImage orig = ImageIO.read(url);
          if (orig == null)
               throw new IOException("Can't load " + url);
          int transparency = orig.getColorModel().getTransparency();
          BufferedImage compat = gc.createCompatibleImage(orig.getWidth(), orig.getHeight(), transparency);
          Graphics2D g = compat.createGraphics();
          g.drawRenderedImage(orig, null);
          g.dispose();
          orig.flush();
          return compat;
class WaveController extends DukeView {
     private JCheckBox check = new JCheckBox("wave");
     public WaveController(Duke duke) {
          super(duke);
          check.setSelected(duke.isWaving());
     protected void getInSynchWithDuke() {
          check.setSelected(getDuke().isWaving());
     protected void buildGUI(JFrame frame) {
          check.addActionListener(new ActionListener(){
               public void actionPerformed(ActionEvent evt) {
                    getDuke().setWaving(check.isSelected());
          frame.getContentPane().add(check);