Java Beans Tutorial, Pt. 4: Converting Event Handlers: ItemListener and WindowListener

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[Tutorial Contents]

Now things get interesting. You're going to add methods to ChoiceApplet0 that can be called by typing their names in the TextField. Alternately, you can select the names from the Choice component.

Start by changing the names of the Strings that appear in the Choice component:

public void init()
{
  theChoice.add("big");
  theChoice.add("small");
  theChoice.add("unknown");
  add(theChoice);
  ...
}

Next, upgrade the event handling for Choice; use an ActionListener similar to the one used by the TextField:

  theChoice.addItemListener(new ItemListener() {
    public void itemStateChanged(ItemEvent e) {
      System.out.println(
        "ENTER----->theChoice ActionListener");
      String mName = (String) e.getItem();
      System.out.println("mName: " + mName);
      String pString =  
        e.paramString();  // useful for Debugging
      System.out.println("pString: " + pString);

      theTextField.setText(mName);

      System.out.println(
        "EXIT------>theChoice ActionListener");
    }
  });

Currently, only diagnostic information is printed in the event handler, but eventually you'll used the String returned by getItem to do a method lookup. One interesting side effect in the handler is that the TextField is set to whatever string was just selected in the Choice component:

    theTextField.setText(mName);

Now that you've upgraded the event handling mechanism for Choice to JDK 1.1, you can get rid of the old action and handleEvent methods:

public boolean action(Event event, Object arg)
{
  System.out.println("ENTER----->action");
  if (event.target == theChoice)
    System.out.println(
      theChoice.getSelectedItem() +  " selected");
  System.out.println("EXIT------>action");
  return true;
}

public boolean handleEvent(Event e)
{
  System.out.println(e.toString());
  return super.handleEvent(e);
}

Finally a couple minor upgrades are in order. Change the following deprecated calls in main. Change:

   theFrame.resize(500,300);
   theFrame.show();

   theFrame.setSize(500,300);
   theFrame.setVisible(true);

These kinds of changes are covered in: How to Convert Programs to the 1.1 AWT API In fact the updateAWT script mentioned in this document was used to upgrade the ChoiceApplet to use JDK 1.1 events.

Contining the upgrade, modify the appletFrame class to implement the WindowListener interface.

  class appletFrame extends Frame 
      implements WindowListener {
    appletFrame() {
      appletFrame() {
        addWindowListener(this);
      }
      ...
    }

public void windowClosed(WindowEvent event) {
}

public void windowDeiconified(WindowEvent event) {
}

public void windowIconified(WindowEvent event) {
}

public void windowActivated(WindowEvent event) {
}

public void windowDeactivated(WindowEvent event) {
}

public void windowClosing(WindowEvent event) {
  System.out.println("ENTER-----appletFrame.windowClosing");
  this.setVisible(false);
  dispose();
  System.out.println("EXIT-----appletFrame.windowClosing");
  System.exit(0);
}

This now does what the old handleEvent method did inside appletFrame. However, note that the updateAWT script changed another deprecated method call. The line:

    this.hide();

    this.setVisible(false);

You can use TestChoiceApplet03.htmlto test the applet.

Program Source Code

A makefile for this lesson automates source code compilation.

You may want to look at the final source file for ChoiceApplet03.java to verify any changes.