Java Beans Tutorial, Pt. 4: Bound Properties

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Bound Properties

By Greg Voss, JavaSoft

[Tutorial Contents]

If you're building Beans, you want to know about bound properties. An object that supports bound properties can notify other objects when certain property values change. Such a notification is called a property change event. This section starts by adding support for bound properties to the evolving NervousText example. The example code shows how to fire property events, how components maintain listeners wishing to be notified of change events, and how to respond to property change event notices.

In this section you will be adding bound property support to NervousText04. An object that supports bound properties can notify other objects when certain properties change. Each time a bound property changes, it notifies registered listener objects by calling specified property change handler methods defined for the listener. These methods are called with the new and old values of the property, as well as name of the property that has changed.

Step 1. Add import Statement to Support Property Change Events

For a Bean to generate or receive property change events, you must add the following import statement to the top of the file.

import java.beans.*;

The package and import statements at the top of the file should be as follows:

package sun.beanbox.beans;
import java.awt.event.*;
import java.awt.Graphics;
import java.awt.Font;
import java.awt.*;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.io.IOException;
import java.beans.*;

Step 2. Add Code to Track and Notify Interested Property Change Listeners

Add the following code to maintain a list of listeners potentially wanting to be notified of text property changes:

public void addPropertyChangeListener(
              PropertyChangeListener l) {
  support.addPropertyChangeListener(l);
}

public void removePropertyChangeListener(
              PropertyChangeListener l) {
  support.removePropertyChangeListener(l);
}

private PropertyChangeSupport support = 
          new PropertyChangeSupport(this);

The last statement in this listing declares an instance variable, support, for NervousText04 objects. This variable holds a collection of listener objects that will be notified about property changes to the text property. The two supporting methods, addPropertyChangeListener and removePropertyChangeListener provide a public interface, allowing interested listeners to register themselves with NervousText04 objects.

Step 3. Add Code to Fire text Property Change Events

Modify the text property setter method to fire a property change event whenever it is called. To do this, you need to save copies of both the old and new values of the property being changed. To save the old property, add the following line to the end of the setText method in NervousText04:

  String oldstring = s;

Recall that "s" is the value of the text property accessed by the getText and setText getter and setter methods for the text property. The instance variable s is declared at the top of the class:

  public class NervousText04 extends Panel 
    implements Runnable, MouseListener {
    ...
    String s = null;
    ...

Now that you have both old a new values saved in local variables, add the following line to the end of the setText method:

  support.firePropertyChange("text", 
    oldstring, newstring);

The full method now looks like this:

  public void setText(String newstring){
    String oldstring = s; 
    s=new String(newstring);
    separated= new char[s.length()];
    s.getChars(0,s.length(),separated,0);
    support.firePropertyChange("text", 
      oldstring, newstring);
  }

Step 4. Define Event Handler Methods for Property Change Events

Define a handler method that can potentially be called by a listener object to report that a text property has changed.

  public void reportChange(PropertyChangeEvent evt) {
  System.err.println(
    "ENTER---> NervousText04 reportChange");

  String oldValue = evt.getPropertyName() + 
    " := " + evt.getOldValue();
  String newValue = evt.getPropertyName() + 
    " := " + evt.getNewValue();

  System.err.println("New value: " + oldValue);
  System.err.println("Old value: " + newValue);
  System.err.println(
    "EXIT----> NervousText04 reportChange");
  }

Add the above code for reportChange at the end of the class definition for NervousText04 immediately after the declaration for the support variable you added earlier:

  private PropertyChangeSupport support = 
    new PropertyChangeSupport(this);

Now NervousText04 is both a source for property change events and a handler for property change events. We haven't made it a listener. That's because the BeanBox will automatically generate a listener class as an adapter. An adapter is created when you connect any object that generates a property change to a NervousText04 object as a target when building a BeanBox application.

The reportChange method does not actually cause a change to take place in the target object. To do that, define an additional method called makeChange as follows:

  public void makeChange(PropertyChangeEvent evt) {
  System.err.println(
    "ENTER---> NervousText04 makeChange");

  String oldValue = (String)evt.getOldValue(); 
    // (String) cast required since 
  String newValue = (String)evt.getNewValue(); 
    // evt.getOldValue() returns an Object
  setText("*" + newValue + "*");

  System.err.println("New value: " + oldValue);
  System.err.println("Old value: " + newValue);
  System.err.println(
    "EXIT----> NervousText04 makeChange");
  }

Add the method definition for makeChange immediately after the code for reportChange. The actual change in the target NervousText object occurs when the setter method, setText, is called in the makeChange event handler method.

Step 4. Build the JAR and Install it in the BeanBox

Compile NervousText04.java:

  javac -d . NervousText04.java

Create the mainfest:

  echo "Manifest-Version: 1.0" > manifest.tmp
  echo """ >>
 manifest.tmp
  echo "Name: sun/beanbox/beans/NervousText04.class" >>
 manifest.tmp
  echo "Java-Bean: True" >>
 manifest.tmp

The resulting temporary manifest file should look like this:

  Manifest-Version: 1.0

  Name: sun/beanbox/beans/NervousText04.class
  Java-Bean: True

Create the JAR file:

  jar cfm NervousText04.jar manifest.tmp 
    sun/beanbox/beans/NervousText04.class

Remove the temporary manifest and install the JAR in the BeanBox jars directory (substituting your BDK installation directory for BDK_HOME):

  rm manifest.tmp
  cp -p NervousText04.jar BDK_HOME/beans/jars

Step 5. Test NervousText04 in BeanBox

NOTE: Use the exiting application and modify it.

Start up the BeanBox. Add one OurButton, and two NervousText04 Beans. You're going to bind changes in the first NervousText04 Bean to the second. The changes to the first NervousText04 will be effected by pressing OurButton. Change the label for OurButton to "change direction." As before, you're going to make an action event in OurButton cause the next to be reversed in the first NervousText04 Bean.

Program Source Code

A makefile for this lesson automates source code compilation, JAR file construction, and copying JAR files to the appropriate BeanBox directory. You'll have to edit several of the variables in the makefile to indicate the location of your JDK 1.1 and the BDK installation directories.

You may want to look at the final source file for NervousText Bean Version 04 to verify the changes you have made to the original NervousText.java file.