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            <div id=PageTitle>How to Write a Property Change Listener</div>
            <blockquote>

Property-change events occur whenever the value of a
<em>bound property</em> changes
for a <em>bean</em> &#151;
a component that conforms to the
JavaBeans<font size="-2"><sup>TM</sup></font> specification.
You can find out more about beans from the 
<a class="TutorialLink" target="_top" href="../../javabeans/">JavaBeans</a> trail of the
<a class="TutorialLink" target="_top" href="../../java/">Java Tutorial</a>. All Swing components are also beans.

<p>
A JavaBeans property is accessed through its
<em>get</em> and <em>set</em> methods.
For example, <code>JComponent</code> has the 
property <em>font</em> which is accessible
through <code>getFont</code> and
<code>setFont</code> methods.

<p>
A <em>bound</em> property fulfills the special
requirement that, besides the <em>get</em> and <em>set</em>
methods, it fires a property-change event
when its value changes.
If you are interested in more information, see the
<a class="TutorialLink" target="_top" href="../../javabeans/properties/bound.html">Bound Properties</a> page in the JavaBeans trail.

<p>Some scenarios that commonly require property-change listeners
include:

<ul>
<li>You have implemented a formatted text field and need
    a way to detect when the user has entered a new value.
    You can register a property-change listener on
    the formatted text field to listen to
    changes on the <em>value</em> property.
    See <code>FormattedTextFieldDemo</code> in
<a class="TutorialLink" target="_top" href="../components/formattedtextfield.html#value">How to Use Formatted Text Fields</a>    for an example of this.
<p>
<li>You have implemented a dialog and need to
    know when a user has clicked one of the dialog's
    buttons or changed a selection in the dialog.
    See <code>DialogDemo</code> in
<a class="TutorialLink" target="_top" href="../components/dialog.html#stayup">How to Make Dialogs</a>    for an example of registering a property-change
    listener on an option pane to listen to changes to
    the <em>value</em> property.  You can also check
    out <code>FileChooserDemo2</code> in
<a class="TutorialLink" target="_top" href="../components/filechooser.html#advancedexample">How to Use File Choosers</a>    for an example of how to register a property-change
    listener to listen to changes to the
    <em>directoryChanged</em> and
    <em>selectedFileChanged</em> properties.
<p>
<li>You need to be notified when the component that has
    the focus changes.  You can register a property-change
    listener on the keyboard focus manager to listen to
    changes to the <em>focusOwner</em> property.
    See <code>TrackFocusDemo</code> and <code>DragPictureDemo</code> in
<a class="TutorialLink" target="_top" href="../misc/focus.html#trackingFocus">How to Use the Focus Subsystem</a>    for examples of this.
</ul>


<p>Although these are some of the more common uses for
property-change listeners, you can register a property-change
listener on the bound property of any component that conforms
to the JavaBeans specification.

<p>You can register a property change listener in two ways.
The first uses the method
<code>addPropertyChangeListener(PropertyChangeListener)</code>.
When you register a listener this way, you are notified
of every change to every bound property for that object.
In the <code>propertyChange</code> method, you can get the name
of the property that has changed using the
<code>PropertyChangeEvent</code> <code>getPropertyName</code>
method, as in the following code snippet:

<blockquote><pre>
KeyboardFocusManager focusManager =
   KeyboardFocusManager.getCurrentKeyboardFocusManager();
focusManager.addPropertyChangeListener(new FocusManagerListener());
...
public FocusManagerListener() implements PropertyChangeListener {
    public void propertyChange(PropertyChangeEvent e) {
        String propertyName = e.getPropertyName();
        if ("focusOwner".equals(propertyName) {
            ...
        } else if ("focusedWindow".equals(propertyName) {
            ...
        }
    }
    ...
}
</pre></blockquote>

<p>
The second way to register a property change listener uses the method
<code>addPropertyChangeListener(String, PropertyChangeListener)</code>.
The <code>String</code> argument is the name of a property.  Using this
method means that you only receive notification when
a change occurs to that particular property. So, for example,
if you registered a property change listener like this:

<blockquote><pre>
aComponent.addPropertyChangeListener("font",
                                     new FontListener());
</pre></blockquote>

<p>
<code>FontListener</code> only receives notification when the
value of the component's
<em>font</em> property changes.  It does <em>not</em>
receive notification when the value changes for <em>transferHandler</em>,
<em>opaque</em>, <em>border</em>, or any other property.

<p>
The following example shows how to register a property
change listener on the <em>value</em> property of a
formatted text field using the two-argument version of
<code>addPropertyChangeListener</code>:

<blockquote><pre>
<em>//...where initialization occurs:</em>
double amount;
JFormattedTextField amountField;
...
amountField.addPropertyChangeListener("value",
                                      new FormattedTextFieldListener());
...
class FormattedTextFieldListener implements PropertyChangeListener {
    public void propertyChanged(PropertyChangeEvent e) {
        Object source = e.getSource();
        if (source == amountField) {
            amount = ((Number)amountField.getValue()).doubleValue();
            ...
        }
        <em>...//re-compute payment and update field...</em>
    }
}
</pre></blockquote>

</blockquote>
<h3>
<a name="api">The Property Change Listener API</a>
</h3>
<blockquote>

<p align=center><a name="addpropertychangelistener">Registering
   a PropertyChangeListener</a>
<p>
<table border=1>
<tr>
<th align=left>Method</th>
<th align=left>Purpose</th>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/javax/swing/JComponent.html#addPropertyChangeListener(java.beans.PropertyChangeListener)">addPropertyChangeListener(PropertyChangeListener)</a></td>
<td>Add a property-change listener to the listener list.
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/javax/swing/JComponent.html#addPropertyChangeListener(java.lang.String, java.beans.PropertyChangeListener)">addPropertyChangeListener(String, PropertyChangeListener)</a></td>
<td>Add a property-change listener for a specific property.
    The listener is called only when there is a change to the
    specified property.
</td>
</tr>

</table>

<p align=center><a name="propertychangelistener">The
   PropertyChangeListener Interface</a>
<p><em>Because <code>PropertyChangeListener</code>
   has only one method, it has no corresponding
   adapter class.</em>
<table border=1>
<tr>
<th align=left>Method</th>
<th align=left>Purpose</th>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/beans/PropertyChangeListener.html#propertyChange(java.beans.PropertyChangeEvent)">propertyChange(PropertyChangeEvent)</a></td>
<td>Called when the listened-to bean changes a bound property.
</td>
</tr>
</table>

<p align=center><a name="propertychangeevent">The PropertyChangeEvent Class</a>
<p>
<table border=1>
<tr>
<th align=left>Method</th>
<th align=left>Purpose</th>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/beans/PropertyChangeEvent.html#getNewValue()">Object getNewValue()</a><br>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/beans/PropertyChangeEvent.html#getOldValue()">Object getOldValue()</a></td>
<td>Return the new, or old, value of the property, respectively.
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/beans/PropertyChangeEvent.html#getPropertyName()">String getPropertyName()</a></td>
<td>Return the name of the property that was changed.
</td>
</tr>

</table>
</blockquote>

<a name="eg">
<h3>Examples that Use Property Change Listeners</h3>
</a>
<blockquote>
The following table lists the
examples that use property-change listeners.

<p>

<table>
<tr>
<th align=left> Example
</th>
<th align=left> Where Described
</th>
<th align=left> Notes
</th>
</tr>

<tr valign=top>
<td>
<a href="../components/examples/index.html#FormattedTextFieldDemo"><code>FormattedTextFieldDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/formattedtextfield.html#value">How to Use Formatted Text Fields</a></td>
<td> A property-change listener is registered on 
     several formatted text fields to track changes
     to the <em>value</em> property.
</td>
</tr>

<tr valign=top>
<td>
<a href="../components/examples/index.html#DialogDemo"><code>DialogDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/dialog.html#stayup">How to Make Dialogs</a></td>
<td> The
<a class="TutorialLink" target="_top" href="../components/examples/CustomDialog.java"><code>CustomDialog</code></a> class registers a property-change listener on
an option pane to listen to the <em>value</em>
and <em>inputValue</em> properties.
</td>
</tr>

<tr valign=top>
<td>
<a href="../components/examples/index.html#FileChooserDemo2"><code>FileChooserDemo2</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/filechooser.html#advancedexample">How to Use File Choosers</a></td>
<td> The
<a class="TutorialLink" target="_top" href="../components/examples/ImagePreview.java"><code>ImagePreview</code></a> class registers a property-change listener on
the file chooser to listen to the 
<em>directoryChanged</em> and
<em>selectedFileChanged</em> properties.
</td>
</tr>

<tr valign=top>
<td>
<a href="../misc/examples/index.html#TrackFocusDemo"><code>TrackFocusDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../misc/focus.html#trackingFocus">How to Use the Focus Subsystem</a></td>
<td> A property-change listener is registered
on the keyboard focus manager to track changes
to the <em>focusOwner</em> property.
</td>
</tr>
</table>
</blockquote>
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