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

One or more component events are fired
by a <code>Component</code> object
just after the component is hidden, made visible, moved, or resized.
An example of a component listener
might be in a GUI builder tool
that's displaying information about the size 
of the currently selected component,
and that needs to know when the component's size changes.
You shouldn't need to use component events
to manage basic layout and rendering.

<p>
The component-hidden and component-shown events
occur only as the result of calls to a <code>Component</code>'s
<code>setVisible</code> method.
For example, a window might be miniaturized into an icon (iconified)
without a component-hidden event being fired.

<p>
The following example demonstrates component events.
The window contains a panel that has a label and a check box.
The check box controls whether the label is visible.
A text area displays a message every time
the window, panel, label, or check box
fires a component event.

<p>
<p><center><IMG SRC="../../figures/uiswing/events/ComponentEventDemo.png" WIDTH="334" HEIGHT="235" ALIGN="BOTTOM" ALT=""></center></p>
<p>
<blockquote><hr><strong>Try this:</strong>&nbsp;
<ol>
<li> <a href="http://java.sun.com/docs/books/tutorialJWS/uiswing/events/examples/ComponentEventDemo.jnlp">Run
     ComponentEventDemo</a> using
     <a href=http://java.sun.com/products/javawebstart>
     Java<sup><font size=-2>TM</font></sup> Web Start</a>.
     Or, to compile and run the example yourself,
     consult the
     <a href="examples/index.html#ComponentEventDemo">example index</a>.
<li> When the window appears, one or more component-shown
     events have been fired.
<li> Click the check box to hide the label.
     <br>
     The label fires a component-hidden event.
     The panel fires component-moved and component-resized events.
     The check box fires a component-moved event.

<li> Click the check box again to show the label.
     <br>
     The label fires a component-shown event.
     The panel fires component-moved and component-resized events.
     The check box fires a component-moved event.

<li> Iconify and then deiconify the window.
     <br>
     You do <em>not</em> get component-hidden or -shown events.
     If you want to be notified of iconification events,
     you should use a window listener or a window state listener.

<li> Resize the window.
     <br>
     You'll see component-resized
     (and possibly component-moved) events
     from all four components &#151; label, check box, panel, and frame.
     If the frame and panel's layout manager didn't make every component
     as wide as possible,
     the panel, label, and check box wouldn't have been resized.
</ol>
<hr></blockquote>

<p>

You can find the demo's code in 

<a class="SourceLink" target="_blank" href="examples/ComponentEventDemo.java"><code>ComponentEventDemo.java</code></a>.
Here is just the code related to handling component events:

<blockquote><pre>
public class ComponentEventDemo ... implements ComponentListener {
    static JFrame frame;
    JLabel label;
    ...
    public ComponentEventDemo() {
        ...
        JPanel panel = new JPanel(new BorderLayout());
        label = new JLabel("This is a label", JLabel.CENTER);
        label.addComponentListener(this);
        panel.add(label, BorderLayout.CENTER);

        JCheckBox checkbox = new JCheckBox("Label visible", true);
        checkbox.addComponentListener(this);
        panel.add(checkbox, BorderLayout.PAGE_END);
        panel.addComponentListener(this);
        ...
        frame.addComponentListener(this);
    }
    ...
    public void componentHidden(ComponentEvent e) {
	displayMessage("componentHidden event from "
		       + e.getComponent().getClass().getName());
    }

    public void componentMoved(ComponentEvent e) {
        Component c = e.getComponent();
        displayMessage("componentMoved event from "
                       + c.getClass().getName()
                       + "; new location: "
                       + c.getLocation().x
                       + ", "
                       + c.getLocation().y);
    }

    public void componentResized(ComponentEvent e) {
        Component c = e.getComponent();
        displayMessage("componentResized event from "
                       + c.getClass().getName()
                       + "; new size: "
                       + c.getSize().width
                       + ", "
                       + c.getSize().height);
    }

    public void componentShown(ComponentEvent e) {
	displayMessage("componentShown event from "
		       + e.getComponent().getClass().getName());
    }

    public static void main(String[] args) {
        ...
        //Create and set up the window.
        frame = new JFrame("ComponentEventDemo");
        ...
        JComponent newContentPane = new ComponentEventDemo();
        frame.setContentPane(newContentPane);
        ...
    }
}
</pre></blockquote>

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

<p align=center><a name="componentlistener">The ComponentListener
   Interface</a>
<p><em>All of these methods are also in the adapter class,
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/awt/event/ComponentAdapter.html"><code>ComponentAdapter</code></a>.</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/awt/event/ComponentListener.html#componentHidden(java.awt.event.ComponentEvent)">componentHidden(ComponentEvent)</a></td>
<td> Called after the listened-to component is hidden
     as the result of the <code>setVisible</code> method being called.
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/awt/event/ComponentListener.html#componentMoved(java.awt.event.ComponentEvent)">componentMoved(ComponentEvent)</a></td>
<td> Called after the listened-to component moves,
     relative to its container.
     For example, if a window is moved,
     the window fires a component-moved event,
     but the components it contains do not.
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/awt/event/ComponentListener.html#componentResized(java.awt.event.ComponentEvent)">componentResized(ComponentEvent)</a></td>
<td> Called after the listened-to component's
     size (rectangular bounds) changes.
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/awt/event/ComponentListener.html#componentShown(java.awt.event.ComponentEvent)">componentShown(ComponentEvent)</a></td>
<td> Called after the listened-to component becomes visible
     as the result of the <code>setVisible</code> method being called.
</td>
</tr>
</table>

<p align=center><a name="componentevent">The ComponentEvent
   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/awt/event/ComponentEvent.html#getComponent()">Component getComponent()</a></td>
<td> Returns the component that fired the event.
     You can use this instead of the <code>getSource</code> method.
</td>
</tr>
</table>

</blockquote>

<a name="eg">
<h3>Examples that Use Component Listeners</h3>
</a>
<blockquote>
The following table lists the
examples that use component 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="examples/index.html#ComponentEventDemo"><code>ComponentEventDemo</code></a>
</td>
<td> This section
</td>
<td> Reports all component events that occur on several components
     to demonstrate the circumstances under which component events are fired.

</td>
</tr>
</table>
        </blockquote>
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