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            <div id=PageTitle>How to Write an Action Listener</div>
            <blockquote>

Action listeners are probably the easiest &#151; and most
common &#151; event handlers to implement.
You implement an action listener
to respond to the user's indication
that some implementation-dependent action should occur.

<p>

When the user clicks a 
<a class="TutorialLink" target="_top" href="../components/button.html">button</a>, chooses a 
<a class="TutorialLink" target="_top" href="../components/menu.html">menu item</a> or presses Enter in a 
<a class="TutorialLink" target="_top" href="../components/textfield.html">text field</a>, an action event occurs.
The result is that an <code>actionPerformed</code> message 
is sent to all action listeners 
that are registered on the relevant component.
<p>

Here is the action event handling code from an applet named
<code>Beeper</code>:

<blockquote><pre>
public class Beeper ...  implements ActionListener {
    ...
    <em>//where initialization occurs:</em>
        button.addActionListener(this);
    ...
    public void actionPerformed(ActionEvent e) {
	Toolkit.getDefaultToolkit().beep();
    }
}
</pre></blockquote>

The <code>Beeper</code> applet is described
in this trail's introduction to events,
<a href="intro.html">Introduction to Event Listeners</a>.
You can find the entire program in 

<a class="SourceLink" target="_blank" href="examples/Beeper.java"><code>Beeper.java</code></a>.
The other example described in that section,

<a class="SourceLink" target="_blank" href="examples/MultiListener.java"><code>MultiListener.java</code></a>,
has two action sources and two action listeners,
with one listener listening to both sources
and the other listening to just one.

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

<p align=center><a name="actionlistener">The ActionListener
   Interface</a>
<p><em>Because <code>ActionListener</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/awt/event/ActionListener.html#actionPerformed(java.awt.event.ActionEvent)">actionPerformed(actionEvent)</a></td>
<td> Called just after the user 
     informs the listened-to component
     that an action should occur.
</td>
</tr>
</table>

<p align=center><a name="actionevent">The ActionEvent 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/ActionEvent.html#getActionCommand()">String getActionCommand()</a></td>
<td>Returns the string associated with this action.
    Most objects that can fire action events
    support a method called <code>setActionCommand</code>
    that lets you set this string.
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/awt/event/ActionEvent.html#getModifiers()">int getModifiers()</a></td>
<td>Returns an integer representing
    the modifier keys the user was pressing
    when the action event occurred.
    You can use the <code>ActionEvent</code>-defined constants 
    <code>SHIFT_MASK</code>,
    <code>CTRL_MASK</code>,
    <code>META_MASK</code>, and
    <code>ALT_MASK</code>
    to determine which keys were pressed.
    For example, if the user Shift-selects a menu item,
    then the following expression is nonzero:
    <blockquote><pre>
actionEvent.getModifiers() & ActionEvent.SHIFT_MASK
    </pre></blockquote>
</td>
</tr>

<tr>
<td>
<a class="APILink" target="_blank" href="http://java.sun.com/javase/6/docs/api/java/util/EventObject.html#getSource()">Object getSource()</a><br>
(<em>in <code>java.util.EventObject</code></em>)
</td>
<td>Returns the object that fired the event.
</td>
</tr>
</table>

</blockquote>
<h3>
<a name="eg">Examples that Use Action Listeners</a>
</h3>
<blockquote>

The following table lists some of the many
examples that use action 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#Beeper"><code>Beeper</code></a>
</td>
<td> This section and
     <a href="intro.html">Introduction to Event Listeners</a>
</td>
<td> Contains one button with one action listener
     that beeps when you click the button.

</td>
</tr>
<tr valign=top>
<td> <a href="examples/index.html#MultiListener"><code>MultiListener</code></a>
</td>
<td> <a href="intro.html">Introduction to Event Listeners</a>
</td>
<td> Registers two different action listeners on one button.
     Also registers the same action listener on two different buttons.

</td>
</tr>
<tr valign=top>
<td> <a href="../components/examples/index.html#RadioButtonDemo"><code>RadioButtonDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/button.html#radiobutton">How to Use Radio Buttons</a></td>
<td> Registers the same action listener on five radio buttons.
     The listener uses the <code>getActionCommand</code> method
     to determine which radio button fired the event.

</td>
</tr>

<tr valign=top>
<td> <a href="../components/examples/index.html#MenuDemo"><code>MenuDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/menu.html">How to Use Menus</a></td>
<td> Shows how to listen for action events on menu items.
</td>
</tr>

<tr valign=top>
<td> <a href="../misc/examples/index.html#DragPictureDemo2"><code>DragPictureDemo2</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../dnd/intro.html#cut">Adding Cut/Copy/Paste Support to Data Transfer</a></td>
<td>Uses <code>setActionCommand</code> to attach the cut,
    copy, and paste actions to the menu.  Then uses as
    action listener to forward the cut/copy/paste actions
    to the currently focused component.
</td>
</tr>

<tr valign=top>
<td> <a href="../components/examples/index.html#TextDemo"><code>TextDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/textfield.html">How to Use Text Fields</a></td>
<td> An applet that registers an action listener on a text field.
</td>
</tr>

<tr valign=top>
<td> <a href="../components/examples/index.html#IconDemoApplet"><code>IconDemoApplet</code>
</a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/icon.html">How to Use Icons</a></td>
<td> Loads an image in an action listener.
     Because loading an image can take a while,
     this program uses a <code>SwingWorker</code>
     to load the image in a background thread.

</td>
</tr>

<tr valign=top>
<td> <a href="../components/examples/index.html#TableDialogEditDemo"><code>TableDialogEditDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/table.html">How to Use Tables</a></td>
<td> Registers an action listener
     through a factory method
     on the OK button of a color chooser dialog.

</td>
</tr>

<tr valign=top>
<td> <a href="../components/examples/index.html#SliderDemo"><code>SliderDemo</code></a>
</td>
<td>
<a class="TutorialLink" target="_top" href="../components/slider.html">How to Use Sliders</a></td>
<td> Registers an action listener on a timer
     that controls an animation loop.

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