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        <b>Trail:</b> Creating a GUI with JFC/Swing
        <br><b>Lesson:</b> Writing Event Listeners
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            <div class="linkLESSON"><a href="index.html">Writing Event Listeners</a></div>
<div class="nolinkAHEAD">Introduction to Event Listeners</div>
<div class="linkAHEAD"><a href="generalrules.html">General Information about Writing Event Listeners</a></div>
<div class="linkAHEAD"><a href="eventsandcomponents.html">Listeners Supported by Swing Components</a></div>
<div class="linkAHEAD"><a href="handling.html">Implementing Listeners for Commonly Handled Events</a></div>
<div class="linkBHEAD"><a href="actionlistener.html">How to Write an Action Listener</a></div>
<div class="linkBHEAD"><a href="caretlistener.html">How to Write a Caret Listener</a></div>
<div class="linkBHEAD"><a href="changelistener.html">How to Write a Change Listener</a></div>
<div class="linkBHEAD"><a href="componentlistener.html">How to Write a Component Listener</a></div>
<div class="linkBHEAD"><a href="containerlistener.html">How to Write a Container Listener</a></div>
<div class="linkBHEAD"><a href="documentlistener.html">How to Write a Document Listener</a></div>
<div class="linkBHEAD"><a href="focuslistener.html">How to Write a Focus Listener</a></div>
<div class="linkBHEAD"><a href="internalframelistener.html">How to Write an Internal Frame Listener</a></div>
<div class="linkBHEAD"><a href="itemlistener.html">How to Write an Item Listener</a></div>
<div class="linkBHEAD"><a href="keylistener.html">How to Write a Key Listener</a></div>
<div class="linkBHEAD"><a href="listdatalistener.html">How to Write a List Data Listener</a></div>
<div class="linkBHEAD"><a href="listselectionlistener.html">How to Write a List Selection Listener</a></div>
<div class="linkBHEAD"><a href="mouselistener.html">How to Write a Mouse Listener</a></div>
<div class="linkBHEAD"><a href="mousemotionlistener.html">How to Write a Mouse-Motion Listener</a></div>
<div class="linkBHEAD"><a href="mousewheellistener.html">How to Write a Mouse-Wheel Listener</a></div>
<div class="linkBHEAD"><a href="propertychangelistener.html">How to Write a Property Change Listener</a></div>
<div class="linkBHEAD"><a href="tablemodellistener.html">How to Write a Table Model Listener</a></div>
<div class="linkBHEAD"><a href="treeexpansionlistener.html">How to Write a Tree Expansion Listener</a></div>
<div class="linkBHEAD"><a href="treemodellistener.html">How to Write a Tree Model Listener</a></div>
<div class="linkBHEAD"><a href="treeselectionlistener.html">How to Write a Tree Selection Listener</a></div>
<div class="linkBHEAD"><a href="treewillexpandlistener.html">How to Write a Tree-Will-Expand Listener</a></div>
<div class="linkBHEAD"><a href="undoableeditlistener.html">How to Write an Undoable Edit Listener</a></div>
<div class="linkBHEAD"><a href="windowlistener.html">How to Write Window Listeners</a></div>
<div class="linkAHEAD"><a href="api.html">Listener API Table</a></div>
<div class="linkAHEAD"><a href="problems.html">Solving Common Event-Handling Problems</a></div>
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            <div id=PageTitle>Introduction to Event Listeners</div>
            <blockquote>

If you've read
<a class="TutorialLink" target="_top" href="../learn/example2.html#handlingEvents">Handling Events</a> or any of the
component how-to pages,
you probably already know the basics of event listeners.

<p>
Let's look at one of the simplest event handling
examples possible.  It's called Beeper, and it features a button
that beeps when you click it. 
You can <a href="http://java.sun.com/docs/books/tutorialJWS/uiswing/events/examples/Beeper.jnlp">run Beeper</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#Beeper">example index</a>.

<p>
<p><center><IMG SRC="../../figures/uiswing/events/Beeper.png" WIDTH="200" HEIGHT="80" ALIGN="BOTTOM" ALT=""></center></p>


You can find the entire program in

<a class="SourceLink" target="_blank" href="examples/Beeper.java"><code>Beeper.java</code></a>.
Here's the code that implements the event handling
for the button:

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

The <code>Beeper</code> class implements the 
<a href="actionlistener.html"><code>ActionListener</code></a>
interface, which contains one method:
<code>actionPerformed</code>.
Since <code>Beeper</code> implements <code>ActionListener</code>,
a <code>Beeper</code> object can register as a listener
for the action events that buttons fire.
Once the <code>Beeper</code> has been registered
using the <code>Button</code> <code>addActionListener</code> method,
the <code>Beeper</code>'s <code>actionPerformed</code> method
is called every time the button is clicked.

</blockquote>
<h3>
<a name="example2">
A More Complex Example
</a>
</h3>
<blockquote>

The event model,
which you saw at its simplest in the preceding example,
is quite powerful and flexible.
Any number of event listener objects
can listen for all kinds of events
from any number of event source objects.
For example, a program
might create one listener per event source.
Or a program might have a single listener
for all events from all sources.
A program can even have more than one listener
for a single kind of event from a single event source.

<p><center><IMG SRC="../../figures/uiswing/events/2eventsource.gif" WIDTH="365" HEIGHT="85" ALIGN="BOTTOM" ALT="Event source with multiple listeners"></center></p>
<h4 align=center>Multiple listeners
can register to be notified of events
of a particular type from a particular source.  
Also, the same listener can listen to notifications from
different objects.
<font color=red>[PENDING: Update this diagram with
an additional event source that also sends notification to
the top-most event listener.]</font></h4>

<p>Each event is represented by an object that gives
information about the event and identifies the event
source.  Event sources are often components or models,
but other kinds of objects can also be event sources.

<p>
Whenever you want to detect events from a particular component,
first check the how-to section for that component.  A list of the
component how-to sections is 
<a class="TutorialLink" target="_top" href="../components/componentlist.html">here</a>.
The how-to sections give examples of handling the
events that you're most likely to care about.  In
<a class="TutorialLink" target="_top" href="../components/colorchooser.html">How to Use Color Choosers</a>, for instance, you'll find an example
of writing a change listener to track when the color
changes in the color chooser.

<p>
The following example demonstrates that event listeners
can be registered on multiple objects and that
the same event can be sent to multiple listeners.
The example contains two event sources 
(<code>JButton</code> instances)
and two event listeners.
One of the event listeners
(an instance of a class called <code>MultiListener</code>)
listens for events from both buttons.
When it receives an event, 
it adds the event's "action command"
(which is set to the text on the button's label)
to the top text area.
The second event listener
(an instance of a class called <code>Eavesdropper</code>)
listens for events on only one of the buttons.
When it receives an event,
it adds the action command 
to the bottom text area.

<p>

<p><center><IMG SRC="../../figures/uiswing/events/MultiListener.gif" WIDTH="400" HEIGHT="300" ALIGN="BOTTOM" ALT=""></center></p>
<h4 align=center><font color=red>[PENDING: Updated screenshot forthcoming.]</font></h4>

<blockquote><hr><strong>Try this:</strong>&nbsp;<ol>
<li> <a href="http://java.sun.com/docs/books/tutorialJWS/uiswing/events/examples/MultiListener.jnlp">Run
     MultiListener</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#MultiListener">example index</a>.

<li> Click the <em>Blah blah blah</em> button.  Only the
     <code>MultiListener</code> object is
     registered to listen to this button.

<li> Click the <em>You don't say!</em> button.  Both 
     the <code>MultiListener</code> object and the
     <code>Eavesdropper</code> object are registered to
     listen to this button.
<hr></blockquote>

You can find the entire program in

<a class="SourceLink" target="_blank" href="examples/MultiListener.java"><code>MultiListener.java</code></a>.
Here's the code that implements the event handling
for the button:

<blockquote><pre>
public class MultiListener ... implements ActionListener {
    ...
    <em>//where initialization occurs:</em>
        button1.addActionListener(this);
        button2.addActionListener(this);

        button2.addActionListener(new Eavesdropper(bottomTextArea));
    }

    public void actionPerformed(ActionEvent e) {
        topTextArea.append(e.getActionCommand() + newline);
    }
}

class Eavesdropper implements ActionListener {
    ...
    public void actionPerformed(ActionEvent e) {
        myTextArea.append(e.getActionCommand() + newline);
    }
}
</pre></blockquote>

In the above code, 
both <code>MultiListener</code> and <code>Eavesdropper</code>
implement the <code>ActionListener</code> interface
and register as action listeners 
using the <code>JButton</code> <code>addActionListener</code> method.
Both classes' implementations of the <code>actionPerformed</code> method
are similar:
they simply add the event's action command 
to a text area. 
</blockquote>
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