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Trail: Creating a GUI with JFC/Swing
Lesson: Writing Event Listeners

How to Write a Change Listener

Change events occur whenever a component changes state. For example, a button fires a change event every time the button is pressed. The look-and-feel implementation of the button listens for change events so that it can react appropriately to the state change (repainting itself, for example). Although nothing's stopping you from registering for change events on a button, most programs don't need to do so.

Two Swing components rely on change events for basic functionality -- sliders and color choosers. To learn when the value in a slider(in the Creating a GUI with JFC/Swing trail) changes, you need to register a change listener. Similarly, you need to register a change listener on a color chooser(in the Creating a GUI with JFC/Swing trail) to be informed when the user chooses a new color.

Here is an example of change event handling code for a slider:

//...where initialization occurs:
framesPerSecond.addChangeListener(new SliderListener());
...
class SliderListener implements ChangeListener {
    public void stateChanged(ChangeEvent e) {
        JSlider source = (JSlider)e.getSource();
        if (!source.getValueIsAdjusting()) {
            int fps = (int)source.getValue();
	    ...
        }    
    }
}
This snippet is from a program named SliderDemo. You can find the source and image files for the program, along with instructions for compiling and running it, in How to Use Sliders(in the Creating a GUI with JFC/Swing trail).

The Change Event API

The ChangeListener(in the API reference documentation) interface has just one method, so it has no corresponding adapter class. Here's the method:
void stateChanged(ChangeEvent)
Called when the listened-to component changes state.
The stateChanged method has a single parameter: a ChangeEvent(in the API reference documentation) object. To get the component that fired the event, use the getSource method which ChangeEvent inherits from EventObject. The ChangeEvent class defines no additional methods.

Examples that Use Change Listeners

The following table lists the examples that use change listeners.

Example Where Described Notes
SliderDemo and
SliderDemo2
How to Use Sliders(in the Creating a GUI with JFC/Swing trail) Registers a change listener on a slider that controls animation speed. The change listener ignores the change events until the user releases the slider.
ColorChooserDemo and
ColorChooserDemo2
How to Use Color Choosers(in the Creating a GUI with JFC/Swing trail) Uses a change listener on the selection model of a color chooser to learn when the user changes the current color.
ConverterRangeModel(in a .java source file)
and its subclass,
FollowerRangeModel(in a .java source file)
The Anatomy of a Swing-Based Program(in the Creating a GUI with JFC/Swing trail) Implement custom models for the sliders used in the Converter demo. Both models explicitly fire change events when necessary.


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