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How to Write a Change Listener

A change listener is similar to a property change listener. A change listener is registered on an object — typically a component, but it could be another object, like a model — and the listener is notified when the object has changed. The big difference from a property change listener is that a change listener is not notified of what has changed, but simply that the source object has changed. Therefore, a change listener is most useful when it is only necessary to know when an object has changed in any way.

Three Swing components rely on change events for basic functionality — sliders, color choosers and spinners. To learn when the value in a slider changes, you need to register a change listener. Similarly, you need to register a change listener on a color chooser to be informed when the user chooses a new color. You register a change listener on a spinner, a component introduced in release 1.4, to be notified when the spinner's value changes.

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();
	    ...
        }    
    }
}

You can find the source file for SliderDemo in the example index for Using Swing Components.

The Change Listener API

The ChangeListener Interface

Because ChangeListener has only one method, it has no corresponding adapter class.

Method	Purpose
stateChanged(ChangeEvent)	Called when the listened-to component changes state.

The ChangeEvent Class

Method	Purpose
Object getSource() (in java.util.EventObject)	Returns the object that fired the event.

The following table lists the examples that use change listeners.

Example	Where Described	Notes
SliderDemo and SliderDemo2 SliderDemo3	How to Use Sliders	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	Uses a change listener on the selection model of a color chooser to learn when the user changes the current color.
SpinnerDemo3	Detecting Spinner Value Changes in How to Use Spinners.	Uses a change listener on a date-field spinner to change the color of the text as the spinner's date changes.
SpinnerDemo4	Detecting Spinner Value Changes in How to Use Spinners.	Uses a change listener on a spinner to cycle through the gray scale as the spinner's value changes.
ConverterRangeModel and its subclass, FollowerRangeModel	Using Models	Implement custom models for the sliders used in the Converter demo. Both models explicitly fire change events when necessary.

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