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, such as 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's an example of change event-handling code for a slider: ^[4]

^[4] You can find the source file, SliderDemo.java , on the CD at: JavaTutorial/uiswing/components/example-1dot4/index.html#SliderDemo .

  //...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();             ...         }     } } 

The Change Listener API

Because ChangeListener has only one method (see Table 5), it has no corresponding adapter class. Table 6 describes the method in the ChangeEvent class. Also see the ChangeListener API documentation at: http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/event/ChangeListener.html. The ChangeEvent API documentation is online at: http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/event/ChangeEvent.html.

Table 5. The ChangeListener Interface

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

Table 6. The ChangeEvent Class

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

Examples That Use Change Listeners

The following examples use change listeners.

Example	Where Described	Notes
SliderDemo and SliderDemo2 SliderDemo3	How to Use Sliders (page 348)	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 (page 167)	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 (page 363)	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 (page 363)	Uses a change listener on a spinner to cycle through the gray scale as the spinner's value changes.
ConverterRangeModel and its subclass, FollowerRangeModel	An Example: Converter (page 51)	Implements custom models for the sliders used in the Converter demo. Both models explicitly fire change events when necessary.