How to Write a Change Listener
Trail: Creating a GUI With JFC/Swing
Lesson: Writing Event Listeners
Section: Implementing Listeners for Commonly Handled Events

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.

Several Swing components (including JTabbedPane, JViewPort) 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, 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.

Examples that Use Change Listeners

The following table lists the examples that use change listeners.

Example Where Described Notes
SliderDemo and
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
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.
and its subclass,
How to Use Models Implement custom models for the sliders used in the Converter demo. Both models explicitly fire change events when necessary.

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