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Trail: Creating a GUI with JFC/Swing
Lesson: Writing Event Listeners
Section: Implementing Listeners for Commonly Handled Events
Writing Event Listeners
Introduction to Event Listeners
General Information about Writing Event Listeners
Listeners Supported by Swing Components
Implementing Listeners for Commonly Handled Events
How to Write an Action Listener
How to Write a Caret Listener
How to Write a Change Listener
How to Write a Component Listener
How to Write a Container Listener
How to Write a Document Listener
How to Write a Focus Listener
How to Write an Internal Frame Listener
How to Write an Item Listener
How to Write a Key Listener
How to Write a List Data Listener
How to Write a List Selection Listener
How to Write a Mouse Listener
How to Write a Mouse-Motion Listener
How to Write a Mouse-Wheel Listener
How to Write a Property Change Listener
How to Write a Table Model Listener
How to Write a Tree Expansion Listener
How to Write a Tree Model Listener
How to Write a Tree Selection Listener
How to Write a Tree-Will-Expand Listener
How to Write an Undoable Edit Listener
How to Write Window Listeners
Listener API Table
Solving Common Event-Handling Problems
Home Page > Creating a GUI with JFC/Swing > Writing Event Listeners
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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.

### Examples that Use Change Listeners

> 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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