The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Property-change events occur whenever the value of a bound property changes for a bean — a component that conforms to the JavaBeans™ specification. You can find out more about beans from the JavaBeans trail of the Java Tutorial. All Swing components are also beans.
A JavaBeans property is accessed through its get and set methods. For example, JComponent
has the property font which is accessible through the getFont
and setFont
methods.
Besides the get and set methods, a bound property fires a property-change event when its value changes. For more information, see the Bound Properties page in the JavaBeans trail.
Some scenarios that commonly require property-change listeners include:
FormattedTextFieldDemo
in
How to Use Formatted Text Fields for an example of this.DialogDemo
in
How to Make Dialogs for an example of registering a property-change listener on an option pane to listen to changes to the value property. You can also check out FileChooserDemo2
in
How to Use File Choosers for an example of how to register a property-change listener to listen to changes to the directoryChanged and selectedFileChanged properties.TrackFocusDemo
and DragPictureDemo
in
How to Use the Focus Subsystem for examples of this.Although these are some of the more common uses for property-change listeners, you can register a property-change listener on the bound property of any component that conforms to the JavaBeans specification.
You can register a property change listener in two ways. The first uses the method addPropertyChangeListener(PropertyChangeListener)
. When you register a listener this way, you are notified of every change to every bound property for that object. In the propertyChange
method, you can get the name of the property that has changed using the PropertyChangeEvent
getPropertyName
method, as in the following code snippet:
KeyboardFocusManager focusManager = KeyboardFocusManager.getCurrentKeyboardFocusManager(); focusManager.addPropertyChangeListener(new FocusManagerListener()); ... public FocusManagerListener implements PropertyChangeListener { public void propertyChange(PropertyChangeEvent e) { String propertyName = e.getPropertyName(); if ("focusOwner".equals(propertyName) { ... } else if ("focusedWindow".equals(propertyName) { ... } } ... }
The second way to register a property change listener uses the method addPropertyChangeListener(String, PropertyChangeListener)
. The String
argument is the name of a property. Using this method means that you only receive notification when a change occurs to that particular property. So, for example, if you registered a property change listener like this:
aComponent.addPropertyChangeListener("font", new FontListener());
FontListener
only receives notification when the value of the component's font property changes. It does not receive notification when the value changes for transferHandler, opaque, border, or any other property.
The following example shows how to register a property change listener on the value property of a formatted text field using the two-argument version of addPropertyChangeListener
:
//...where initialization occurs: double amount; JFormattedTextField amountField; ... amountField.addPropertyChangeListener("value", new FormattedTextFieldListener()); ... class FormattedTextFieldListener implements PropertyChangeListener { public void propertyChanged(PropertyChangeEvent e) { Object source = e.getSource(); if (source == amountField) { amount = ((Number)amountField.getValue()).doubleValue(); ... } ...//re-compute payment and update field... } }
Registering a PropertyChangeListener
Method | Purpose |
---|---|
addPropertyChangeListener(PropertyChangeListener) | Add a property-change listener to the listener list. |
addPropertyChangeListener(String, PropertyChangeListener) | Add a property-change listener for a specific property. The listener is called only when there is a change to the specified property. |
The PropertyChangeListener Interface
Because PropertyChangeListener
has only one method, it has no corresponding adapter class.
Method | Purpose |
---|---|
propertyChange(PropertyChangeEvent) | Called when the listened-to bean changes a bound property. |
Method | Purpose |
---|---|
Object getNewValue() Object getOldValue() |
Return the new, or old, value of the property, respectively. |
String getPropertyName() | Return the name of the property that was changed. |
void setPropagationId() | Get or set the propagation ID value. Reserved for future use. |
The following table lists the examples that use property-change listeners.
Example | Where Described | Notes |
---|---|---|
FormattedTextFieldDemo |
How to Use Formatted Text Fields | A property-change listener is registered on several formatted text fields to track changes to the value property. |
DialogDemo |
How to Make Dialogs | The
CustomDialog class registers a property-change listener on an option pane to listen to the value and inputValue properties. |
FileChooserDemo2 |
How to Use File Choosers | The
ImagePreview class registers a property-change listener on the file chooser to listen to the directoryChanged and selectedFileChanged properties. |
TrackFocusDemo |
How to Use the Focus Subsystem | A property-change listener is registered on the keyboard focus manager to track changes to the focusOwner property. |