PropertyChangeEvent

PropertyChangeEvent is an event class in the JavaBeans framework used to signal that a bound property on a bean has changed. It is commonly used to notify interested listeners when a property value is updated, enabling data binding and UI updates.

The class is defined in the java.beans package and extends java.util.EventObject, implementing Serializable. A PropertyChangeEvent carries information about the change: the source object that fired the event, the name of the property that changed, and the old and new values. The typical constructor is PropertyChangeEvent(Object source, String propertyName, Object oldValue, Object newValue). Accessors include getPropertyName(), getOldValue(), and getNewValue(); the getSource() method is inherited from EventObject.

Usage typically involves a bean that supports property change listeners through a PropertyChangeSupport object. When a property's value changes, the bean fires a PropertyChangeEvent to registered listeners by calling firePropertyChange with the property name and the old and new values. Listeners implement PropertyChangeListener and respond in their propertyChange(PropertyChangeEvent evt) method, which can inspect the property name and the change details to take appropriate action.

Notes: oldValue and newValue are of type Object and may be null. A bean is free to fire an event only when the change is meaningful, and some properties may trigger many changes while others may remain stable. PropertyChangeEvent plays a central role in JavaBeans data binding and in GUI frameworks that reflect model changes in views.

See also: PropertyChangeListener, PropertyChangeSupport, and related vetoable change mechanisms.