Developing Java Beans

Handling Events for Specific Properties

If a Bean component supports bound properties, it is required to support the binding mechanism described earlier. This means that it must provide an addPropertyChangeListener() method for any client object that wants to receive bound property changes via the PropertyChangeListener interface. The same holds true for constrained properties. In that case, the Bean must provide an addVetoableChangeListener() method for any client object that wants to receive VetoableChangeEvent notifications via the VetoableChangeListener interface. But the Bean also has the option of providing specific registration methods for listeners to register for property change and vetoable change notifications for specific properties. There is no requirement to support these mechanisms, but they may be useful under some circumstances.

There is a design pattern for registering and unregistering event listeners for changes to specific bound properties. The method signatures are as follows:

public void add<PropertyName>Listener(PropertyChangeListener p);
public void remove<PropertyName>Listener(PropertyChangeListener p);

There is also a design pattern for registering and unregistering event listeners for vetoable change events for specific constrained properties. The method signatures are as follows:

public void add<PropertyName>Listener(VetoableChangeListener p);
public void remove<PropertyName>Listener(VetoableChangeListener p);

This is convenient if a source object supports many properties and you are only interested in one. It eliminates unwanted PropertyChangeEvent and VetoableChangeEvent notifications. If you’re interested in changes for multiple properties, however, this technique probably doesn’t add any value. All of the property changes will still be directed to your one and only implementation of the propertyChange() or vetoableChange() methods. If you wanted to direct various property change events to different methods, you could combine these patterns with the adapter technique described in Chapter 3.