Developing Java Beans

Constrained Properties

To this point we’ve assumed that all the property changes that take place are acceptable. Often this will not be the case; it’s possible that an attempt to set a property to a given value will be unacceptable and therefore rejected. The simple case is when the object that owns the property wants to reject the change, but there may be times when another object wants a chance to voice its disapproval. Properties that go through this approval process are known as constrained properties.

The design pattern for setting and getting constrained properties is similar to the design pattern for properties that are not constrained. The difference is that the set method declares that it throws the exception java.beans.PropertyVetoException. The method signatures look as follows:

public <PropertyType> get<PropertyName>();
public void set<PropertyName>(<PropertyType> value)
            throws java.beans.PropertyVetoException;

Binding to Non-Specific Constrained Properties

Those objects that want to participate in the approval process for a change to a constrained property must implement the java.beans.VetoableChangeListener interface. This interface contains a method called vetoableChange() that takes a single parameter of type PropertyChangeEvent. This method may throw the java.beans.PropertyVetoException if it wants to reject the change. Objects that support constrained properties must provide methods for registering and unregistering VetoableChangeListener objects. The methods ...