Implementing a Constrained Property
Constrained properties fire a PropertyChangeEvent whenever its value is about to be changed. Any listener can veto the event, thereby preventing the change. This example bean implements a single constrained integer property called ``property.'' 

 int property;
 public int getProperty() {
   return property;
 }
 public void setProperty(int newValue) throws 
   PropertyVetoException {
     try {
       vceListeners.fireVetoableChange(
         "property", 
         new Integer(property), 
         new Integer(newValue));
       property = newValue;
     } catch (PropertyVetoException e) {
       throw e;
     }
 }
     
 // Create the listener list.
 VetoableChangeSupport vceListeners = 
   new VetoableChangeSupport(this);
     
 // The listener list wrapper methods.
 public synchronized void 
         addVetoableChangeListener(
          VetoableChangeListener listener) {
   vceListeners.addVetoableChangeListener(
                                  listener);
 }
 public synchronized void 
               removeVetoableChangeListener(
                       VetoableChangeListener 
                                  listener) {
   vceListeners.removeVetoableChangeListener(
                                   listener);
 }