With this example you are going to learn about a very important event monitoring component, the VetoableChangeListener. The VetoableChangeListener is , in many ways, similar to the PropertyChangeListener class. The main difference is that PropertyChangeListener is applied to bound properties. On the other hand the VetoableChangeListener is applied to constraint properties.
A bound property is just as simple as a property. A constraint property is a property that can change its state only if the event listener allows it to. For example, if the new value of the property is bigger that the listener can allow, it can refuse to give the new value to the property.
In order to work with a VetoableChangeListener one should take the following steps:
- Use the
addVetoableChangeListenermethod of an event manager and give the new class as an argument to bundle an event with aVetoableChangeListener. - Create a new class that implements the
VetoableChangeListenerinterface. Override thevetoableChangemethod and examine the old and the new value of the property. If you are not happy with the new value you can throw aPropertyVetoException
Let’s take a look a the code:
package com.javacodegeeks.snippets.desktop;
import java.awt.Component;
import java.awt.KeyboardFocusManager;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyVetoException;
import java.beans.VetoableChangeListener;
public class VetoableChangeListenerExample {
public static void main(String[] argv) {
KeyboardFocusManager.getCurrentKeyboardFocusManager().addVetoableChangeListener(
new FocusVetoableChangeListener());
}
}
class FocusVetoableChangeListener implements VetoableChangeListener {
@Override
public void vetoableChange(PropertyChangeEvent evt) throws PropertyVetoException {
Component oldComp = (Component) evt.getOldValue();
Component newComp = (Component) evt.getNewValue();
if ("focusOwner".equals(evt.getPropertyName())) {
if (oldComp == null) {
System.out.println(newComp.getName());
} else {
System.out.println(oldComp.getName());
}
} else if ("focusedWindow".equals(evt.getPropertyName())) {
if (oldComp == null) {
System.out.println(newComp.getName());
} else {
System.out.println(oldComp.getName());
}
}
boolean vetoFocusChange = false;
if (vetoFocusChange) {
throw new PropertyVetoException("message", evt);
}
}
}
This was an example on how to use VetoableChangeListener .
