e012. listening for a vetoable property change event.txt

这里面包含了一百多个JAVA源文件

A vetoable property change event is fired when a constrained property is changed. A listener can veto the change by throwing PropertyVetoException and preventing the change. 
    // Register for property change events on the bean
    bean.addVetoableChangeListener(new MyVetoableChangeListener());
    
    class MyVetoableChangeListener implements VetoableChangeListener {
        // This method is called every time the property value is changed
        public void vetoableChange(PropertyChangeEvent evt) throws PropertyVetoException {
            // Get the old value of the property
            Object oldValue = evt.getOldValue();
    
            // Get the new value of the property
            Object newValue = evt.getNewValue();
    
            // Determine if the change should be vetoed, thereby preventing the change
            boolean veto = false;
            if (veto) {
                throw new PropertyVetoException("the reason for the veto", evt);
            }
        }
    }