How do I listen for a constrained property change?

The constrained property change is fired when a bean’s value is about to change. When a VetoableChangeListener veto the value change the bean’s value will be rolled-back to the previous value. In this example we have a constrained property called interest.

package org.kodejava.example.bean;

import java.beans.VetoableChangeSupport;
import java.beans.PropertyVetoException;

public class VetoBean {
    private double interest;

    private VetoableChangeSupport vcs = new VetoableChangeSupport(this);

    public VetoBean() {
        vcs.addVetoableChangeListener(new VetoChangeListener());
    }

    public double getInterest() {
        return interest;
    }

    public void setInterest(double interest) {
        try {
            vcs.fireVetoableChange("interest", new Double(this.interest), new Double(interest));

            this.interest = interest;
        } catch (PropertyVetoException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        VetoBean bean = new VetoBean();
        bean.setInterest(10.99);
        bean.setInterest(15.99);
        
        //
        // PropertyVetoException will be thrown beacuse the interest value 
        // should not exceed 20.00.
        //
        bean.setInterest(20.99);
    }
}
package org.kodejava.example.bean;

import java.beans.VetoableChangeListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyVetoException;

public class VetoChangeListener implements VetoableChangeListener {
    /**
     * This method gets called when a constrained property is changed.
     *
     * @param evt a PropertyChangeEvent object describing the
     *            event source and the property that has changed.
     * @throws java.beans.PropertyVetoException
     *          if the recipient wishes the property
     *          change to be rolled back.
     */
    public void vetoableChange(PropertyChangeEvent evt) throws PropertyVetoException {
        String eventName = evt.getPropertyName();
        if (eventName.equalsIgnoreCase("interest")) {
            double interest = ((Double) evt.getNewValue()).doubleValue();
            if (interest > 20.00) {
                throw new PropertyVetoException("Interest must be below 20.00", evt);                
            }
            System.out.println("Interest applied = " + interest);
        }
    }
}

How do I listen for bean’s property change event?

In this example we’ll listen to bean’s property change event. We create a small bean named MyBean, adds attributes and getter/setter. We want to know or to get notified when the bean property name is changed.

First we need the add a PropertyChangeSupport field to the bean, with this object we will fire the property change event. When we need to listen for the change we have to create an implementation of a PropertyChangeListener. In this example we’ll just use the MyBean class as the listener.

The PropertyChangeListener has a method called propertyChange and inside this method we’ll implementing the code to get the event fired by the PropertyChangeSupport.

package org.kodejava.example.bean;

import java.beans.PropertyChangeSupport;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeEvent;
import java.io.Serializable;

public class MyBean implements PropertyChangeListener, Serializable {
    private Long id;
    private String name;

    private PropertyChangeSupport pcs = new PropertyChangeSupport(this);

    public MyBean() {
        pcs.addPropertyChangeListener(this);
    }

    /**
     * This method gets called when a bound property is changed.
     *
     * @param evt A PropertyChangeEvent object describing the event source
     *            and the property that has changed.
     */
    public void propertyChange(PropertyChangeEvent evt) {
        System.out.println("Name      = " + evt.getPropertyName());
        System.out.println("Old Value = " + evt.getOldValue());
        System.out.println("New Value = " + evt.getNewValue());
    }

    public static void main(String[] args) {
        MyBean bean = new MyBean();
        bean.setName("My Initial Value");
        bean.setName("My New Value");
        bean.setName("My Yet Another Value");
    }


    //~ --------------------------------------------- Bean's Getters and Setters

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        String oldValue = this.name;
        this.name = name;

        //
        // Fires a property change event
        //
        pcs.firePropertyChange("name", oldValue, name);
    }
}