Cyclic Property Change Listeners

Question

I have a cyclic property change listener set up. I have 2 objects of class A. I have made one object listen to changes of the other and vice versa. Here's the code:

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class CyclicPCLTest {

    public static class A implements PropertyChangeListener{
       private PropertyChangeSupport propertyChangeSupport;
       private String date;

        public A(String date) {
            this.date = date;
            propertyChangeSupport=new PropertyChangeSupport(this);
        }

        public String getDate() {
           return date;
       }

       public void setDate(String date) {
           String oldDate=this.date;
           this.date = date;
           propertyChangeSupport.firePropertyChange(new PropertyChangeEvent(this, "date", oldDate, this.date));
       }

        public void addPropertyChangeListener(PropertyChangeListener listener){
            propertyChangeSupport.addPropertyChangeListener(listener);
        }

        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            setDate ((String) evt.getNewValue());
        }
    }


    public static void main(String[] args) {
        List<A> as= Arrays.asList(new A[]{new A("01/02/2011"), new A("01/02/2011")});
        as.get(0).addPropertyChangeListener(as.get(1));
        as.get(1).addPropertyChangeListener(as.get(0));

        as.get(0).setDate("02/02/2011");
        System.out.println("date of the other A "+as.get(1).getDate());

        as.get(1).setDate("03/02/2011");
        System.out.println("date of the other A "+as.get(0).getDate());
    }
}

The code works and this is the output:

date of the other A 02/02/2011
date of the other A 03/02/2011

I am wondering how it does? Shouldn't it give me a stackoverflow error? When the first A is updated it notifies the second A, the second A then gets updated and notifies the first A. This should go on for ever.

Answer 1

Thats because firePropertyChange will only fire the event if oldValue is not equal to newValue

After 2 recursive runs, when the code reaches

propertyChangeSupport.firePropertyChange(new PropertyChangeEvent(this, "date", oldDate, this.date));

the oldDate and this.date essentially become same (that is oldDate.equals(this.date) == true ) and it doesn't propagate the event to the listeners

Check the documentation of firePropertyChange method

Fire an existing PropertyChangeEvent to any registered listeners. No event is fired if the given event's old and new values are equal and non-null.