The following constructor:
http://download.oracle.com/javase/7/docs/api/java/beans/PropertyChangeEvent.html#PropertyChangeEvent%28java.lang.Object,%20java.lang.String,%20java.lang.Object,%20java.lang.Object%29

Does not specify thrown NullPointerException for passed null source object
However the parent constructor which is used by the implementation specifies:

http://download.oracle.com/javase/7/docs/api/java/util/EventObject.html#EventObject%28java.lang.Object%29

The following code illustrates this issue:

import java.beans.PropertyChangeEvent;

/**
 *
 */
public class PropChangeEvent {

    public static void main(String[] args) {

        /**
         * Constructs a new PropertyChangeEvent.
            Parameters:
            source - The bean that fired the event.
            propertyName - The programmatic name of the property that was changed.
            oldValue - The old value of the property.
            newValue - The new value of the property.
         */
        new PropertyChangeEvent(null, null, null, null);
    }

}

The output will be:

Exception in thread "main" java.lang.IllegalArgumentException: null source
at java.util.EventObject.<init>(EventObject.java:56)
at java.beans.PropertyChangeEvent.<init>(PropertyChangeEvent.java:60)
at bugs.PropChangeEvent.main(PropChangeEvent.java:20)