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Bound Properties
 
  UC ingot Bound properties inform listeners that their value has changed.  Thus, you can take an action when the setter method is called.
 
  • To be a bound property, the bean must send a PropertyChange event when the property changes.
  • Thus, the bean must support listeners.  Listeners can register themselves with the 

    • addPropertyChangeListener(PropertyChangeListener listener)
    and
    removePropertyChangeListener(PropertyChangeListener listener)
    methods.
    • The PropertyChangeListener interface has a single method, public void propertyChange(PropertyChangeEvent evt).  This method is called when a bound property is changed.
  • But, there is a convenience method available to make your life easier.  Create an instance field of type PropertyChangeSupport, and pass the constructor a reference to the this object (an instance of the current object).  Then you can proxy your dirty work to it.


    • PropertyChangeSupport pcs = new PropertyChangeSupport(this);

    public void addPropertyChangeListener(PropertyChangeListener listener) {

      pcs.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {

      pcs.removePropertyChangeListener(listener);
    }

    Whenever an event happens, simply call this method of the PropertyChangeSupport object:

    firePropertyChange(String propertyName, Object oldValue, Object newValue) 

    There are a few different signatures of this method in JDK 1.4.* (see the online API documentation  for details).  One takes ints for the new and old values, another takes booleans, and a fourth takes an event object itself.  Like the ResultSet methods in the slide about indexed properties, this allows the programmer to choose the optimal level of complexity.
     

    • This way you don't have to deal with the dirty work of managing listeners.
    • Remember that Swing components are beans.  So if your bean extends a Swing component, you can simply piggy back off of its event handling structure.
  • Beans that want to be notified of chagnes to bound properties must implement the PropertyChangeListener interface.
    • Thus, they must implement the one and only method of this interface, void propertyChange(PropertyChangeEvent event) .
    • This method is called whenever the value of the bound property is changed.  It is passed an Event object, which contains useful information about the change.
      • public Object getOldValue() - gets the previous value of the property.
      • public Object getNewValue() - gets the new value of the property.
  • Example
    • If we change our database with setDatabase(), or change our username and password, we may want to fire an event to any other beans that we're using in our Database bean.


     Packaging Beans in a Jar file