subclassing event objects

Yes, it's possible, and the source-listener architecture is a good design. Consider a component JSpin, a spin button that does not exist in swing. It provides events when a spin action is performed. The listner class can be:

public interface SpinListener extends EventListener

{

public void spinPerformed(SpinEvent event);

}

The spin event:

public class SpinEvent extends EventObject

{

public static final int UP_SPIN_PERFORMED = 0;

public static final int DOWN_SPIN_PERFORMED = 1;

private int id;

public SpinEvent(Object source, int id)

{

super(source);

this.id = id;

}

public int getId()

{

return id;

}

}

Actually, the implementation is yours to decide. Take two buttons on a panel, listen to some state changes or button acttions and then you can fire your own spin events to listeners. Thus you should implement these two methods:

public synchronized void addSpinListener(SpinListener listener)

public synchronized void removeSpinListener(SpinListener

listener)

And fire:

protected synchronized void fireSpinListenerSpinPerformed(

SpinEvent event)

{

Vector list;

if (spinListenerList != null)

{

list = (Vector) spinListenerList.clone();

for (int counter = 0; counter < list.size(); counter++)

{

((SpinListener) list.elementAt(counter)).

spinPerformed(event);

}

}

}

Kurta

h230561 at 2007-6-29 0:32:57 >

The reason is: to add your event specific knowledge to the event class. Of course you might define your own event class that is not a subclass of EventObject. For me it's just a convention, like implementing the empty interface EventListener.

To know more: http://developer.java.sun.com/developer/onlineTraining/Beans/JBShortCourse/beans.html

Kurta

h230561 at 2007-6-29 0:32:57 >

The two main benefits here (albeit minimal) are the functionality inherited from EventObject and the fact that it makes your unique event type standard and thus interchangeable with other listeners and objects in the Swing/AWT API. Often, implementers of event types won't care about the main function ( getSource() ) offered by EventObject but will extend anyway to get the latter mentioned benefit. It makes your object ten times more reusable someday, later on, even when you can't envision an immediate need.

boothben at 2007-6-29 0:32:57 >