Building Reusable Java Widgets

An introduction to writing pluggable do-it-yourself widgets for the Java programmer.
2.4. Event Multicasters

Event multicasters handle asynchronous broadcasting of events to listeners. You will be relieved to know that you do not have to write your own multicaster from scratch. Instead you will need to create a subclass of the existing AWTEventMulticaster and write a few methods so that it can handle your new events. When you create your multicaster follow these steps:

  • Implement the listener interface that you created in Listing 3.

  • Create the add and remove methods using that same listener.

  • Create the methods defined in the listener interface. These methods simply forward the messages to the appropriate listeners.

Take a look at Listing 4 to see how I created the multicaster for this example. Don't let the multicaster scare you. The code is very basic, cookie-cutter-style. You will be relying on the superclass to do all of the hard multicasting work. All that your multicaster must do is be aware of your new events and listeners.

2.5. Hooking it all up

We have now created all of the necessary components,, and what remains is to connect it up properly. The first order of business is to complete the EmailWidget class. As we left it, it only had a constructor. Next you must give it the ability to add listeners. Here is the addEmailEntryEventListener method:

public void addEmailEntryListener(
        EmailEntryListener e ) {
_emailEntryListener = MyMulticaster.add(
        _emailEntryListener, e ) ;

Notice that the widget has an instance variable that maintains its listener. If you look closely, you will see that this variable is actually an instance of your multicaster class. Any widget can potentially have many listeners. Your multicaster will maintain the list of listeners and ensure that they get their appropriate events. Finally, you must handle the internal events (from the buttons) and generate your new event.

public void actionPerformed( ActionEvent e ) {
        EmailEntryEvent newEvent ;
        if ( _emailEntryListener == null ) return ;
        if ( e.getSource() == _doneButton ) {
                newEvent = new EmailEntryEvent(
                                getEmailAddress()) ;
        _emailEntryListener.done( newEvent );
        else if ( e.getSource() == _cancelButton ) {
                newEvent = new EmailEntryEvent() ;
                _emailEntryListener.cancel( newEvent );
In this code you generate the widget specific events. When the done button is pressed, a “done” event is created that stores the e-mail address. Otherwise a “cancel” event is generated. Notice also how the event is dispatched: the corresponding method is called on the multicaster (_emailEntryListener is an instance of MyMulticaster). The multicaster then forwards the method call to all of the registered listeners. The EmailEntry widget is now ready to be used by any Java program. Listing 5 shows a simple applet that uses the e-mail entry widget and intercepts the events. Take a look at the e-mail entry widget in Figure 7.

Figure 7. The EmailEntryWidget in Use as an Applet

One important note: the e-mail entry class generates a done event. How is this different from the action event generated by the done button? The difference is subtle, but important. If you rely upon detecting the events of a specific button, other classes need to know the details of the inner workings of your class. If this changes, every class that uses it has to change as well.

2.6. Exercises for the Reader

Here are a few ideas for expanding the EmailEntry widget that you might enjoy trying:

  • Collect more information: Expand the widget by collecting more information from the user. Add checkboxes, additional text fields and so on. Remember that your event class has to maintain this information and pass it on to the listeners.

  • Add error checking: Check for things like the e-mail address being complete and reasonable. Be sure that you are broadcasting the event only if the form is completed correctly.