Database Programming with JDBC & Java, Second Edition

Distributed Listeners

Swing uses an event model that enables UI delegates to monitor their models for changes. Under this model, a UI delegate registers itself with its model as a listener. It listens for specific events, including property changes, that may require it to redraw itself. This event model, however, is sufficiently abstract to allow any object to listen for changes in a model. In fact, any object that has interesting things happen to it—a change in a value, a change in its internal structure, etc.—can allow other objects to listen for when those things occur. When one of those things occur, the object of interest notifies its listeners of the occurrence.

The example most people are familiar with is a button component. When you place a button on a screen, your application probably wants to know when someone clicks on the button so that an appropriate action can be performed. A button supports ActionListener listeners. An ActionEvent is an event that occurs when a user requests a GUI component to do its thing. The user request usually comes in the form of hitting the Enter key or clicking on the component. Using the following code, an application can register what should happen when the button is clicked:

JButton button = new JButton("Save");

button.addActionListener(new ActionListener( ) {
    public void actionPerformed(ActionEvent evt) {
        save( );
    }
});

This code creates an anonymous class that listens to the button for ActionEvent occurrences. This example is mercifully ...