How Event Handling Works

Let us illustrate how the event-handling mechanism works, using textField1 from the example of Fig. 11.9. We have two remaining open questions from Section 11.5:

1. How did the event handler get registered?
2. How does the GUI component know to call actionPerformed rather than some other event-handling method?

The first question is answered by the event registration performed in lines 4346 of the application. Figure 11.13 diagrams JTextField variable textField1, TextFieldHandler variable handler and the objects to which they refer.

Figure 11.13. Event registration for JTextField textField1.

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Registering Events

Every JComponent has an instance variable called listenerList that refers to an object of class EventListenerList (package javax.swing.event). Each object of a JComponent subclass maintains a references to all its registered listeners in the listenerList. For simplicity, we have diagramed listenerList as an array below the JTextField object in Fig. 11.13.

When line 43 of Fig. 11.9

 textField1.addActionListener( handler );

executes, a new entry containing a reference to the TextFieldHandler object is placed in textField1's listenerList. Although not shown in the diagram, this new entry also includes the listener's type (in this case, ActionListener). Using this mechanism, each lightweight Swing GUI component maintains its own list of listeners that were registered to handle the component's events.

Event-Handler Invocation

The event-listener type is important in answering the second question: How does the GUI component know to call actionPerformed rather than another method? Every GUI component supports several event types, including mouse events, key events and others. When an event occurs, the event is dispatched to only the event listeners of the appropriate type. Dispatching is simply the process by which the GUI component calls an event-handling method on each of its listeners that are registered for the particular event type that occurred.

Each event type has one or more corresponding event-listener interfaces. For example, ActionEvents are handled by ActionListeners, MouseEvents are handled by MouseListeners and MouseMotionListeners, and KeyEvents are handled by KeyListeners. When an event occurs, the GUI component receives (from the JVM) a unique event ID specifying the event type. The GUI component uses the event ID to decide the listener type to which the event should be dispatched and to decide which method to call on each listener object. For an ActionEvent, the event is dispatched to every registered ActionListener's actionPerformed method (the only method in interface ActionListener). For a MouseEvent, the event is dispatched to every registered MouseListener or MouseMotionListener, depending on the mouse event that occurs. The MouseEvent's event ID determines which of the several mouse event-handling methods are called. All these decisions are handled for you by the GUI components. All you need to do is register an event handler for the particular event type that your application requires and the GUI component will ensure that the event handler's appropriate method gets called when the event occurs. [Note: We discuss other event types and event-listener interfaces as they are needed with each new component we introduce.]