Common GUI Event Types and Listener Interfaces

In Section 11.5, you learned that information about the event that occurs when the user presses Enter in a text field is stored in an ActionEvent object. There are many different types of events that can occur when the user interacts with a GUI. The information about any GUI event that occurs is stored in an object of a class that extends AWTEvent. Figure 11.11 illustrates a hierarchy containing many event classes from the package java.awt.event. Some of these are discussed in this chapter and Chapter 22. These event types are used with both AWT and Swing components. Additional event types that are specific to Swing GUI components are declared in package javax.swing.event.

Figure 11.11. Some event classes of package java.awt.event.

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Let's summarize the three parts to the event-handling mechanism that you saw in Section 11.5the event source, the event object and the event listener. The event source is the particular GUI component with which the user interacts. The event object encapsulates information about the event that occurred, such as a reference to the event source and any event-specific information that may be required by the event listener for it to handle the event. The event listener is an object that is notified by the event source when an event occurs; in effect, it "listens" for an event and one of its methods executes in response to the event. A method of the event listener receives an event object when the event listener is notified of the event. The event listener then uses the event object to respond to the event. The event-handling model described here is known as the delegation event modelan event's processing is delegated to a particular object (the event listener) in the application.

For each event-object type, there is typically a corresponding event-listener interface. An event listener for a GUI event is an object of a class that implements one or more of the event-listener interfaces from packages java.awt.event and javax.swing.event. Many of the event-listener types are common to both Swing and AWT components. Such types are declared in package java.awt.event, and some of them are shown in Fig. 11.12. Additional event-listener types that are specific to Swing components are declared in package javax.swing.event.

Figure 11.12. Some common event-listener interfaces of package java.awt.event.

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Each event-listener interface specifies one or more event-handling methods that must be declared in the class that implements the interface. Recall from Section 10.7 that any class which implements an interface must declare all the abstract methods of that interface; otherwise, the class is an abstract class and cannot be used to create objects.

When an event occurs, the GUI component with which the user interacted notifies its registered listeners by calling each listener's appropriate event-handling method. For example, when the user presses the Enter key in a JTextField, the registered listener's actionPerformed method is called. How did the event handler get registered? How does the GUI component know to call actionPerformed rather than another event-handling method? We answer these questions and diagram the interaction in the next section.