I.4. Common GUI Event Types and Listener Interfaces

In Section I.3, you learned that information about the event that occurs when the user presses Enter in a text field is stored in an ActionEvent object. Many different types of events can occur when the user interacts with a GUI. The event information is stored in an object of a class that extends AWTEvent (from package java.awt). Figure I.3 illustrates a hierarchy containing many event classes from the package java.awt.event. Additional event types are declared in package javax.swing.event.

Fig. I.3. Some event classes of package java.awt.event.

Let’s summarize the three parts to the event-handling mechanism that you saw in Section I.3—the event source, the event object and the event listener. The event source is the 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’s 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. This event-handling model is known as the delegation event model—an event’s processing is delegated to an object (the event listener) in the app.