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Understanding Event Handling in Java: Observers, Listeners, and the Event Hierarchy

In this lesson, we explore event handling in Java, focusing on how event objects are processed within graphical components. Java 2 employs a delegation model where observable components generate events that are sent to registered listeners. We will examine the event hierarchy, including various event types like ActionEvent and MouseEvent, and understand the roles of listener interfaces and adapter classes. Finally, we present a practical example: an applet with a button labeled "Click Me" that beeps when clicked, demonstrating how event handling works in practice.

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Understanding Event Handling in Java: Observers, Listeners, and the Event Hierarchy

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Presentation Transcript

  1. ProgrammingLanguages

  2. The Java Event Model In this lesson, we look at the content of these objects and how they are processed in Java 2.

  3. Events In Java 2 • Java 2 handles events that are generated in a graphical component by delegating to objects that have registered an interest in that type of event. • The observable component that generates events, and the observer is the object that has registered to receive events. • Event observer objects are called listeners and implement listener interfaces.

  4. The Event Hierarchy • The root of the Java 2 event hierarchy is the EventObject class • java.uti1.EventObject +--java.awt.AWTEvent +-- java.awt.event.ActionEvent +-- java.awt.event.AdjustmentEvent +-- java.awt.event.AncestorEvent +-- java.awt.event.ComponentEvent +-- java.awt.event.InputMethodEvent +-- java.awt.event.InternalFrameEvent +-- java.awt.event.InvocationEvent +-- java.awt.event.ItemEvent +-- java.awt.event.TextEvent

  5. Events that descend from ComponentEvent • For typical programming, user probably works with only a few of the events: ComponentEvent • java.awt.event.ComponentEvent +-- java.awt.event.ContainerEvent +-- java.awt.event.FocusEvent +-- java.awt.event.PaintEvent +-- java.awt.event.WindowEvent +-- java. awt.event.InputEvent +-- java.awt.event.MouseEvent +-- java.awt.event.KeyEvent

  6. Listeners And Adapters • Java defines a number of listener interfaces that are named according to the events they process. • Java also includes classes called adapters that provide empty methods for the more complex interfaces. • Each class that can generate one of these events has a method to add a listener for that event.

  7. Events, Listener interfaces, and Adapter classes.

  8. An Java Example with Event-Handling • This Java Applet has a button, with caption “Click Me”. When user clicks the button, it beeps.

  9. An Java Example with Event-Handling

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