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8 GUI Event Handling
8.1 Objectives
In this module, you will learn how to handle events triggered when the user interacts with your GUI application. After completing this module, youll be able to develop GUI
applications that responds to user interaction.
After completing this lesson, you should be able to: 1. Enumerate the delegation event model components
2. Explain how the delegation event model works 3. Create GUI applications that interact with the user
4. Discuss benefits of adapter classes 5. Discuss advantages of using inner and anonymous classes
8.2 The Delegation Event Model
The Delegation event model describes how your program can respond to user interaction. To understand the model, let us first study its three important components.
1. Event Source The event source refers to the GUI component that generates the event. For example,
if the user presses a button, the event source in this case is the button. 2. Event ListenerHandler
The event listener receives news of events and processes users interaction. When a button is pressed, the listener may handle this by displaying an information useful to
the user. 3. Event Object
When an event occurs i.e., when the user interacts with a GUI component, an event object is created. The object contains all the necessary information about the event
that has occurred. The information includes the type of event that has occurred, say, the mouse was clicked. There are several event classes for the different categories of
user action. An event object has a data type of one of these classes.
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Here now is the delegation event model.
Figure 8.1: Delegation Event Model
Initially, a listener should be registered with a source so that it can receive information about events that occur at the source. Only registered listeners can receive notifications
of events. Once registered, a listener simply waits until an event occurs.
When something happens at the event source, an event object describing the event is created. The event is then fired by the source to the registered listeners.
Once the listener receives an event object i.e., a notification from the source, it goes to work. It deciphers the notification and processes the event that occurred.
8.2.1 Registration of Listeners
The event source registers a listener through the addTypeListener methods. void addTypeListenerTypeListener listenerObj
Type depends on the type of event source. It can be Key, Mouse, Focus, Component, Action and others.
Several listeners can register with one event source to receive event notifications.
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A registered listener can also be unregistered using the removeTypeListener methods. void removeTypeListenerTypeListener listenerObj
8.3 Event Classes
An event object has an event class as its reference data type. At the root of event class heirarchy is the EventObject class, which is found in the java.util package. An immediate
subclass of the EventObject class is the AWTEvent class. The AWTEvent class is defined in java.awt package. It is the root of all AWT-based events. Here are some of the AWT
event classes.
Event Class Description
ComponentEvent Extends AWTEvent. Instantiated when a component is moved, resized, made visible or hidden.
InputEvent Extends ComponentEvent. The abstract root event class for all
component-level input event classes. ActionEvent
Extends AWTEvent. Instantiated when a button is pressed, a list item is double-clicked, or a menu item is selected.
ItemEvent Extends AWTEvent. Instantiated when an item is selected or
deselected by the user, such as in a list or a checkbox. KeyEvent
Extends InputEvent. Instantiated when a key is pressed, released or typed.
MouseEvent Extends InputEvent. Instantiated when a mouse button is pressed,
released, or clicked pressed and released, or when a mouse cursor enteres or exits a visible part of a component.
TextEvent Extends AWTEvent. Instantiated when the value of a text field or a
text area is changed. WindowEvent
Extends ComponentEvent. Instantiated when a Window object is opened, closed, activated, deactivated, iconified, deiconified, or when
focus is transferred into or out of the window.
Table 23: Event classes
Take note that all AWTEvent subclasses follow this naming convention: TypeEvent
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8.4 Event Listeners
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