EVENT HANDLING MODEL

• GUIs are event driven (i.e. they generate events when the user of the program interacts with the GUI). Some common interactions are moving the mouse, clicking the mouse, clicking a button, typing in a text field, selecting an item from a menu, closing a window etc.

• When a user interaction occurs, an event is sent to the program. GUI event information is stored in an object of a class that extends AWTEvent.

• AWTEvents are used with both AWT components and Swing components.

Java.lang.Object Action Event Container Event

Java.util.EventObject Adjustment Event Focus Event

Java.awt.AWTEvent Item Event Paint Event

Component Event Window Event

Input Event

Key Event Mouse Event

• There are three parts to the event handling mechanism
• Event Source
• Event Object
• 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. This information includes a reference to the event source and any event-specific information that may be required by the event listener to handle the event.

• The event listener is an object that is notified by the event source when and event occurs. The event listener receives an event object when it is notified of the event, then uses the object to respond to the event. The event source is required to provide methods that enable listeners to be registered and unregistered. The event source also is required to maintain a list of its registered listeners and be able to notify its listeners when an event occurs.

• The programmer must perform two key tasks to process a graphical user interface event in a program. 1) Register an event listener for the GUI component that is expected to generate the event. 2) Implement and event handling method (or set of event-handling methods), called event handlers.

• An event listener for a GUI event is an object of a class that implements one or more of the event-listener interfaces from package java.awt.evetn and package javax.swing.event. Many of the event-listener are common to both packages. The main event that all listeners extend from is the java.util.EventListener .

Additional event listeners can be found in the javax.swing.event package.

• An event listener object listens for specific types of events generated by event sources in a program. An event handler is a method that is called in response to a particular type of event. Each event interface specifies one or more event-handling methods that must be defined in the class that implements the event-listener interface. REMEMBER that interfaces define abstract methods. Any class that implements an interface must define all the methods of that interface.

• When an event occurs, the GUI component with which the user interacted notifies its registered listeners by calling each listeners appropriate event handling method. For example, when the user presses the Enter Key in a JTextField, the registered listener’s actionPerformed method is called.

Example