What is Event Handling?

Almost all programs must respond to commands from the user in order to be useful. Java's AWT uses event driven programming to achieve processing of user actins: one that underlies all modern window systems programming. Within the AWT, all user actions belong to an abstract set of things called events. An event describes, in sufficient detail, a particular user action. Rather than the program actively collecting user-generated events, the Java run time notifies the program when an interesting event occurs. Programs that handle user interaction in this fashion are said to be event driven.

There are three parts to the event model in Java:

Event object - this is an instance of a Java class that contains the characteristics of the event. For example, if the event is generated from clicking a mouse button, the event object would contain information such as the coordinates of the mouse cursor, which button was clicked, how many times it was clicked, etc.

Dispatching class/method - this is an object, which detects that an event has occurred and is then responsible for notifying other objects of the event, passing the appropriate event object to those objects. These other objects are "listeners" for the event. Most AWT components, such as Button, List, Textfield, etc. are examples of dispatching classes.

A Button, for instance, is capable of notifying other components when it is "pushed." These classes will typically have a set of two methods that can be invoked by would-be "listeners": one to tell the class that the object wants to listen and another to tell the class that the object no longer wants to listen.

These methods are conventionally named i.e. addxxListener (to add an object as a listener) or removexxListener (to remove the object as a listener). Here ‘xx’ is the specific type of event to listen for. In the case of the Button, this would be "Action" to indicate the action of pushing the button. (So the methods would be addActionListener and removeActionListener).

Listener Interface - for the dispatching to work properly, the dispatching class must be able to rely on each of its listeners to contain the method that it executes when the event occurs. This is easily accomplished in Java through the use of an Interface class. The important point is that a class, which is going to be a listener, must implement that interface.

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