In life, you encounter events that force you to suspend other activities and respond to them immediately. In Java, events represent all activity that goes on between the user and the application. Java's Abstract Windowing Toolkit (AWT) communicates these actions to the programs using events.
Components of an Event: Can be put under the following categories.
1. Event Object: When the user interacts with the application by clicking a mouse button or pressing a key an event is generated. The Operating System traps this event and the data associated with it. For example, info about time at which the event occurred, the event types (like keypress or mouse click). This data is then passed on to the application to which the event belongs. In Java, events are represented by objects, which describe the events themselves. And Java has a number of classes that describe and handle different categories of events.
2. Event Source: An event source is the object that generated the event. Example if you click a button an ActionEvent Object is generated. The object of the ActionEvent class contains information about the event.
3. Event-Handler: Is a method that understands the event and processes it. The event-handler method takes the Event object as a parameter. Java uses Event-Delegation Model :with JDK1.1 onwards; you can specify the objects that are to be notified when a specific event occurs. If the event is irrelevant, it is discarded. The four main components based on this model are Event classes, Event Listeners, Explicit event handling and Adapters. Let us take a closer look at them one by one.
Event Classes: The EventObject class is at the top of the event class hierarchy. It belongs to the java.util package. While most of the other event classes are present in java.awt.event package. The getSource() method of the EventObject class returns the object that initiated the event. The getId () method returns the nature of the event. For example, if a mouse event occurs, you can find out whether the event was click, a press, a move or release from the event object. AWT provides two conceptual types of events: Semantic and low-level events.
Java & J2EE 
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