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Send Sending. Log in with Facebook. Many other user operations could also be cited as examples. Events may also occur that are not directly caused by interactions with a user interface. For example, an event may be generated when a timer expires, a counter exceeds a value, a software or hardware failure occurs, or an operation is completed. You are free to define events that are appropriate for your application. Event Sources. A source is an object that generates an event.
This occurs when the internal state of that object changes in some way. Sources may generate more than one type of event. A source must register listeners in order for the listeners to receive notifications about a specific type of event. Each type of event has its own registration method. Here is the general form:. Here, Type is the name of the event, and el is a reference to the event listener.
For example, the method that registers a keyboard event listener is called addKeyListener. The method that registers a mouse motion listener is called addMouseMotionListener. When an event occurs, all registered listeners are notified and receive a copy of the event object.
This is known as multicasting the event. In all cases, notifications are sent only to listeners that register to receive them. Some sources may allow only one listener to register. The general form of such a method is this:. When such an event occurs, the registered listener is notified. This is known as unicasting the event. A source must also provide a method that allows a listener to unregister an interest in a specific type of event.
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The model for event processing in version 1. In order for a program to catch and process GUI events, it must subclass GUI components and override either action or handleEvent methods. Returning "true" from one of these methods consumes the event so it is not processed further; otherwise the event is propagated sequentially up the GUI hierarchy until either it is consumed or the root of the hierarchy is reached.
The result of this model is that programs have essentially two choices for structuring their event-handling code: Each individual component can be subclassed to specifically handle its target events.
The result of this is a plethora of classes.
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