JAVA 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 listener to receive notification about a specific type of event.

 Each type of notification has its own registration method. The general form is shown below:

public void addTypeListner (TypeListner el)

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 ().  

 When an event occurs, all registered listeners are notified and receive a copy of the event object. This is known as multicasting the event.

 Some sources may allow only one listener to register. Its general form is:

public void addTypeListner (TypeListner el) throws  java.util.TooManyListenersException

When some 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 for a specific type of event. Its general form is:

public void removeTypeListner (TypeListner el)

Here, Type is the name of the event, and el is a reference to the event listener. For example, the method to remove a keyboard event listener is removeKeyListener ().