Java AWT Event

• Event classes are stored in java.awt.event package

Event handling mechanism

⟶Delegation Event Model

• It is a modern approach.
• It uses the concept of source and listener.
• Source generates an event and sends it to one or more listeners.
• Listener must register with the source to receive the event notification.

→Now what is an event?

• An event is an object that describes state change in source.
• for example:-   Suppose  a checkbox is registered with the ItemListener , so whenever the checkbox is selected or de-selected the state of the checkbox changes, therefore whenever the checkbox changes it's state it' s  listener is notified which in turn contains a method called itemStateChanged(ItemEvent ie) which has default parameter of ItemEvent object which in turn describes the event generated by the object.

→How to register a source with the listener

• A source is an object that generates an event if there is some change in the internal state of that object.
• To register a source with the listener addTypeListener(TypeListener object) method is used.
• for example :- addActionListener(this), addItemListener(this)... like this way you can register components with their specified listeners. 
• Here the TypeListener specifies object of class implementing listener interface.
• for example :- To implementing ActionListener, addActionListener(new ActionListener(){public void actionPerformed(){}}); 

 →Explanation of  how an even occurs

• When an even occurs all the registered listeners are verified. this is known as multi-casting the event.
• Some sources may allow only one listener to register by using following method. addTypeListener(TypeListener obj)throws java.util.TooManyListenerException 
• It is the type of uni-casting the listener.
• To un-register with a specific type of event listener, you can use the following method public void removeTypeListener(TypeListener object).

→Event Classes

• At the root of java event class hierarchy we have EventObject which is in package java.util.EventObject
• EventObject class contains two methods getSource() and toString().
• AWTEvent class belongs to java.awt package and is a sub-class of EventObject class
• AWTEvent is a super class of all awt based events.
• It has a function getId() which is used to determine the type of event. 

Summary

• EventObject is super class of all events
• AWT is super class of all awt based events
• java.awt.event package defines various events generated  by various user interfaces.