Event Handling In Java - Relax Back Event Handling In A Nut Shell (
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Semantic events are defined at a higher-level to encapsulate the
semantics of user interface components model. Now let us see what are the
various semantic event classes and what they generate:
An
ActionEvent object is generated when a component is activated
An
AdjustmentEvent Object is generated when scrollbars and other adjustment
elements are used.
A
TextEvent object is generated when text of
a component is modified.
An
ItemEvent is generated when an item
from a list, a choice or checkbox is selected.
Low-Level Events is
one that represents a low-level input or windows-system occurrence on a visual
component on the screen. The various low-level event classes and what they
generate are as follows:
A
ContainerEvent Object is generated
when component are added or removed from container.
A
ComponentEvent object is generated when a component is resized, moved
etc.
A
FocusEvent object is generated when component receives
focus for input.
A
KeyEvent object is generated when key on
keyboard is pressed, released etc.
A
WindowEvent object is
generated when a window activity, like maximizing or close occurs.
A
MouseEvent object is generated when a mouse is used.
A
PaintEvent object is generated when component is painted.
Event
Listeners: An object delegates the task of handling an event to an
event
listener. When an event occurs, an event object of the appropriate type (as
illustrated below) is created. This object is passed to a
Listener. A
listener must
implement the interface that has the method for event
handling. A component can have multiple listeners, and a listener can be removed
using removeActionListener () method. Next question in your mind must be what is
an interface?.
An Interface contains constant values and method
declaration. The difference between classes and interface is that the methods in
an interface are only declared and not implemented, that is, the methods do not
have a body. What is the Need for interface? Are interfaces are used to define
behavior protocols (standard behavior) that can be implemented by any class
anywhere in the class hierarchy. The java.awt.event package contains definitions
of all event classes and listener interface. The
semantic listener
interfaces define by AWT for the above mentioned semantic events
are:
ActionListener
AjdustmentListener
ItemListener
TextListener
The
low-level event listeners are as
follows:
ComponentListener
ContainerListener
FocusListener
KeyListener
MouseListener
MouseMotionListener
WindowsListener.
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