Event Handling
What is an Event?
Change in the state of an object is known as event i.e. event
describes the change in state of source. Events are generated as
result of user interaction with the graphical user interface
components. For example, clicking on a button, moving the
mouse, entering a character through keyboard, selecting an item
from list, scrolling the page are the activities that causes an
event to happen.
Types of Event
The events can be broadly classified into two categories:
Foreground Events - Those events which require the
direct interaction of user.They are generated as
consequences of a person interacting with the graphical
components in Graphical User Interface. For example,
clicking on a button, moving the mouse, entering a
character through keyboard,selecting an item from list,
scrolling the page etc.
Background Events - Those events that require the
interaction of end user are known as background events.
Operating system interrupts, hardware or software failure,
timer expires, an operation completion are the example of
background events.
What is Event Handling?
Event Handling is the mechanism that controls the event and
decides what should happen if an event occurs. This mechanism
have the code which is known as event handler that is executed
when an event occurs. Java Uses the Delegation Event Model to
handle the events. This model defines the standard mechanism
to generate and handle the events. Let's have a brief
introduction to this model.
The Delegation Event Model has the following key participants
namely:
Source - The source is an object on which event occurs.
Source is responsible for providing information of the
occurred event to it's handler. Java provide as with classes
for source object.
Listener - It is also known as event handler. Listener is
responsible for generating response to an event. From java
implementation point of view the listener is also an object.
Listener waits until it receives an event. Once the event is
received, the listener process the event and then returns.
The benefit of this approach is that the user interface logic is
completely separated from the logic that generates the event.
The user interface element is able to delegate the processing of
an event to the separate piece of code. In this model, Listener
needs to be registered with the source object so that the listener
can receive the event notification. This is an efficient way of
handling the event because the event notifications are sent only
to those listener that want to receive them.
Steps involved in event handling
The User clicks the button and the event is generated.
Now the object of concerned event class is created
automatically and information about the source and the
event get populated with in same object.
Event object is forwarded to the method of registered
listener class.
the method is now get executed and returns.
Event and Listener (Java Event Handling)
Changing the state of an object is known as an event.
For example, click on button, dragging mouse etc. The
java.awt.event package provides many event classes and
Listener interfaces for event handling.
Java Event classes and Listener interfaces
Event Classes Listener Interfaces
ActionEvent ActionListener
MouseEvent MouseListener and
MouseMotionListener
MouseWheelEvent MouseWheelListener
KeyEvent KeyListener
ItemEvent ItemListener
TextEvent TextListener
AdjustmentEvent AdjustmentListener
WindowEvent WindowListener
ComponentEvent ComponentListener
ContainerEvent ContainerListener
FocusEvent FocusListener
Registration Methods
For registering the component with the Listener, many classes provide
the registration methods. For example:
o Button
o public void addActionListener(ActionListener a){}
o MenuItem
o public void addActionListener(ActionListener a){}
o TextField
o public void addActionListener(ActionListener a){}
o public void addTextListener(TextListener a){}
o TextArea
o public void addTextListener(TextListener a){}
o Checkbox
o public void addItemListener(ItemListener a){}
o Choice
o public void addItemListener(ItemListener a){}
o List
o public void addActionListener(ActionListener a){}
o public void addItemListener(ItemListener a){}
Java Event Handling Code
We can put the event handling code into one of the following places:
1. Within class
2. Other class
3. Anonymous class
Java event handling by implementing ActionListener
import java.awt.*;
import java.awt.event.*;
class AEvent extends Frame implements
ActionListener
{
TextField tf;
AEvent()
{
tf=new TextField();
tf.setBounds(60,50,170,20);
Button b=new Button("click me");
b.setBounds(100,120,80,30);
//register listener
b.addActionListener(this);//passing current
instance
//add components and set size, layout and
visibility
add(b);
add(tf);
setSize(300,300);
setLayout(null);
setVisible(true);
}
public void actionPerformed(ActionEvent
e)
{
tf.setText("Welcome");
}
public static void main(String args[])
{
new AEvent();
}
}
public void setBounds(int xaxis, int yaxis, int width, int height); have been used in the
above example that sets the position of the component it may be button, textfield etc.
