File Handling

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Events and Listeners
In event-driven programming, code is executed upon
activation of events
An event can be defined as a type of signal to the program that
something has happened
The event is generated by external user actions such as:
Moving the mouse
Clicking the button
Pressing a key
Sliding the scrollbar
Choosing an item from a menu
Events are responded to by event listeners

Event Handling Model
• To process an event
– Register an event listener
– Implement event handler
• Method that is called in response to an event
• Each event handling interface has one or more event
handling methods that must be defined

The Event Handling process
When an event is triggered, the JAVA runtime first
determines its source and type
If a listener for this type of event is registered with the
source, an event object is created
For each listener to this type of an event
The JAVA runtime invokes the appropriate event handling
method to the listener and passes the event object as the
parameter

Selected User Actions
User Action
Click a button
Select a new item
Select an item from a List
Window opened, closed
Mouse pressed, released
Key released, pressed
Event Type Generated
ActionEvent
ItemEvent, ActionEvent
ListSelectionEvent
WindowEvent
MouseEvent
KeyEvent
Source Object
JButton
JComboBox
JList
Window
MouseEvent
KeyEvent

Java AWT Event Listener Interfaces
 ActionListener
 ItemListener
 KeyListener
 MouseListener
 WindowListener
 ListSelectionListener

Event Listener and Action Listener
• An Event Listener, once set to an applet object waits for some
action to be performed on it
– It mouse click, mouse hover, pressing of keys, click of
button, etc
• ActionListener is an interface that could be implemented in
order to determine how certain event should be handled
• When implementing an interface, all methods in that interface
should be implemented, ActionListener interface has one
method to implement named actionPerformed()

How to Implement a Listener Interface
Use the implements keyword in the class declaration
Register the object as a listener for a component’s event, using
the component’s addXListener method. (where X is the type of
event).
Declare and fully define all methods for the interface that you
are implementing

Three Steps of Event Handling
Prepare to accept events
• import package java.awt.event
Start listening for events
• include appropriate methods
Respond to events
• implement appropriate abstract method

Prepare to accept events
• Import package java.awt.event
• Applet manifests its desire to accept events by promising to
“implement” certain methods
• Example:
– “ActionListener” for Button events
– “AdjustmentListener” for Scrollbar events

Start listening for events
• To make the applet “listen” to a particular event, include the
appropriate “addxxxListener”.
• Examples:
addActionListener(this)
– shows that the applet is interested in listening to events
generated by the pushing of a certain button
addAdjustmentListener(this)
– shows that the applet is interested in listening to events
generated by the sliding of a certain scroll bar

Respond to events
• The appropriate abstract methods are implemented.
• Example:
– actionPerformed() is automatically called whenever the user
clicks the button
• Thus, implement actionPerformed() to respond to the button event
– adjustmentValueChanged() is automatically invoked whenever
the user slides the scroll bar thumb
• So adjustmentValueChanged() needs to be implemented
• In actionPerformed(ActionEvent evt), ActionEvent is
a class in java.awt.event

ActionEvent
In Java, most components have a special event called an
ActionEvent
This is loosely speaking the most common or canonical
event for that component
A good example is a click for a button
To have any component listen for ActionEvents, you must
register the component with an ActionListener
– e.g. button.addActionListener(new MyAL());

ActionPerformed
The actionPerformed method has the following signature:
» void actionPerformed(ActionEvent)
The object of type ActionEvent passed to the event
handler is used to query information about the event
Some common methods are:
getSource()
object reference to component generating event
getActionCommand()
some text associated with event (text on button, etc)

Event Handler Code
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Test2 extends JFrame implements
ActionListener
{
JFrame myFrame;
JTextField myTextField1;
JButton myButton1;
JLabel myLabel;
JLabel myLabel1;
public void simple()
{
myFrame=new JFrame("My Frame");
myFrame.setLayout(new FlowLayout());
myLabel=new JLabel("Simple GUI Frame");
myLabel1=new JLabel("Enter Input");
myTextField1=new JTextField(10);
myButton1=new JButton("Clear");
myFrame.setLayout(new GridLayout(15,1));
myFrame.add(myLabel);
myFrame.add(myLabel1);
myFrame.add(myTextField1);
myFrame.add(myButton1);
myFrame.setSize(400,400);
myFrame.setVisible(true);
myButton1.addActionListener(this);
}
public void actionPerformed(ActionEvent evt)
{
JButton source=(JButton)evt.getSource();
if(source == myButton1)
myTextField1.setText("");
}
public static void main(String[] args)
{
Test2 test = new Test2();
test.simple();
test.setDefaultCloseOperation(JFrame.EXIT_ON_CL
OSE);
}}

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