The Delegation Event Model
- The modern
approach to handling events is based on the delegation event model, which
defines standard and consistent mechanisms to generate and process events.
- Its concept is
quite simple: a source generates an event and sends it to one or more
listeners.
- In the scheme,
the listener simply waits until it receives an event. Once received, the listener processes
event and then returns. The
advantage of this design is that the application logic that processes
events is clearly separated from the user interface logic that generates
those events.
- In the
delegation event model, listeners must register with a source in order to
receive an event notification. This
provides an important benefit: notifications are sent only to listeners
that want to receive them.
Event:
- An event is an
object that describes some change in source. It can be generated when a person
interacts with an element in a GUI.
For example pressing a button, clicking a mouse, double clicking on
a list box entry or closing window.
- Events may
also occur that are not directly caused by interactions with a user
interface. For example, an event
may be generated when a time expires, counter exceeds a value, a software
or hardware failure occurs.
Event Source:
A source generates events. It has three main responsibilities
1.
It
must provide methods that allow listeners to register and unregister for
notifications about a specific type of events.
2.
It
must generate the event
3.
It
must send the event to all registered listeners. The event may be a unicast to a single
listeners or multicast to several listeners.
It is possible for a source to generate several types of events.
4.
Each type of event has its own registration method. Here is the
general form:
public void addTypeListener(TypeListener
el)
Event Listeners:
A listener receives event
notifications. It has three main
responsibilities
1.
It
must register to receive notifications account specific events. It does so by calling the appropriate registration
method of source.
2.
It
must implement an interface to receive events of that type.
3.
It
must unregister if it is no longer to proceed those notifications. It does so by calling appropriate unregistration
method of the source.
Event
Classes
The
classes that represent events are at the core of Java’s event handling
mechanism.
At
the root of the Java event class hierarchy is EventObject, which is in java.util.
It is
the superclass for all events. Its one constructor is shown here:
EventObject(Object src). Here,
src is the object that generates this event.
|
Method |
Description |
|
Object getSource( ) |
Returns
the object that generated the event |
|
String
toString( ) |
Returns
the string equivalent of the event |
To
summarize:
■ EventObject is a
superclass of all events.
■ AWTEvent is a
superclass of all AWT events that are handled by the delegation
event
model.
The
package java.awt.event defines several types of events that are
generated by
various
user interface elements
Main
Event Classes in java.awt.event
|
Event
Class |
Generated
When |
|
ActionEvent |
Generated
when a button is pressed, a list item is double-clicked, or a menu item is
selected. |
|
AdjustmentEvent |
Generated
when a scroll bar is manipulated. |
|
ComponentEvent |
Generated
when a component is hidden, moved, resized, or becomes visible. |
|
ContainerEvent |
Generated
when a component is added to or removed from a container. |
|
FocusEvent |
Generated
when a component gains or loses keyboard focus. |
|
InputEvent |
A
mouse or key event occurs. |
|
ItemEvent |
Generated
when a check box or list item is clicked; also occurs when a choice selection
is made or a checkable menu item is selected or deselected. |
|
KeyEvent |
Generated
when input is received from the keyboard. |
|
MouseEvent |
Generated
when the mouse is dragged, moved, clicked,pressed, or released; also
generated when the mouse enters or exits a component. |
|
MouseWheelEvent |
Generated
when the mouse wheel is moved. |
|
TextEvent |
Generated
when the value of a text area or text field is changed. |
|
WindowEvent |
Generated
when a window is activated, closed, deactivated, deiconified, iconified,
opened, or quit. |
Sources
of Events
The
table lists some of the user interface components that can generate the events
|
Event
Source |
Description |
|
Button
|
Generates
action events when the button is pressed. |
|
Checkbox
|
Generates
item events when the check box is selected or deselected. |
|
Choice
|
Generates
item events when the choice is changed. |
|
List
|
Generates
action events when an item is double-clicked; generates item events when an
item is selected or deselected. |
|
Menu
Item |
Generates
action events when a menu item is selected; generates item events when a
checkable menu item is selected or deselected. |
|
Scrollbar
|
Generates
adjustment events when the scroll bar is manipulated. |
|
Text
components |
Generates
text events when the user enters a character. |
|
Window
|
Generates
window events when a window is activated, closed, deactivated, deiconified,
iconified, opened, or quit. |
Event
Listener Interfaces
The
delegation event model has two parts: sources and listeners. Listeners are
created by implementing one or more of the interfaces defined by the java.awt.event
package. When an event occurs, the event source invokes the appropriate
method defined by the listener and provides an event object as its argument. Table
lists commonly used listener interfaces and provides a brief description of the
methods that they define.
|
Interface |
Description |
Methods |
|
ActionListener
|
Defines
one method to receive action events. |
void
actionPerformed(ActionEvent ae) |
|
AdjustmentListener
|
Defines
one method to receive adjustment events. |
void
adjustmentValueChanged(AdjustmentEvent ae) |
|
ComponentListener
|
Defines
four methods to recognize when a component is hidden, moved, resized, or
shown. |
void
componentResized(ComponentEvent ce) void
componentMoved(ComponentEvent ce) void
componentShown(ComponentEvent ce) void
componentHidden(ComponentEvent ce) |
|
ContainerListener
|
Defines
two methods to recognize when a component is added to or removed from a
container. |
void
componentAdded(ContainerEvent ce) void
componentRemoved(ContainerEvent ce) |
|
FocusListener
|
Defines
two methods to recognize when a component gains or loses keyboard focus. |
void
focusGained(FocusEvent fe) void
focusLost(FocusEvent fe) |
|
ItemListener
|
Defines
one method to recognize when the state of an item changes. |
void
itemStateChanged(ItemEvent ie) |
|
KeyListener
|
Defines
three methods to recognize when a key is pressed, released, or typed. |
void
keyPressed(KeyEvent ke) void
keyReleased(KeyEvent ke) void
keyTyped(KeyEvent ke) |
|
MouseListener
|
Defines
five methods to recognize when the mouse is clicked, enters a component,
exits a component, is pressed, or is released. |
void
mouseClicked(MouseEvent me) void
mouseEntered(MouseEvent me) void
mouseExited(MouseEvent me) void
mousePressed(MouseEvent me) void
mouseReleased(MouseEvent me) |
|
MouseMotionListener
|
Defines
two methods to recognize when the mouse is dragged or moved. |
void
mouseDragged(MouseEvent me) void
mouseMoved(MouseEvent me) |
|
TextListener
|
Defines
one method to recognize when a text value changes. |
void
textChanged(TextEvent te) |
|
WindowFocusListener
|
Defines
two methods to recognize when a window gains or loses input focus. |
void
windowGainedFocus(WindowEvent we) void
windowLostFocus(WindowEvent we) |
|
WindowListener
|
Defines
seven methods to recognize when a window is activated, closed, deactivated,
deiconified, iconified, opened, |
void
windowActivated(WindowEvent we) void
windowClosed(WindowEvent we) void
windowClosing(WindowEvent we) void
windowDeactivated(WindowEvent we) void
windowDeiconified(WindowEvent we) void
windowIconified(WindowEvent we) void
windowOpened(WindowEvent we) |
java.awt.Event package : Listener interfaces to handle events.
All the interfaces and the methods in them.
Getting ready to handle events
Step 1: Import classes from the package:
import java.awt.event.*;
Step 2: Implement the appropriate listener interfaces.
public class sample extends Applet implements listenerinterface1, listenerinterface2{
} //Several listener interfaces can be listed, separated by commas.
Step 3: Register the listener object with the appropriate event source.
event_source.addXXXListener(event_listener_object);
Step 4: Define all the methods of the implemented listener interfaces.
MouseEvent
Four steps:
Step 1: Include the following import statement:
import java.awt.event.*;
Step 2: Include Implements MouseListener :
public class w2 extends Applet implements MouseListener {
}
Step 3: Register addMouseListener method in the applet.
this.addMouseListener (this);
MouseMotion Events:
Moving and dragging the mouse generates an object of the MouseEvent class.
Keyboard Events
Whenever a key is pressed, an object of the KeyEvent class is generated.
Virtual Key Codes
Refer to the key on the keyboard returned by event.getKeyCode()
Produce a numeric value - but - use the symbol in the program.
No guarantee that the numeric value produced is the same on all platforms.
// Demonstrate the mouse event handlers.
import java.awt.*;
import java.awt.event.*;
import
java.applet.*;
/*<applet
code="MouseEvents" width=300 height=100> </applet>*/
public class MouseEvents extends Applet implements
MouseListener, MouseMotionListener {
String
msg = "";
int
mouseX = 0, mouseY = 0; // coordinates of mouse
public
void init() {
addMouseListener(this);
addMouseMotionListener(this);
}
//
Handle mouse clicked.
public
void mouseClicked(MouseEvent me) {
//
save coordinates
mouseX
= 0;
mouseY
= 10;
msg =
"Mouse clicked.";
repaint();
}
//
Handle mouse entered.
public
void mouseEntered(MouseEvent me) {
//
save coordinates
mouseX
= 0;
mouseY
= 10;
msg =
"Mouse entered.";
repaint();
}
//
Handle mouse exited.
public
void mouseExited(MouseEvent me) {
//
save coordinates
mouseX
= 0;
mouseY
= 10;
msg =
"Mouse exited.";
repaint();
}
//
Handle button pressed.
public
void mousePressed(MouseEvent me) {
//
save coordinates
mouseX
= me.getX();
mouseY
= me.getY();
msg =
"Down";
repaint();
}
//
Handle button released.
public
void mouseReleased(MouseEvent me) {
//
save coordinates
mouseX
= me.getX();
mouseY
= me.getY();
msg =
"Up";
repaint();
}
//
Handle mouse dragged.
public
void mouseDragged(MouseEvent me) {
//
save coordinates
mouseX
= me.getX();
mouseY
= me.getY();
msg =
"*";
showStatus("Dragging
mouse at " + mouseX + ", " + mouseY);
repaint();
}
//
Handle mouse moved.
public
void mouseMoved(MouseEvent me) {
//
show status
showStatus("Moving
mouse at " + me.getX() + ", " + me.getY());
}
//
Display msg in applet window at current X,Y location.
public
void paint(Graphics g) {
g.drawString(msg,
mouseX, mouseY);
}
}
Sample output:
THE// Demonstrate the key event handlers.
import java.awt.*;
import java.awt.event.*;
import
java.applet.*;
/*
<applet code="SimpleKey" width=300 height=100> </applet>
*/
public
class SimpleKey extends Applet implements KeyListener {
String
msg = "";
int X
= 10, Y = 20; // output coordinates
public
void init() {
addKeyListener(this);
requestFocus();
// request input focus
}
public
void keyPressed(KeyEvent ke) {
showStatus("Key
Down");
}
public
void keyReleased(KeyEvent ke) {
showStatus("Key
Up");
}
public
void keyTyped(KeyEvent ke) {
msg
+= ke.getKeyChar();
repaint();
}
//
Display keystrokes.
public
void paint(Graphics g) {
g.drawString(msg,
X, Y);
}
}
JAVA LIBRARY
Sample output:
// Demonstrate some
virtual key codes.
import java.awt.*;
import java.awt.event.*;
import
java.applet.*;
/*<applet
code="KeyEvents" width=300 height=100> </applet> */
public
class KeyEvents extends Applet implements KeyListener {
String
msg = "";
int
X = 10, Y = 20; // output coordinates
public
void init() {
addKeyListener(this);
requestFocus();
// request input focus
}
public
void keyPressed(KeyEvent ke) {
showStatus("Key
Down");
int
key = ke.getKeyCode();
switch(key)
{
case
KeyEvent.VK_F1:
msg
+= "<F1>";
break;
case
KeyEvent.VK_F2:
msg
+= "<F2>";
break;
case
KeyEvent.VK_F3:
msg
+= "<F3>";
break;
case
KeyEvent.VK_PAGE_DOWN:
msg
+= "<PgDn>";
break;
case
KeyEvent.VK_PAGE_UP:
msg
+= "<PgUp>";
break;
case
KeyEvent.VK_LEFT:
msg
+= "<Left Arrow>";
break;
case
KeyEvent.VK_RIGHT:
msg
+= "<Right Arrow>";
break;
}
repaint();
}
public
void keyReleased(KeyEvent ke) {
showStatus("Key
Up");
}
public
void keyTyped(KeyEvent ke) {
msg
+= ke.getKeyChar();
repaint();
}
//
Display keystrokes.
public
void paint(Graphics g) {
g.drawString(msg,
X, Y);
}
}
Sample output:
Reviewed by Asst. Prof. Sunita Rai
on
August 24, 2022
Rating:
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