Basics of Event Handling, The AWT Event Hierarchy, Semantic and Low- Level Events in the AWT, Low-Level Event Types

1. JAVA
EventHandling
Prepared by
Miss. Arati A. Gadgil

2. EventHandling
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.
Foreground Events - Those events which require the direct
interaction of user
Background Events - Those events that require the interaction
of end user are known as background events
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3. Source - The source is an object on which event occurs
Listener - It is also known as event handler. Listener is
responsible for generating response to an event.
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. 3

4. 4
Event Class Hierarchy
Each high level event class extends java.awt.AWTEvent.
java.lang.Object
| ---java.util.EventObject
| ---java.awt.AWTEvent
| ---java.awt.event.ActionEvent
| ---java.awt.event.ItemEvent
| ---java.awt.event.AdjustmentEvent
| ---java.awt.event.TextEvent
| ----java.awt.event.ComponentEvent
| ---java.awt.event.InputEvent |
| | ---java.awt.event.KeyEvent
| | ---java.awt.event.MouseEvent
| +---java.awt.event.FocusEvent
| +---java.awt.event.ContainerEvent
| +---java.awt.event.WindowEvent

5. 5
Semantic and Low-Level Events in the AWT
A semantic event is one that expresses what the user is doing, such as
"clicking that button"; hence, an ActionEvent is a semantic event.
Low-level events are those events that make this possible. In the case of
a button click, this is a mouse down, a series of mouse moves, and a
mouse up (but only if the mouse up is inside the button area). Or it might
be a keystroke, which happens if the user selects the button with
the TAB key and then activates it with the space bar. Similarly, adjusting
a scrollbar is a semantic event, but dragging the mouse is a low-level
event.
Semantic event classes in the java.awt.event package:
ActionEvent (for a button click, a menu selection, selecting a list item,
or ENTER typed in a text field)
AdjustmentEvent (the user adjusted a scrollbar)
ItemEvent (the user made a selection from a set of checkbox or list

6. 6
Five low-level event classes are commonly used:
•KeyEvent (a key was pressed or released)
•MouseEvent (the mouse button was pressed, released, moved, or
dragged)
•MouseWheelEvent (the mouse wheel was rotated)
•FocusEvent (a component got focus, or lost focus). See page 321 for
more information about the focus concept.
•WindowEvent (the window state changed)

7. AWT ActionListener Interface
Method
void actionPerformed(ActionEvent e)
Invoked when an action occurs.
The Event listener represent the interfaces
responsible to handle events.
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8. AWT MouseListener Interface
Methods
•void mouseClicked(MouseEvent e)
Invoked when the mouse button has been clicked (pressed and
released) on a component.
•void mouseEntered(MouseEvent e)
Invoked when the mouse enters a component
•void mouseExited(MouseEvent e)
Invoked when the mouse exits a component.
•void mousePressed(MouseEvent e)
Invoked when a mouse button has been pressed on a component.
•void mouseReleased(MouseEvent e)
Invoked when a mouse button has been released on a
component.
.
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9. AWT WindowListener Interface
Methods
void windowActivated(WindowEvent e)
Invoked when the Window is set to be the active Window
void windowClosed(WindowEvent e)
Invoked when a window has been closed as the result of calling dispose
on the window.
void windowClosing(WindowEvent e)
Invoked when the user attempts to close the window from the window's
system menu.
void windowDeactivated(WindowEvent e)
Invoked when a Window is no longer the active Window
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10. void windowDeiconified(WindowEvent e)
Invoked when a window is changed from a
minimized to a normal state
void windowIconified(WindowEvent e)
Invoked when a window is changed from a
normal to a minimized state.
void windowOpened(WindowEvent e)
Invoked the first time a window is made visible.
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11. import java.awt.*;
import java.awt.event.*;
class frmw extends Frame implements WindowListener
{
frmw()
{
super("window Listener");
addWindowListener(this);
setSize(300,300);
setVisible(true);
}
public void windowActivated(WindowEvent e)
{
System.out.println("Activated");
}
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12. public void windowDeactivated(WindowEvent e)
{
System.out.println("Deactivated");
}
public void windowIconified(WindowEvent e)
{
System.out.println("Iconified");
}
public void windowDeiconified(WindowEvent e)
{
System.out.println("Deiconified");
}
public void windowOpened(WindowEvent e)
{
System.out.println("opened");
} 12

13. public void windowClosed(WindowEvent e)
{
System.out.println("closed");
}
public void windowClosing(WindowEvent e)
{
System.out.println("closing");
}
public static void main(String []a)
{
frmw k=new frmw();
}
}
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14. AWT MouseMotionListener Interface
Methods
void mouseDragged(MouseEvent e)
Invoked when a mouse button is pressed on a component and then
dragged
void mouseMoved(MouseEvent e)
Invoked when the mouse cursor has been moved onto a component
but no buttons have been pushed
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15. AWT FocusListener Interface
Methods
void focusGained(FocusEvent e)
Invoked when a component gains the keyboard focus
void focusLost(FocusEvent e)
Invoked when a component loses the keyboard focus
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16. Thank You
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