Dacj 2-2 b

Event-handling

Pre-Assessment Questions
1. Identify the package that defines the JApplet class in Java.
a. javax.swing
b. java.awt
c. java.io
d. java.net

2. Which method suspends the execution of an applet?
a. init()
b. start()
c. stop()
d. destroy()

©NIIT Java Programming Lesson 2B / Slide 1 of 27

Event-handling

Pre-Assessment Questions (Contd.)
3. Which class enables you to create a drop-down menu in Java?
a. List
b. Choice
c. Label
d. TabbedPane

4. Identify an intermediate-level Swing container.
a. JPanel
b. JFrame
c. JDialog
d. JComboBox


Event-handling

Pre-assessment Questions (Contd.)
5. Which layout manager allows specified components to span multiple rows and
columns?
• FlowLayout manager
• BorderLayout Manager
• GridLayout Manager
• GridBagLayout Manager


Event-handling

Solutions to Pre-assessment
Questions
• a. javax.swing
• c. stop()
• b. Choice
• a. JPanel
• d. GridBagLayout Manager


Event-handling

Objectives
• In this lesson, you will learn about:
• Event-handling in Java
• Event Classes
• EventListener Interfaces
• Adapter Classes


Event-handling

Event-Handling in Java
• An object that describes a change of state in a source component is called an
event.
• The source components can be the elements of the Graphical User Interface
(GUI).
• Events are supported by the classes and interfaces defined in the
java.awt.event package.
• Identifying the Source Of Events:
• An event source is an object that generates an event.
• An event source registers some listeners, which receive notifications about
a specific type of event generated by that particular event source.
• All the registered listeners are notified about the generation of an event
and receive a copy of the event object.
• This is known as multicasting the event.
• Some sources allow only single listener to register.
• This is known as unicasting of event.

Event-handling

Event-Handling in Java (Contd.)
• Various event sources and the types of events they generate:

Event Source Description

Checkbox Creates an item event when a check
box is selected or deselected.

Button Creates an action event when a
button is pressed.

List Creates an action event when an item
is double-clicked; creates an item
event when an item is selected or
deselected.


Event-handling


Event Source Description

Scrollbar Creates an adjustment event when
a scroll bar is scrolled.

Text components Creates a text event when a text
character is entered in the text
component.

Window Creates a window event when a
window is activated, deactivated,
opened, closed, or quit.


Event-handling

• Event Listeners and Event Handlers
• An event listener listens for a specific event and is notified when that
specific event occurs.
• An event listener registers with one or more event sources to receive
notifications about specific types of events and processes the events.
• An event-handler is called by the event listener whenever a specific event
occurs.
• Event listeners are interfaces and the event-handler is a method declared
in the event listener interface that is implemented by the application.
• The syntax of a method that registers an event listener with an event
source is:
public void addTYPEListener(TYPEListener obj)


Event-handling

• The Delegation Event Model
• The delegation event model is based on the concept that source generates
the event and notifies one or more event listeners.
• The delegation event model allows you to specify the objects that are to
be notified when a specific event occurs.
• In delegation event model, the event listener has to be registered with a
source in order to get notified about the occurrence of a particular event.


Event-handling

Event Classes
• The Java event class hierarchy:


Event-handling

Event Classes (Contd.)
• The Action Event class
•ActionEvent is generated by an AWT component, such as a button,
when a component-specific action is performed.
•The action event is generated when a button is pressed, a
list item is double-clicked, or a menu item is selected.
•The following syntax shows the declaration of the constructor of
the ActionEvent class is:
public ActionEvent(Object source, int id, String
command)
•The main methods included in the Action Event class are:
•String getActionCommand()
•int getModifiers()


Event-handling

• The MouseEvent class
• The MouseEvent class extends the java.awt.event.InputEvent class.
• The mouse event indicates that a mouse action has occurred on a
component.
• The mouse events include:
• Pressing a mouse button
• Releasing a mouse button
• Clicking a mouse button
• Entering of mouse in a component area
• Exiting of mouse from a component area
• The mouse event class defines some integer constants that can be used to
identify several types of mouse events.


Event-handling

• Various integer constants of the Event class:

Constants Description

MOUSE_CLICKED Identifies the event of mouse clicking.

MOUSE_DRAGGED Identifies the event of dragging of
mouse.

MOUSE_MOVED Identifies the event of mouse moving.

MOUSE_PRESSED Identifies the event of mouse
pressing.


Event-handling


Constants Description

MOUSE_RELEASED Identifies the event of mouse
releasing.

MOUSE_ENTERED Identifies the event of mouse
entering an AWT component.

MOUSE_EXITED Identifies the event of mouse exiting
an AWT component.


Event-handling

•The following syntax shows the declaration of one of the constructors
of the MouseEvent class:
public MouseEvent(Component source, int eventType, long
when, int modifiers, int x, int y, int clickCount,
boolean triggersPopup )
•Various methods of the MouseEvent class:

Methods Description

public int getX() Returns the horizontal x coordinate of
the mouse position relative to a source
component.
public int getY() Returns the vertical y coordinate of the
mouse position relative to a source
component.


Event-handling


Methods Description

public point getPoint() Returns the Point object. The Point
object contains the x and y coordinates
of the mouse position relative to a
source component.

public void translatePoint(int x, int Translates the coordinates of a mouse
y) event to a new position by adding x and
y offsets.

public int getClickCount() Returns the number of mouse clicks
associated with an event.


Event-handling

Event Listener Interfaces
• An event listener registers with an event source to receive
notifications about the events of a particular type.
• Various event listener interfaces defined in the java.awt.event
package:

Interface Description

ActionListener Defines the actionPerformed() method
to receive and process action events.

MouseListener Defines five methods to receive
mouse events, such as when a mouse
is clicked, pressed, released, enters,
or exits a component.
MouseMotionListener Defines two methods to receive
events, such as when a mouse is
dragged or moved.


Event-handling

Event Listener Interfaces (Contd.)
Interface Description

AdjustmentListner Defines the adjustmentValueChanged()
method to receive and process the
adjustment events.

TextListener Defines the textValueChanged()
method to receive and process an
event when the text value changes.

WindowListener Defines seven window methods to
receive events.

ItemListener Defines the itemStateChanged()
method when an item has been
selected or deselected by the user.

Event-handling

Demonstration-Using the
ActionListener Interface
• Problem Statement
Create an applet that contains a button. When you click the button,
the label of the button is changed from "Click here" to "Button
clicked".
• Solution
• To solve the above problem, perform the following tasks:
• Code the application
• Compile and execute the application


Event-handling

Using the MouseListener Interface
• The MouseListener interface is implemented for receiving various mouse
events, such as when you press, click, release, enter, and exit a
component.
• Various public methods declared in the MouseListener interface:
Methods Description

void mouseClicked(MouseEvent me) Performs an action when the mouse
button clicks a component.
void mousePressed(MouseEvent me) Performs an action when mouse button
presses a component.
void mouseReleased(MouseEvent me) Performs an action when the mouse
button releases a component.
void mouseEntered(MouseEvent me) Performs an action when a mouse enters
a component area.
void mouseExited(MouseEvent me) Performs an action when a mouse exits a
component area.

Event-handling

Adapter Classes
• The Java programming language provides adapter classes, which implement
the event listener interfaces containing more than one event-handling
method.
• An adapter class provides an empty implementation of the event-handling
methods in an event listener interface.
• The adapter classes are useful because they allows you to receive and
process only some of the events that are handled by a particular event
listener interface.
• You can define a class that acts as an event listener by extending one of the
adapter classes and overriding its methods to handle a particular type of
event.


Event-handling

Adapter Classes (Contd.)
• Various adapter classes:

Adapter Class Description

KeyAdapter Provides empty implementation of
the methods of the KeyListener
interface.
MouseAdapter Provides empty implementation of
the methods of the MouseListener
interface.

MouseMotionAdapter Provides empty implementation of
the methods of the
MouseMotionListener interface.


Event-handling

• Various adapter classes: (contd.)

Adapter Class Description

WindowAdapter Provides empty implementation of the
methods of the WindowListener and
WindowFocusListener interfaces.

FocusAdapter Provides empty implementation of the
methods of the FocusListener
interface.


Event-handling

• Using the MouseAdapter Class
• The MouseAdapter class provides empty implementation of mouse
event-handling methods, such as mouseClicked(),
mouseEntered(), mouseExited(), mousePressed(), and
mouseReleased().
• A class that acts as a listener for mouse events extends the
MouseAdapter class and overrides the required methods.

•Using the MouseMotionAdapter Class
• The MouseMotionAdpater class provides empty implementation of
methods, such as mouseDragged() and mouseMoved().
• A class that acts as a listener for mouse motion events extends the
MouseAdapter class and overrides the required method.


Event-handling

Summary
In this lesson, you learned:
• The components of an event-driven program are:
• Event Source: Refers to AWT components such as Button, List,
Checkbox, and Scrollbar that generate events.
• Event Listener- Refers to any object that receives messages or
events.
• Event Handler- Refers to the method that receives and processes the
event. The event handler method takes an Event object as a
parameter.
• The various event classes in Java, such as the ActionEvent and MouseEvent
classes.
• To handle action events, you need to register the listener object that
implements the ActionListener interface.
• To handle mouse events, you need to register the listener object that
implements the MouseListener interface.
• Events generated by mouse are of two types, mouse events and mouse
motion events.

Event-handling

Summary (Contd.)
• Mouse events include, pressing a mouse button, releasing a mouse
button, clicking a mouse button, entering of a mouse in a component
area, exiting of a mouse from a component area.
• Mouse motion events include, moving a mouse and dragging a mouse.
• Adapter classes provides an empty implementation of all the methods
in an event listener interface.


Dacj 2-2 b

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