Event handling

Programming in Java
Event Handling
By
Ravi Kant Sahu
Asst. Professor

Lovely Professional University, Punjab

Outlines
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Delegation Event Model
ActionListener
ItemListener
KeyListener
MouseListener
MouseMotionListener
WindowListener

Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

Introduction
• An event can be defined as a signal to the program that something has
happened.
• Events are triggered either by external user actions, such as mouse
movements, button clicks, and keystrokes, or by internal program
activities, such as a timer.
• The program can choose to respond to or ignore an event.
• The component that creates an event and fires it is called the source
object or source component.
• For example, a button is the source object for a button-clicking action
event.

Introduction
• An event is an instance of an event class.
• The root class of the event classes is java.util.EventObject.
•

We can identify the source object of an event using the getSource()
method in the EventObject class.

• The subclasses of EventObject deal with special types of events, such
as action events, window events, component events, mouse events,
and key events.


Delegation event MoDel


The Delegation Event Model
• The delegation event model defines standard and consistent
mechanisms to generate and process events.

Principle:
• A source generates an event and sends it to one or more listeners.
• The listener waits until it receives an event.
• Once an event is received, the listener processes the event and then
returns.
Advantage:
• The application logic that processes events is cleanly separated
from the user interface logic that generates those events.
• A user interface element is able to “delegate” the processing of an
event to a separate piece of code.

• 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 a state change in a source.
• It can be generated as a consequence of a person interacting with
the elements in a graphical user interface.
• For Example, pressing a button, entering a character via the
keyboard, selecting an item in a list, and clicking the mouse.
• Events may also occur that are not directly caused by interactions
with a user interface.
• For example, an event may be generated when a timer expires, a
counter exceeds a value, a software or hardware failure occurs, or
an operation is completed.


Event Source
• An Event source is an object that generates an event.
• This occurs when the internal state of that object changes in some
way.
• Sources may generate more than one type of event.
• A source must register listeners in order for the listeners to receive
notifications about a specific type of event.
• Each type of event has its own registration method.
public void addTypeListener(TypeListener el)

• When an event occurs, all registered listeners are notified and
receive a copy of the event object. This is known as multicasting
the event.
• In all cases, notifications are sent only to listeners that register to
receive them.
• Some sources may allow only one listener to register.
public void addTypeListener(TypeListener el) throws
java.util.TooManyListenersException


Event Listener
• A listener is an object that is notified when an event occurs. It has two
major requirements.
• First, it must have been registered with one or more sources to receive
notifications about specific types of events.
• Second, it must implement methods to receive and process these
notifications.
• The methods that receive and process events are defined in a set of
interfaces found in java.awt.event.
• For example, the MouseMotionListener interface defines two methods
to receive notifications when the mouse is dragged or moved.

Listener interfaces


Listener API Table
Listener Interface

Listener Methods

ActionListener

actionPerformed(ActionEvent)

ItemListener

itemStateChanged(ItemEvent)

MouseListener

mouseClicked(MouseEvent)
mouseEntered(MouseEvent)
mouseExited(MouseEvent)
mousePressed(MouseEvent)
mouseReleased(MouseEvent)

MouseMotionListener

KeyListener

mouseDragged(MouseEvent)
mouseMoved(MouseEvent)
keyPressed(KeyEvent)
keyReleased(KeyEvent)
keyTyped(KeyEvent)


ActionListener
• Action listeners are most common event handlers to implement.
• An action event occurs, whenever an action is performed by the user.
• We implement an action listener to define what should be done when
an user performs certain operation.
Examples: When the user clicks a button, chooses a menu item,
presses Enter in a text field.
•

The result is that an actionPerformed message is sent to all action
listeners that are registered on the relevant component.


• To write an Action Listener, follow the steps given below:
• Declare an event handler class and specify that the class either
implements an ActionListener interface or extends a class that
implements an ActionListener interface.
For example:
public class MyClass implements ActionListener {
• Register an instance of the event handler class as a listener on one
or more components.
For example:
someComponent.addActionListener(instanceOfMyClass);
• Include code that implements the methods in listener interface.
For example:
public void actionPerformed(ActionEvent e)
{ ...//code that reacts to the action... }

ActionEvent Class
Method

Purpose

String getActionCommand()

Returns the string associated with this
action. Most objects that can fire action
events support a method called
setActionCommand that lets you set
this string.

Object getSource()

Returns the object that fired the event.


ItemListener Interface
• Item events are fired by components that implement the
ItemSelectable interface.
• Generally, ItemSelectable components maintain on/off state for one or
more items.
• The Swing components that fire item events include buttons like
check boxes, check menu items, toggle buttons and combo boxes etc.
• ItemListener Interface has only one method.

public void itemStateChanged (ItemEvent)


ItemEvent class
Method
Object getItem()

Purpose
Returns the component-specific
object associated with the item
whose state changed. Often this is a
String containing the text on the
selected item.

ItemSelectable getItemSelectable() Returns the component that fired
the item event. You can use this
instead of the getSource method.
int getStateChange()

Returns the new state of the item.
The ItemEvent class defines two
states: SELECTED and
DESELECTED.


KeyListener Interface
• Key events indicate when the user is typing at the keyboard.
• Key events are fired by the component with the keyboard focus
when the user presses or releases keyboard keys.
• Notifications are sent about two basic kinds of key events:
– The typing of a Unicode character
– The pressing or releasing of a key on the keyboard


• The first kind of event is called a key-typed event.
• To know when the user types a Unicode character ? whether by
pressing one key such as 'a' or by pressing several keys in sequence ?
• The second kind is either a key-pressed or key-released event.
• To know when the user presses the F1 key, or whether the user
pressed the '3' key on the number pad, you handle key-pressed events.


Methods of KeyListener Interface
Method

Purpose

keyTyped(KeyEvent)

Called just after the user types a
Unicode character into the listenedto component.

keyPressed(KeyEvent)

Called just after the user presses a
key while the listened-to
component has the focus.

keyReleased(KeyEvent)

Called just after the user releases a
key while the listened-to
component has the focus.


KeyEvent class
Method

int getKeyChar()

Purpose
Obtains the Unicode character associated
with this event.

int getKeyCode()

Obtains the key code associated with this
event. The key code identifies the
particular key on the keyboard that the user
pressed or released. For example, VK_A
specifies the key labeled A, and
VK_ESCAPE specifies the Escape key.

boolean isActionKey()

Returns true if the key firing the event is an
action key. Examples of action keys
include Page Up, Caps Lock, the arrow and
function keys.


MouseListener Interface
• Mouse events notify when the user uses the mouse (or similar input
device) to interact with a component.
• Mouse events occur when the cursor enters or exits a component's
onscreen area and when the user presses or releases one of the mouse
buttons.


Methods of MouseListener Interface
Method

Purpose

mouseClicked(MouseEvent)

Called just after the user clicks the
listened-to component.

mouseEntered(MouseEvent)

Called just after the cursor enters
the bounds of the listened-to
component.

mouseExited(MouseEvent)

Called just after the cursor exits the
bounds of the listened-to
component.

mousePressed(MouseEvent)

Called just after the user presses a
mouse button while the cursor is
over the listened-to component.

mouseReleased(MouseEvent)

Called just after the user releases a
mouse button after a mouse press
over the listened-to component.


MouseEvent class
Method

Purpose

int getClickCount()

Returns the number of quick, consecutive
clicks the user has made (including this
event). For example, returns 2 for a double
click.

int getButton()

int getX()
int getY()

Returns which mouse button, if any, has a
changed state. One of the following
constants is returned: NOBUTTON,
BUTTON1, BUTTON2, or BUTTON3.
Return the (x,y) position at which the event
occurred, relative to the component that
fired the event.


MouseAdapter Class
• MouseAdapter class provides an empty implementation of all the
methods in MouseListener interface. This class exists as convenience
for creating listener objects.
• Extend this class to create a MouseEvent listener and override the
methods for the events of interest.
• Create a listener object using the extended class and then register it
with a component using the component's addMouseListener method.
• When a mouse button is pressed, released, or clicked (pressed and
released), or when the mouse cursor enters or exits the component,
the relevant method in the listener object is invoked and the
MouseEvent is passed to it.

MouseMotionListener Interface
• Mouse-motion events notify when the user uses the mouse (or a
similar input device) to move the onscreen cursor.
• If an application requires the detection of both mouse events and
mouse-motion events, use the MouseInputAdapter class.
• It implements the MouseInputListener a convenient interface that
implements both the MouseListener and MouseMotionListener
interfaces.


Methods of MouseMotionListener Interface
Method

mouseDragged(MouseEvent)

mouseMoved(MouseEvent)

Purpose
Called in response to the user moving
the mouse while holding a mouse
button down. This event is fired by the
component that fired the most recent
mouse-pressed event, even if the
cursor is no longer over that
component.
Called in response to the user moving
the mouse with no mouse buttons
pressed. This event is fired by the
component that's currently under the
cursor.


WindowListener Interface
• The listener interface for receiving window events.
• The class that is interested in processing a window event either
implements this interface (and all the methods it contains) or
extends the abstract WindowAdapter class (overriding only the
methods of interest).
• The listener object created from that class is then registered
with a Window using the window's addWindowListener ()
method.


Methods of WindowListener
Method

Purpose

void windowClosing
(WindowEvent e)

Invoked when the user attempts to close the
window from the window's system menu.

void windowOpened
(WindowEvent e)

Invoked the first time a window is made visible.

void windowClosed
(WindowEvent e)

Invoked when a window has been closed as the
result of calling dispose on the window.

void windowIconified
(WindowEvent e)

Invoked when a window is changed from a
normal to a minimized state.

void windowDeiconified(W Invoked when a window is changed from a
indowEvent e)
minimized to a normal state.
void windowActivated
(WindowEvent e)

Invoked when the Window is set to be the active
Window.

void windowDeactivated(W Invoked when a Window is no longer the active
indowEvent e)
Window.

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