Java Programming :Event Handling(Types of Events)

Event Handling is the mechanism that controls the event and decides what should happen if an event occurs. This mechanism have the code which is known as event handler that is executed when an event occurs. Java Uses the Delegation Event Model to handle the events.

1. Event Handling
SimmiS
Department of Computer Science(UG)
KristuJayantiCollege ,Bangalore

2. EventhandlingisfundamentaltoJavaprogramming because
itisusedtocreateeventdrivenprogramseg
Applets
GUIbasedwindowsapplication
WebApplication
Eventhandlingmechanism havebeenchanged
significantlybetweentheoriginalversionofJava(1.0)andall
subsequentversionsofJava,beginningwithversion1.1.
Themodernapproachtohandlingeventsisbasedon the
delegationeventmodel,
EventHandling

3. What is an Event?
Changeinthestateofanobjectisknownaseventi.e.eventdescribesthe
changeinstateofsource.
Eventsaregeneratedasresultofuserinteractionwiththegraphicaluser
interfacecomponents.
Forexample,clickingonabutton,movingthemouse,enteringacharacter
throughkeyboard,selectinganitemfromlist,scrollingthepagearethe
activitiesthatcausesaneventtohappen.

4. TypesofEvent
Theeventscanbebroadlyclassifiedintotwocategories:
ForegroundEvents-Thoseeventswhichrequirethedirectinteractionofuser.
Theyaregeneratedasconsequencesofapersoninteractingwiththegraphical
componentsinGraphicalUserInterface.
Forexample,clickingonabutton,movingthemouse,enteringacharacterthrough
keyboard,selectinganitemfromlist,scrollingthepageetc.
BackgroundEvents-Thoseeventsthatrequiretheinteractionofenduserareknownas
backgroundevents.
Operatingsysteminterrupts,hardwareorsoftwarefailure,timerexpires,anoperation
completionaretheexampleofbackgroundevents.

5. EventHandlingisthemechanismthatcontrolstheeventanddecideswhat
shouldhappenifaneventoccurs.
Thismechanismhavethecodewhichisknownaseventhandlerthatis
executedwhenaneventoccurs.
JavaUsestheDelegationEventModeltohandletheevents.
Thismodeldefinesthestandardmechanismtogenerateandhandlethe
events.Let'shaveabriefintroductiontothismodel.
WhatisEventHandling?

6. Source & Listener
TheDelegationEventModelhasthefollowingkeyparticipantsnamely:
Source-Thesourceisanobjectonwhicheventoccurs.
Sourceisresponsibleforprovidinginformationoftheoccurred
eventtoit'shandler.Javaprovidesclassesforsourceobject.
Listener-Itisalsoknownaseventhandler.
Listenerisresponsibleforgeneratingresponsetoanevent.From
javaimplementationpointofviewthelistenerisalsoanobject.
Listenerwaitsuntilitreceivesanevent.Oncetheeventisreceived,the
listenerprocesstheeventanthenreturns.

7. Source & Listener
Itsconceptisquitesimple:asourcegeneratesaneventandsendsittooneor
morelisteners.Inthisscheme,thelistenersimplywaitsuntilitreceivesanevent.
Onceaneventisreceived,thelistenerprocessestheeventandthenreturns.
Thisprovidesanimportantbenefit:notificationsaresentonlytolistenersthat
wanttoreceivethem.
ToperformEventHandling,weneedtoregisterthesourcewiththelistener.

8. Registering Listener
Asourcemustregisterlistenersinorderforthelistenerstoreceive
notificationsaboutaspecifictypeofevent.Eachtypeofeventhasitsown
registrationmethod.Hereisthegeneralform:
publicvoidaddTypeListener(TypeListenerel)
Typeisthenameoftheevent,andelisareferencetotheeventlistener.
Forexample,themethodthatregistersakeyboardeventlisteneriscalled
addKeyListener().Themethodthatregistersamousemotionlisteneriscalled
addMouseMotionListener().

9. DelegationEventModel

10. Event Classes in Java

11. Event Classes in Java

12. Event Classes in Java

13. Event Listener Interfaces
Asexplained,thedelegationeventmodelhastwoparts:sourcesand
listeners.
Listenersarecreatedbyimplementingoneormoreoftheinterfaces
definedbythejava.awt.eventpackage.
Whenaneventoccurs,theeventsourceinvokestheappropriatemethod
definedbythelistenerandprovidesaneventobjectasitsargument

14. Event Listener Interfaces
Listener Interface Methods
ActionListener •actionPerformed()
AdjustmentListener•adjustmentValueChanged()
ComponentListener
•componentResized()
•componentShown()
•componentMoved()
•componentHidden()
ContainerListener
•componentAdded()
•componentRemoved()
FocusListener
•focusGained()
•focusLost()

15. Event Listener Interfaces
ItemListener •itemStateChanged()
KeyListener
•keyTyped()
•keyPressed()
•keyReleased()
MouseListener
•mousePressed()
•mouseClicked()
•mouseEntered()
•mouseExited()
•mouseReleased()
MouseMotionListener
•mouseMoved()
•mouseDragged()
MouseWheelListener •mouseWheelMoved()

16. Java MouseListenerInterface
TheJavaMouseListenerisnotifiedwheneveryouchangethestateof
mouse.ItisnotifiedagainstMouseEvent.
TheMouseListenerinterfaceisfoundinjava.awt.eventpackage.It
hasfivemethods

17. Methods of MouseListenerinterface
The signature of 5 methods found in MouseListenerinterface are given
below:
publicabstractvoidmouseClicked(MouseEvente);
publicabstractvoidmouseEntered(MouseEvente);
publicabstractvoidmouseExited(MouseEvente);
publicabstractvoidmousePressed(MouseEvente);
publicabstractvoidmouseReleased(MouseEvente);

18. MouseListenerSample Program

20. Java KeyListenerInterface
TheJava KeyListeneris notified whenever you change the state
of key.
It is notified against KeyEvent. The KeyListenerinterface is found
in java.awt.eventpackage, and it has three methods

21. Interface declaration
Following is the declaration
forjava.awt.event.KeyListenerinterface:
publicinterfaceKeyListenerextendsEventListener
Sr. no. Method name Description
1. public abstract void
keyPressed(KeyEvente);
Itisinvokedwhenakeyhas
beenpressed.
2. public abstract void
keyReleased(KeyEvente);
Itisinvokedwhenakeyhas
beenreleased.
3. publicabstractvoidkeyTyped
(KeyEvente);
Itisinvokedwhenakeyhas
beentyped

22. THANK YOU