02 Java Oop Class

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ClassesA class (e.g. , Dog) is a k ind of mo ld or temp late to crea te o bjects (e.g ., Mary and DooDoo)

m old = = mo uldAn o bject is an instance of a c lass. The o bject belongs to that class

Dog Dog

r

DogDooDoo

Instance-of

Dog


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What Is an Obj ect?

These rea l-wor ld o bjects all have states and behav iors .Defini tion: An o bject is a sof tware bundle of v ariables

and re lated methods ( function ).

A common vis ual represen tation of asof tware o bject.


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Examp le: Bicyc leThe sof tware bicyc le's c urren t state:

its speed is 10 mph, its peda l cadence is 9 0 rpm , and

its curren t gear is the 5 th gear .

The sof tware bicyc le wo uld also have me thods tobrake,change the pedal cadence, and change gears.


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W hat Are Classes ?Defini tion:

A class is a blueprin t or pro totypeDefines the varia bles and me thods common to all o bjects of a

cer tain k ind.The Benefi ts of C lasses

Reusa bility -- Sof tware programmers use the same c lass , and thesame code to crea te many o bjects.


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Obj ects vs. Classes

Obj ec ts:E.g. , a rea l car.

Classes: or typesE.g. , BM W -500

A Class i s a bluepr int o r t empla t e of s ome obj ec t s

Obj ec t i s an In s t ance of s ome class.

Obj ec t ( , ) -- (Var i able )


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An bst r act Data Type (ADT ) is a user -defined data type that satisfies the fo llowing two condi tions: ( E ncapsulation + Info rm ationHiding )

The represen tation of , and opera tions on, o bjects of the type aredefined in a sing le syn tactic unit; also, other program units cancrea te o bjects of the type.The r ep r esentation of objects of the type is hidden f r om thep r og r am units that use these objects , so the on ly opera tions(methods ) possi ble are those provided in the type's defini tion

which are k nown as interfaces .

ADT --- Data Abstraction

ADT: =(f unction)


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Encaps ulation ( )Hide the o bject's nucleus from o ther o bjects in the program .

The imp lemen tation de tails can change a t any time wi thout affec ting o ther par ts of the program.

It is an idea l represen tation of an o bject, butFor imp lemen tation or efficiency reasons , an o bject may wish to expose

some of i ts var iables or hide some of i ts methods .

Benefi ts:M odula r ity

The so urce code for an o bject can be wri tten and main tained independen tly of the so urce code for o ther o bjects.

Info rm ation hidingAn o bject has a p ublic in terface that other o bjects can use to comm unica tewith it.


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Classes in Java

A class -- is a temp late that descri bes the data and behavio rassocia ted wi th instances of that class .An object -- ins tantiate a c lass yo u crea te an object .An object is an ins tance of some class .The da ta associa ted wi th a class or o bject is s tored invar iables .The behavior associa ted wi th a c lass or o bject isimp lemen ted wi th methods .

Methods are simi lar to the f unctions or proced ures in C.


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public class Bicycle {

private int speed = 10;

private int pedalCadence = 90;

private int currentGear = 5;

// a constructor!

public void brake() { /* ... */ } public void changePedalCadence(int x) { /* */ } public void changeGear(int x) { /* ... */ }

}

Class B icycle in Java

Not all owed in C++ L anguage


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Class HelloW or ld (Applet version)

import java.applet.Applet;import java.awt.Graphics;

public class HelloWorld extends Applet {public void paint(Graphics g) {

g.drawString("Hello world!", 50, 25);}

}

inherit pplet


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W hat Is Inheri tance ? (extends )Subclasses inherit va r iables and m ethods from s u perc lasses.

States:Gear & Speed

Behaviors:Brak e & Accelerate

Subclasses canAdd va r iables and m ethods .Ove rr ide inheri ted m ethods .

Benefi ts:Provide specia lized behaviors

Reuse the code in the s u perc lassA bstract classes -- define "generic" behaviors.


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Man k ind extends Anima lpublic class Animal {

private int height = 0;private int weight = 0;public void talk( ) { System.out.println(Arhh");}

}

public class Mankind extends Animal {private int iq = 120;public void talk( ) { System.out.println("Hello");}

}

M ankind is-a Anima l

Sho uld be indifferen t files

One Java fi le can on ly have one p ubl ic c lass


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Man k ind inheri ts Anima l (C++ )class Animal {

int height = 0;int weight = 0;

public:void talk( ) { System.out.println("Won");}

} ;

class Mankind :public Animal {private: int iq = 120;

public: void talk( ) { System.out.println("Hello");}} ;

Classescan bein samefile

In C++ , ";" is req uired to termina te a c lass


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Rec tang le Con tains Poin tpublic class Point {

private int x = 0;private int y = 0;

}

public class Rectangle {private int width = 0;private int height = 0;private Point origin = new Point();

}

Sho uld be indifferen t files

Car has-a W hee l


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public class Point {private int x = 0;

private int y = 0;

// a constructor! public Point(int xxx, int y) {

x = xxx;this.y = y;

}}

. . .Point p = new Point(44,78);

Poin t and i t's const r ucto r

this.ythis.x


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Class may have many Cons tr uctorspubl i c class Rec t angle {

int wi dth = 0;int h eight = 0;Point or igin ;// fo ur cons t ruc to rs

publ i c Rec t angle () { or igin = new Point( 0, 0) ; }publ i c Rec t angle ( Point p) { or igin = p; }publ i c Rec t angle (int w, int h) { thi s (n ew Point( 0, 0), w, h) ; }publ i c Rec t angle ( Point p, int w, int h)

{ or igin = p; width = w; height = h; }

// a metho d fo r moving th e rec t anglepubl i c voi d move(int x, int y) { or igin . x = x; or igin . y = y; }// a metho d fo r computing th e area of th e rec t anglepubl i c int area () { re t ur n wi dth * h eight ; }

}


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W hat Are Messages?Message in an o bject.Sof tware o bjects in terac t and comm unica te wi th each o ther by sending messages to each o ther.

W hen o bject A wan ts o bject B to perform one of B's me thods , o bject A sends a message to o bject B.


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Messaging

Three componen ts comprise a message:1. The o bject to whom the message is addressed (Yo ur Bicyc le).2. The name of the me thod to perform (changeGears).3. Any parame ters needed by the me thod ( lowerGear).

Benefi ts:Message passing s u ppor ts all interac tions between o bjects.Messages can be sen t and received between processes in the samemachine or in differen t machines.

you rB icycle .changeGea r s(lowe r Gea r)


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Obj ect-Orien ted Techno logy

Obj ect-O rien ted Analysis ( OOA )Goa l: Unders tand the domain

Obj ect-O rien ted Design ( OO D)Goa l: Design a so lution , a mode l of the domain in which thedesired ac tivities occ ur

Obj ect-O rien ted Programming ( OO P)Goa l: Imp lemen t the so lution

No te: A Good Design is 2/3 Before Yo u Hit the Key board


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It's a process , not a wa terfa ll

Design is an i tera tive ac tivi tyStar t in OOA, go to OO D, forge t some thing in OOAGet to OO P, rea lize yo u didn' t unders tand some thing and go back

The s tages are there to iden tify emphasesFor examp le, OOA is T OTALLY Lang uage -Independen t

Wat er f all m odel

emphases = = emphas i s


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O bject-Orien ted Analysis

Step one: Brains torming Candida te ClassesW rite down a ll the o bjects that related

Focus on the nounsGood o bjects have attributes and ser v ices

Now , filter the candida tesDea l with the in terface later (No t par t of the domain)Are some candida tes attr ibutes of o thers?

Are some subclasses of o thers? (Inheri tance)Are some instances of o thers?


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OOA : CRC CardsStep two: W rite CRC cards and wor k thro ugh scenarios

Class-Responsibility-Collabo r ato r Cards (C unningham and Bec k )Just 3x5 cards

Dat a fi elds ( att r ib ut es )

Although CRC is no t par t of UML , theyadd some very usef ul

insigh ts thro ugho ut adeve lopmen t.


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How to use CRC Cards

Ma k e one for each candida te classInven t scenarios: W hat sho uld these o bjects do?Play the cards

Lay down the card that star ts the scenarioW rite down i ts responsi bilityAdd co lla bora tor o bjects to he l p wi th that responsi bilityPic k u p cards as they leave the scenario


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W hy CRC Cards?

Hel p yo u iden tify o bjects and their responsi bilitiesHe l p yo u unders tand in terac tions between o bjects

Cards form a usef ul record of ear ly design ac tivi tyCards wor k we ll in gro u p si tuations and are unders tanda ble bynon -technica l stak eho lders


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Obj ect-Orien ted Design

Step one: Crea te a UML c lass diagram of yo ur o bjec ts

Step two: Crea te a de tailed descrip tion of theservices to be performed

Peter Coad's "I am a Co unt. I k now how toincremen t "

Activi ty, seq uence , or co lla bora tion UML diagrams


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MD A-Mode l Driven Archi tecture?

A New W ay to Specify and B uild Sys tems2001 OMG ( http://www.omg.org )

U ML M odel ( )

: Analysis , Design , Implemen tation , Dep loymen t, Main tenance , Evo lution & Integra tion wi th later sys tems (

)

R OI ( ):

Programming lang uage y Ne twor k O pera ting Sys tem y Midd leware


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Unified Mode ling Lang uage

There have been O-O gur us for many yearsThree of them wor k ed toge ther to define UML (Threeamigos : Booch , Ru m baugh , J acobson )Has now been approved as a standard by the Obj ect Managemen t Gro u p (OMG)Very powerf ul, many forms of no tation

It's even prova ble! (wi th OCL)

htt p://www.UML.org

am igos = f r iends

htt p://www.omg.org

(Object Const r ain Language )


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Java OOP-classHis tory of the UML( htt p://www. uml.org/ )

Public Feedback

Web - June 1996 Web - June 1996 UML 0.9UML 0.9

Unified Method 0.8 Unified Method 0.8 OOPSLA 95 OOPSLA 95

UML 1.0 UML 1.0 UML partnersUML partners

OMG Acceptance, Nov 1997 Final submission to OMG, Sept 1997 First submission to OMG, Jan 1997

OMG Acceptance, Nov 1997 Final submission to OMG, Sept 1997 First submission to OMG, Jan 1997

UML 1.1UML 1.1

UML 2.0 UML 2.0 Approved 2004 Approved 2004

Minor revision 1999Minor revision 1999 UML 1.3UML 1.3

UML 1.4UML 1.4Minor revision 2001Minor revision 2001

OOSE Other methods OMT Booch method

Minor revision 2003Minor revision 2003 UML 1.5 UML 1.5


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UML Diagrams

U se case diagramsF unctional behavio r of the syste m as seen by the use r .

Class diagramsS tatic st r uctu r e of the syste m: Objects, Att r ibutes, andAssociations.

Activity diagramsDyna m ic behavio r of a syste m , in pa r ticula r the wo r kflow, i.e. a flowcha r t.

Seq uence diagramsDyna m ic behavio r between acto r s and syste m objects.

Statechar t diagramsDyna m ic behavio r of an individual object as FS M ( ).


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UML 12 Diagrams

Behavior :Use CaseSeq uence

Co lla bora tionState Char tActivi ty

Str uctural:ClassComponen t

Dep loymen tObj ect

Mode l Managemen t:Pac k ages (c lass diagram con tains pac k ages)Subsys tems (c lass diagram con tains s ubsys tems)Mode ls (c lass diagram con tains mode ls)


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So, W hat is UML?

UML is a Unified Mode ling Lang uageUML is a se t of no tations , not a sing le me thodo logyMode ling is a way of thin k ing a bout the pro blems using

mode ls organized aro und the rea l wor ld ideas.Res ulted from the convergence of no tations from threeleading Obj ect-O rien ted me thods:

Booch method ( by Grady Booch)OMT ( by James R um baugh)OO SE ( by Ivar Jaco bson)

Yo u can mode l 80% of mos t pro blems by using a bout 20% of the UML

UML is a blueprint for building complex software


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UML Core Conven tionsRec tang les are classes or ins tancesOvals are f unctions or use casesTypes are deno ted wi th non -under lined names

SimpleWatch

Firefighter

Instances are deno ted wi th an under lined namesmyWatch:SimpleWatch

Joe:Firefighter

Diagrams are higraphs Nodes are en tities (e.g. c lasses , states)Arcs are re lationships among en tities (e.g. sender/receiver)Con tainmen t represen ts belonging (e.g. use cases in pac k age)

Hig r aphs are anextension to thefami liar Direc tedGraph s tr ucturewhere nodes areconnec ted by

edges to o ther nodes. Nodesrepresen t entitiesin some domain (inour case , classes ,

pac k ages , methods , etc.).


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Java OOP-classU se Case Diagram examp les

WatchUser WatchRepairPerson

ReadTime

SetTime

ChangeBattery

Actor

Use case

PackageSimpleWatch

U se case diagrams represen t the f unctiona lity of the sys temfrom users poin t of view. ( wha t, how)

A use casedoc

umen

ts

theinterac tion between the

system user and the sys tem.It is high ly de tailed indescri bing wha t is req uired but is free of mos t imp lemen tation de tails and

cons train ts.


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Class Diagram : a simp le W atch

Batteryload()

1

2

Timenow()

PushButtonstatepush()release()

1

1

1

1

1

2

blinkIdxblinkSeconds()blinkMinutes()blinkHours()stopBlinking()referesh()

LCDDisplay

SimpleWatch

C lass

Association

Operations

Class diagrams represen t the s tr ucture of the domain or sys tem

Attributes

Multiplicity


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Java OOP-classSeq uence DiagramObject

Message Activation

Seq uence diagrams represen t the behavior as in terac tionsIt shows seq uence of even ts for a par ticular use case

blinkHours()

blinkMinutes()

incrementMinutes()refresh()

commitNewTime()stopBlinking()

pressButton1()

pressButton 2 ()

pressButtons1And 2 ()

pressButton1()

:WatchUser :Time:LCDDisplay:SimpleWatch

Activation

Collabo r ation Diag r am

Obj ect diagram wi thnum bered messages

Seq uence n um bers of messagesare nes ted by proced ure ca ll


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button1& 2 Pressed

button1& 2 Pressed

button1Pressed

button 2 Pressed

button 2 Pressed

button 2 Pressed

button1Pressed

button1& 2 Pressed Increment Minutes

IncrementHours

BlinkHours

BlinkSeconds

Blink Minutes

IncrementSeconds

StopBlinking

State char t Diagrams for the wa tch

StateInitial state

Final state

Transition

Event

FS M: F inite S tate M achine


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Activi ty DiagramsAn ac tivi ty diagram shows f low con trol within a sys tem

An ac tivi ty diagram is a specia l case of a state cha r t diag r amin which s tates are ac tivi ties (f unctions)Two types of s tates:

A ct ion state:Canno t be decomposed any f ur ther Happens ins tantaneo usly wi th respec t to the leve l of a bstraction used in

the mode lA ct ivi ty state:

Can be decomposed f ur ther The ac tivi ty is mode led by ano ther ac tivity diagram

Business process use case ;

HandleIncident

DocumentIncident

ArchiveIncident


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Classes in UMLClasses descri be o bjec ts

Behavio ur (mem ber f unction signa ture / imp lemen tation)Proper ties (a ttri butes and associa tions)

Associa tion , aggrega tion , dependency , and inheri tancerelationshipsMult iplici ty and naviga tion indica torsRole names

Obj ec ts descri bed by c lasses co lla bora teClass re lations o bject relationsDependencies between c lasses


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U ML ClassClass name

Data mem bers

(attri butes)

Instance me thods

Argumen tsReturn typesClass me thod (s tatic)

Data mem bers , arg umen ts and me thods are specified byvisi bility name : type

Visi bility shown as+ publ ic- priva te# pro tected


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Class Att ri butes

Att ri butes are the ins tance da ta mem bersand c lass da ta mem bers

Class data m em be r s (under lined)

are shared between a ll ins tances(o bjects) of a given c lass

Data types shown af ter ":"

Visi bility shown as+ publ ic- priva te# pro tected

Att ri butecompar tmen t

visi bility name : type

Class name


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Class O pera tions (In terface)

O pera tions are the c lassme thods wi th their arg umen tand re turn types

Publ ic (+) opera tions define theclass in terface

Class me thods ( under lined)can on ly access to c lass da tamem bers , no need for a c lassins tance (o bject)

O pera tionscompar tmen t


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Temp late C lasses

Generic c lasses depending on parame trised types

Type parame ter(s)

O pera tions compar tmen tas usual, but may have

type parame ter ins tead of concre te type


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Class Inheri tance

Base c lass or s u per c lass

Derived c lass or s ubclass

Arrow shows direc tionof dependency ( B inheri ts A)

B inheri ts A's in terface , behavio ur and da ta mem bersB can ex tend A, i.e. add newdata mem bers or mem ber f unctionsB depends on A,A k nows no thing a bout B


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Mult iple Inheri tance ( Java )

The derived c lass inheri tsinterface , behavio ur anddata mem bers of a ll its base c lasses

Extension and overridingwor k s as before

B imp lemen ts the in terface A andis a lso a "co unta ble" c lass since i tinheri ts class Co unta ble

extends

imp lemen ts

class B ex tends Co unta ble imp lemen ts A { /* */ }


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How can two c lasses be re lated?G eneral iz at ion-spec ial iz at ion or IsA

NamedBox Is A BoxDiagram: Triang le on the re lationship line

A ssoc iat ion or HasABox Has A PenDiagram: J ust a re lationship lineA ggregat ion is a part-whole relationship

Diagram: Diamond on the line

Dependency or TalksToDependency is sor t of temporary Has A

Diagram: Dashed line in UML


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Recommended Boo k :

UML Dis tilledSerio us O-O designers D O use UMLUML Dis tilled by M a r tin F owle r is a grea t pract ical introd uction to UMLOfficia l UML book series p ubl ished by Addison -W esley

htt p://www.omg.org


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UML Too ls

Mos t comp lete tool: Rationa l Rose , htt p://www.ra tiona l.comLots of o thers

Toge ther by Obj ect Interna tiona l, htt p://www.oi.comBOO ST (Basic Obj ect-Orien ted S u ppor t Too l) by Noe l Rappin , avai la ble on CD/Co W e bArgo -UML , Obj ectPlant, Posiden , etc.


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O O F eatu r es

Encaps ulationInforma tion Hiding (Da ta Hiding)

Inhe r itancePoly m or phis m

Obj ect Based

Obj ect O r iented( )( )

F or S oftwa r e Reuse


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OO featu r esE ncapsulationInfo rm ation Hiding

The concep t of a bstrac tion isf undamen tal in programming

Sub program / f unc tion

p r ocess abst r action

ADT

Data abst r actionS oftwa r e Reuse

:Inhe r itancePoly m or phis m

: /


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Class elemen ts


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Cons tr uctors

F or purpose of initialization objects have specialmethods called constructors.

To write a constructor you must adhere to two rules :1. The method name must exactly match the class

name.

2. There must not be a return type declared for themethod.

Java has NO destructor.


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The Defa ult Cons tr uctor

E very class has a constructor .

If you don t write one Java provides one for you . This

constructor takes no arguments and has an empty body.

? What If you write one constructor ?


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S tr uct in C/C++

#include st r uct S tudent {

long sid ;

cha r na m e[9] ; /* B ig5 */

float sco r e[13] ; /* 13 */}; /* */int m ain( ) {

st r uct S tudent x ; /* C++ C99 st r uct */

x.sid = 123 ; st r cpy(x.na m e, " " ) ; /* *//* loop x.sco r e[?] */

}

W hy using s tr uct ?Java has no str uct


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S tr uct vs. Class in C++ (1/2 )

#include class S tudent { // st r uct S tudent { ?

long sid ;

};

int m ain( ) {S tudent x ;

x.sid = 123 ; // co m pile , access not allowedcout


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S tr uct vs. Class in C++ (2/2 )#include class S tudent { // p r ivate :

long sid ;

public : showNu m be r ( ) { r etu r n sid ; }setId( long xx ) { sid = xx ; }

};

int m ain( ) {S tudent x, y ;

x.setId(123 ); // OK, public function setId( )

cout


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Class(Object ) Const r ucto r 1/4 (C++ )#include class S tudent { // p r ivate :

long sid ;


};

int m ain( ) {S tudent x, m , n ;S tudent y(456 ); // Err or

x.setId(123 ); r etu r n 0 ;

}Studen t y(456 ) ; !

defa ult cons tr uctor

defa ult cons tr uctor


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Class(Object ) Const r ucto r 2/4 (C++ )#include class S tudent { // p r ivate :

long sid ;


S tudent(long x ) { sid = x ; }};

int m ain( ) {S tudent x ; // Err orS tudent y(456 ); // OK nowx.setId(123 ); r etu r n 0 ;

} Studen t y(456 ) ; OK

cons tr uctor

Studen t x; !defa ult cons tr uctor

:


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#include class S tudent { // p r ivate :

long sid ;

public : showNu m be r ( ) { r etu r n sid ; }setId( long x ) { sid = x ; }

S tudent (long x =0 ) { sid = x ; }};

int m ain( ) {S tudent x, y(456 ); // OK x.setId(123 ); r etu r n 0 ;

}Studen t x;

Defa ult

Class(Object ) Const r ucto r 3/4 (C++ )


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#include class S tudent { // p r ivate :

long sid ;


S tudent(long x ) { sid = x ; cout


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More a bout Classes

Proper ties:Sing le imp lemen tation inheri tance - extends

The inheri ted c lass is ca lled su perc lass .

Multiple in terface inheri tance - implements

All classes inheri t from java. lang. ObjectScope: (nex t slide)


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S cope


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The Life Cyc le of an Obj ect Crea tion (I)Crea ting Obj ects

Rectangle rect = new Rectangle();

Dec laring an Obj ectObj ect dec lara tions can a lso appear a lone :

Rectangle rect;

In Java , classes and in terfaces can be used as da ta types.Decla r ations do not c r eate new objects.

Instantiating an Obj ectThe new opera tor a lloca tes memory for a new o bject.The new opera tor req uires a arg umen t: a ca ll to a cons tr uctor.

The new opera tor crea tes the o bject, and the cons tr uctor ini tializes i t.new Rectangle(100, 200);

The new opera tor re turns a reference to the new ly crea ted o bject.Rectangle rect = new Rectangle(100,200);

Af ter this s tatemen t, rect refers to a Rectangle o bject.


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The Life Cyc le of an Obj ect -- Crea tion (II)

Initializing an Obj ectCons tr uctors have the sa m e na m e as the class and have no r etu r ntype .

public Rectangle(Point p)

public Rectangle(int w, int h)

public Rectangle(Point p, int w, int h)public Rectangle()

Examp le:Rectangle rect = new Rectangle(100, 200);

Rectangle rect = new Rectangle(

new Point(44,78));

A cons tr uctor can tak es no arg umen ts, is ca lled a no-argument

constructor .If a c lass does no t define any cons tr uctors , Java provides a no -argumen t cons tr uctor , called the default constructor.


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The Life Cyc le of an Obj ect -- Access

Using Obj ectsReferencing an Obj ect's Varia bles

objectReference . variable

Examp le:

int areaOfRectangle = rect.height * rect.width;

Calling an Obj ect's Me thodsobjectReference. methodName(); or objectReference. methodName(argumentList);

Examp le:rect.move(15, 37);

int areaOfRectangle =new Rectangle(100, 50).area();


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The Life Cyc le of an Obj ect Clean u p

Cleaning Up Un used Obj ectsAn o bject is e ligi ble for gar bage co llection when there are no morereferences to that o bject.The Gar bage Co llector

Aut oma tica lly frees an o bject, if no t referenced. preven ts memory leak sclaimed de lay ~2 00ms for 4MBMar k-sweep gar bage co llector The gar bage co llector r uns in a low- priori ty thread.

W hen the sys tem r uns o ut of memory.In response to a req uest from a Java program.W hen the sys tem is id le.


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Fina liza tionW hen freeing an o bject, the sys tem wi ll call this firs t.The finalize method is a mem ber of the Object class.Imp lemen ting a finalize me thod to r elease r esou r ces that aren' t under the con trol of the ga r bage collecto r , such as na tive

peers.Scenario: For a file o bject, when freed , need to close it.Pro blem: Ma k e the in terna l JVM imp lemen tation for memory managemen t very comp lica ted.Examp le

public class Rectangle {public Point origin;

. . .protected void finalize () throws Throwable {

origin = null;super.finalize() ;

}}


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Examp le for Overriding (1/2)class Point {

protected int x,y;

Point (int x, int y) { this.x = x; this.y = y;}

void draw (){System.out.println(Point at+x+,+y);

};

}

Subclassing:class Cpoint extends Point {

private Color c;

Cpoint (int i, int j, Color c) {

super (i,j);

this.c =c;

};void draw (){

System.out.println(Color Point at + i + , + j + , + c);};

}


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Examp le for Overriding (2/2)class PointApp {

public static void main() {

Point p = new Point(5,5);

Cpoint cp = new Cpoint(1,1,Red);

p.draw();

cp.draw(); p = cp; p.draw();

// call Points draw() or Cpoint s draw()?}

}

Overr i ding overl oading !


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Over loadingA Method's Name (f unction name)

Java s u ppor ts me thod name overload ing so that mult iple me thods canshare the same name.

class DataRenderer {

void draw(String s) { . . . }

void draw(int i) { . . . }

void draw(float f) { . . . }

}

Ove r loaded methods are differen tiated by the nu m be r and type of thea r gu m ents .

A subclass may override a me thod in i ts su perc lass.The overriding me thod m ust have the same name , return type, and parame ter list as the me thod i t overrides.

Java opera to r overl oading


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Abstrac t Classes in Java

Abstrac t classes crea ted using the a bstrac t k eyword: publ ic a bstrac t class Mo torVehic le { }

In an a bstrac t class , severa l a bstrac t methods are dec lared.

- An a bstrac t method is no t imp lemen ted in the c lass , only dec lared. The body of the me thod is then imp lemen ted in s ubclass.

- An a bstrac t method is decora ted wi th an ex tra a bstrac t k eyword.Abstrac t classes can no t be ins tantiated! So the fo llowing isillega l:

Mo torVehic le m = new Mo torVehic le; // !//

//


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Abstrac t methods

Abstrac t methods are dec lared but do no t con tain animp lemen tation.For examp le, the Mo torVehic le class may have an a bstrac t method gas( ):

publ ic abst r act class Mo torVehic le { priva te do uble speed , maxSpeed;void accToMax( ) { speed = maxSpeed;} // !

publ ic abst r act void gas( ); // ; C++ =0;/* */

}So Mo torVehic le can N OT be ins tantiated

(ie. , can no t be used to crea te an o bject.)


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Examp le of Abstrac t Classes (1/2)

Examp le:abstract class Stack {

abstract void push(Object o);abstract Object pop();

}

public class ArrayStack extends Stack {.... // declare elems[] and top;void push(Object o) { elems[top++] = o; }Object pop() { return elems[--top]; }

}

class LinkedStack extends Stack {.... // declare ...void push(Object o) { .... }Object pop() { .... }

}


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Examp le of Abstrac t Classes (2/2)

// ============================== Another file...static public int testStack(Stack s) { ....

s.push(x); s.push(y);o1 = s.pop(); o 2 = s.pop();// check if o1 == y && o 2 == x.

}...

ArrayStack as = new ArrayStack();testStack(as) ;...

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I nterfacesDefines a se t of me thods that a c lass m ust imp lemen t

An interface is lik e a class with no thing but a bstrac t methodsand fina l and s tatic fie lds (cons tants).An interface can a lso have fie lds, but the fie lds m ust bedec lared as fina l and static.All methods are a bstrac t imp lici tly.

I nterface can be added to a c lass that is a lready a s ubclass of ano ther c lass. ( imp lemen ts)To dec lare an in terface:

publ ic interface Impor tTax { publ ic do uble ca lculateTax( );

}

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Examp le of I nterfaces (1/2)interface Stack {

void push(Object o);

Object pop();

}

public class ArrayStack implements Stack { .... // declare elems[] and top;void push(Object o) { elems[top++] = o; }Object pop() { return elems[--top]; }

}

class LinkedStack implements Stack {

.... // declare ...void push(Object o) { .... }Object pop() { .... }

}

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Examp le of I nterfaces (2/2)

//============================== Another file

...

static public int testStack(Stack s) { ....

s.push(x); s.push(y);o1 = s.pop(); o2 = s.pop();

// check if o1 == y && o2 == x.

}

...

ArrayStack as = new ArrayStack();

testStack(as) ;...

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Difference between Abstrac t Class and I nterface

An a bstrac t class is a c lass con taining severa l a bstrac t methods.Each a bstrac t method is prefixed wi th the k eyword a bstrac t.

An a bstrac t class can no t be ins tantiated but can be ex tended(subclassed).An interface con tains on ly a bstrac t methods and cons tants.Each a bstrac t method is no t prefixed with the k eyword a bstrac t.

An interface can on ly be imp lemen ted.

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How C++ Dea ls wi th I nterfaces

or Abstrac t ClassesUse vir tual f unctions:class Stack {

virtual void push(void* o) = NULL;

virtual void* pop() = NULL; // NULL 0}

// C++ virtual = NULL; = 0;// = NULL; =0; virtual f unction pure vir tual f unction// Java virtual

W hat are the in terna l designs?Key for the designs of C OM/DC OM (MicroSof t)

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Examp le for C++class ArrayStack : Stack {

.... // declare elems[] and top;void push(Object o) { elems[top++] = o; }Object pop() { return elems[--top]; }

}class LinkedStack : Stack {

.... // declare ...

void push(Object o) { .... }Object pop() { .... }}============================== Another file...int testStack(Stack s) { ....

s.push(x); s.push(y);o1 = s.pop(); o 2 = s.pop();// check if o1 == y && o 2 == x.

}...

ArrayStack as = new ArrayStack();testStack(as) ;...

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Pac k ages and Li braryOrganize c lass na m e spaceHierarchica l division m ust be ref lected in direc tory s tr ucture, i.e. ,

package cyc.sys ;import java.io.*;class Login { /* in fact, this is cyc.sys.Login */ }

Defa ult pac k ages ( li brary) -- JDK java. lang - Obj ect, Thread , Excep tion , String ,...java.io - InputStream , Out putStream , ...java.ne t - Ne twor k ing

su ppor t for Soc k et & URL -based o bjectsjava.aw t - Abstrac t window toolk it

defines a simp le, restricted windowing APIjava. util - Hash ta ble, Vec tor , BitSet, Regexp , ..java. tools - Compi lation , De bugging , Doc umen tation

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Naming a Pac k ageThe name of the class in the

pac k age is rea lly

The name of the class in the pac k age is rea lly

Java grap hi cs. Rec t angle

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Access Modifiers(no k eyword): c lass/pac k age access (diffe r ent f r om C++ )

I.e. , package available

public : wor ld access (same as C++) private : class access (same as C++)

protected : subclassing access(same as C++)

static : on ly one copy for a ll instancesabstract : lef t unimp lemen tedfinal : no t overridden

native : me thods imp lemen ted in na tive code.synchronized : descri bed in Threadtransient : da ta that is no t persis tent

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Comparisons

Con trolling Access to Mem bers of a C lass

Specifier Class Subclass Package Worldprivate Xprotected X X Xpublic X X X Xpackage X X

X !

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Staticstatic:

only one copy for the c lass and a ll ins tancesUsually used as a g lo bal varia ble (even no ins tances)

class TestStatic {int x, y, z;int average() { return (x+y+z)/3; }

static int j = 0;static int peek() {

j += 2;x = 3; // error!

}static int k = peek();static { j = j*5; } // done when loading classpublic static void main(String[] args) {

...TestStatic ts = new TestStatic();int ave = ts.average();...

}}

S t ati c metho d

S t ati c bl ock

S t ati c var i able j, kS t ati c metho d

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Fina lFina l varia bles:

The va lue of a fina l varia ble canno t change af ter i t has beeninitialized.

Such varia bles are simi lar to cons tants in o ther programminglang uages.

final Color red = Color.red;

A fina l loca l varia ble that has been dec lared but not yet ini tialized iscalled a blank fina l.

final int blankfinal;. . .blankfinal = 0;

Fina l methods: me thods that canno t be overridden.Preven t clients from overriding.Allow for JIT to op timize.

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Transien t vs. Vo latiletransient : used in o bject seria liza tion to mar k mem ber varia bles that sho uld no t be seria lized.volatile : used to preven t the compi ler from

performing cer tain op timiza tions on a mem ber.

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Nes ted C lassA nes ted c lass is a c lass that is a mem ber of ano ther c lass.

class Encl os ingClass{. . .class ANes t edClass {

. . .}

}Define a c lass wi thin ano ther c lass when the nes ted c lassmak es sense on ly in the con text of i ts enc losing c lass.

For examp le, a text cursor ma k es sense on ly in the con text of a par ticular text componen t.

A nes ted c lass has a specia l privi lege:It has unlimited access to i ts enc losing c lass's mem bers , even for

priva te.Just lik e a me thod that can access priva te.

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Types of Nes ted C lassclass Encl os ingClass{

. . .s t ati c class AS t ati cNes t edClass {

. . .}class Inn er Class {

. . . / * s t ati c i s wr ong in thi s inn er class */}

}

A static nested class is called : a static nested class.A static nested class cannot r efe r di r ectly to instance va r iables o r m ethods defined in its enclosing class

A nonstatic nested class is called an inne r class.An inne r class is associated with an instance of its enclosing class andhas di r ect access to that object's instance va r iables and m ethods.Because an inne r class is associated with an instance, it cannot defineany static m em be r s itself.

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Nes ted vs. Inner To he l p differen tiate the terms nes ted c lass and inner c lass:

The term "nes ted c lass" ref lects the syn tactic re lationship betweentwo c lasses.In con trast, the term "inner c lass" ref lects the re lationship betweenins tances of the two c lasses.

An ins tance of InnerC lass can exis t only wi thin an ins tance of Enc losingC lass ,It has direc t access to ins tance varia bles and me thods of i ts enc losingins tance.

class Encl os ingClass {

. . .class Inn er Class {. . .

}}

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Examp le: Adap tor Using an Inner C lass to Imp lemen t an Adap ter

impor t java. util.Enumera tion;impor t java. util.NoS uchE lemen tExcep tion;

publ i c class S t ack {pr i vat e j ava.u ti l. Vec to r it ems;

// ... code fo r S t ack's metho ds and cons t ruc to rs not s hown...

publ i c Enumeration enumerato r () {re t ur n new S t ack Enum() ;

}

class S t ack Enum i mplement s Enumeration {int curre ntIt em = it ems.s iz e() - 1;publ i c boolea n h asMore Eleme nt s () {

re t ur n ( curre ntIt em >= 0) ;}publ i c Obj ec t n extE leme nt() {if (!h asMore Element s())

th r ow new NoS uchElementEx cep tion() ;else

re t ur n it ems.eleme ntAt( curre ntIt em-- ) ;}

}}

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Anonymo us C lassY ou can decla r e an inne r class without na m ing it.E xa m ple : (Rew r ite the p r evious slide. )

publ i c class S t ack {pr i vat e Vec to r it ems;...//c ode fo r S t ack's metho ds and cons t ruc to rs not s hown...publ i c Enumeration enumera to r () {

re t ur n n ew Enumera tion() {int curre ntIt em = it ems.s iz e() - 1;publ i c boolea n h asMore Eleme nt s () {

re t ur n ( curre ntIt em >= 0) ;}publ i c Obj ec t n extE leme nt() {

if (!h asMore Eleme nt s ())th r ow new NoS uchEleme ntEx cep tion() ;

else re t ur n it ems.eleme ntAt( curre ntIt em-- ) ;}

};}

}

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U pcas ting and Po lymorphism(1/4) U pcas ting: Ta k ing an o bject reference and trea ting i t as areference to i ts base type is ca lled upcast ing , beca use of theway inheri tance trees are drawn wi th the base c lass a t the top.

.

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U pcas ting and Po lymorphism (2/4)Polymorphism : In Java , the princip le that the ac tual type of theo bject determines the me thod to be ca lled is ca lled polymorphism .

class Shape {void draw( ) {}void erase( ) {}

}

class Circ le ex tends Shape {void draw( ) {

Sys tem.o ut.prin tln("Circ le.draw( ) ");}void erase( ) {Sys tem.o ut.prin tln("Circ le.erase( )");}

}

Shape

draw()erase()

Squaredraw()erase()

Triang ledraw()erase()

Circ ledraw()erase()

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class Sq uare ex tends Shape {void draw( ) {

Sys tem.o ut.prin tln("Sq uare.draw( ) ");}

void erase( ) {Sys tem.o ut.prin tln("Sq uare.erase( )");}}

class Triang le ex tends Shape {void draw( ) {

Sys tem.o ut.prin tln("Triang le.draw( )");}void erase( ) {Sys tem.o ut.prin tln("Triang le.erase( )");}

}

U pcas ting and Po lymorphism (3/4)

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publ ic c lass Shapes { publ ic s tatic Shape randShape() {

swi tch((in t) (Ma th.random()*3)) {case 0: return new Circ le();case 1: re turn new Sq uare();case 2: re turn new Triang le();defa ult : re turn new Circ le();}

} publ ic s tatic void main(S tring[] args) {

Shape[] s = new Shape[9];for (in t i = 0; i < s. leng th; i++)

s[i] = randShape();//Ma k e po lymorphism me thod ca lls:

for (in t i = 0; i < s. leng th; i++) s[i].d r aw( );}

}

U pcas ting and Po lymorphism (4/4)

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Polymorphism in C++ (1/2)// polymo.cpp -- CopyWrong by [email protected]

#include class Shape{

public:virtual // run

void draw( ) { cout


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Polymorphism in C++ (2/2)int main(){

Circle * ppp;Shape * fig[9];

// ppp = new Shape(); // errorppp = (Circle *)new Shape();ppp -> draw();ppp = (Circle *)new Line(); ppp -> draw();ppp = new Circle(); ppp -> draw();cout


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http://www.csie.nctu.edu.tw/~tsaiwn /course/ja v a/

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02 Java Oop Class