Java Second Oop

8/16/2019 Java Second Oop

http://slidepdf.com/reader/full/java-second-oop 1/62

Zahid Aslam

Classes and Objects



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Overview

Implementing classes and objects in Java.

Understand important features in Java classes.

Visibility Control

Static Metods and !ttributes

"arameter "assing

Object #elegation

Object Cleanup



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Classes ! class is a collection of felds $data or attributes% and methods

$procedure or function% tat operate on tat data.

centre&centre'radius

area$%circumference$%

Circle



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#e(ning a Class

)e basic synta* for a class de(nition+class ClassName [extends SuperClassName]

{

[fields declaration]

[methods declaration] }

,are bone class +

// Circle.java

// Circle class definition

class Circle {

}



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!dding -ields+ Class Circle wit(elds

!dd felds

// Circle.java

class Circle {

double centreX, centreY; //centre coordinate

double radius; //radius

}

)e (elds $data% are also called te instance variables.



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!dding Metods ! class wit only data (elds are not useful. Objects created by

suc a class cannot respond to any messages.

Metods are declared inside te body of te class butimmediately after te declaration of data (elds.

)e general form of a metod declaration is+

t!e method"ame #!arameter$list%

{

method$bod;}


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Circle Class /*ample

// Circle.java

class Circle {

double centreX, centreY;

double radius;

//&ethods to return circumference and area double circumference#% {

double circum ' ()*.+)radius;

return circum;

}

double area#% {

double area ' *.+ ) radius ) radius;

return area;

}

}

Metod ,ody



Compiling te class

Create a (le Circle.java and enter te code for te classsown in te previous e*ample $0ote+ te (le namesould matc te class name%.

Compiling te class +javac Circle.java

Creates (le Circle.class

Circle becomes a derived data t!e that can be

used in a java !ro-ram.



#ata !bstraction #eclaring te Circle class1 as created a new data type Circle 2

#ata !bstraction

Variables of te data type can be now de(ned in a program.

// Circleest.java// est !ro-ram to test the Circle class

class Circleest {

!ublic static void main# trin- ar-s[] % {

Circle aCircle; Circle bCircle;

}

}



Class of Circle cont. )e de(nitions+ Circle aCircle3 Circle bCircle3 in te above

e*ample did not create Circle objects.

aCircle1 bCircle are simply references to a Circle object1 not anobject itself.

aCircle

"oints to noting

$0ull 4eference%

bCircle

"oints to noting

$0ull 4eference%

null null



Creating objects of a class Objects are created dynamically using te new 5eyword.

aCircle and bCircle refer to Circle objects

bCircle ' ne0 Circle#%;aCircle ' ne0 Circle#%;



Creating objects of a class

// Circleest.java

// est !ro-ram to test the Circle class

class Circleest {

!ublic static void main# trin- ar-s[]%{

Circle aCircle;

Circle bCircle;

aCircle ' ne0 Circle#%;

bCircle ' ne0 Circle#%;

}

}



Creating objects of a class

aCircle ' ne0 Circle#%;bCircle ' ne0 Circle#% ;

bCircle ' aCircle;

"

aCircle

6

bCircle

,efore !ssignment

"

aCircle

6

bCircle

,efore !ssignment



!utomatic garbage collection )e object does not ave a reference and cannot be

used in future.

)e object becomes a candidate for automatic garbagecollection.

Java automatically collects garbage periodically and releaseste memory used to be used in te future.

6



!ccessing Variables andMetods

Similar to C synta* for accessing data de(ned in a structure.

Circle aCircle ' ne0 Circle#%;

// initiali1e centre and radius

aCircle.centreX ' (.2;

aCircle.centreY ' (.2;

aCircle.radius ' +.2;

ObjectName.VariableName

ObjectName.MethodName(parameter-list)

i d i



/*ecuting Metods inObject7Circle

Using Object Metods+


double area;

area ' aCircle.area#%;

sent 8message9 to aCircle



Using Circle Class /*ample

// Circleest.javaclass Circleest {

!ublic static void main#trin- ar-s[]%{

Circle aCircle;


aCircle.centreX ' +2;aCircle.centreY ' (2;

aCircle.radius ' 3;

double area ' aCircle.area#%;

double circumf ' aCircle.circumference#%;

stem.out.!rintln# 45adius'46aCircle.radius64 7rea'46area%;

stem.out.!rintln#45adius'46

aCircle.radius64Circumference '46circumf%;

}

}



Using Circle Class /*ample "rogram Output +

5adius'3.2 7rea'89.3

5adius'3.2 Circumference '*+.2222222222222(



,etter way of Initialising or!ccess #ata Members *1 y1 r

Information :iding is important;

!void giving direct access to attributes.

#e(ne !ccess metods. an interface for te rest of te system to use.

)o initialise7Update a value+

aCircle.setX# +2 % )o read a value+

aCircle.-etX#%

)ese metods are informally called as !ccessors orSetters7<etters Metods.

0ote + Java te*t boo5 uses a di=erent convention from wat isused in tese notes. <et metod in te te*t boo5 ise>uivalent to set metod in tese e*amples.



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!ccessors 2 ?<etters7Setters@

// Circle.java

class Circle {

double centreX, centreY, radius;

//&ethods to set and -et fields

double -etX#% { return centreX;} double -etY#% { return centreY;}

double -et5#% { return radius;}

void setX#double x:in% { centreX ' x:in;}

void setY#double :in% { centreY ' :in;}

void set5#double r:in% { radius ' r:in;}

// ther methods -o here

}

U i t7 t t d



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Using get7set metods /*ample

// Circleest.java

class Circleest {


Circle aCircle;


aCircle.setX#+2%;aCircle.setY#(2%;

aCircle.set5#3%;

double area ' aCircle.area#%;

double circumf ' aCircle.circumference#%;

stem.out.!rintln# 45adius'46aCircle.-et5#%64 7rea'46area%;

stem.out.!rintln#45adius'46

aCircle.-et5#%64Circumference '46circumf%;

}

}



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Object InitialiAation

Ben objects are created1 te initial value of data (elds isun5nown unless its users e*plicitly do so.

In te previous e*amples te initial values were set afterobject creation using synta*+

Object0ame.#ata-ield D E3Object0ame.Set#ata-ield$E%3

In many cases1 it would be useful if tis initialisation can

be carried out by default witout te users e*plicitlyinitialiAing tem.

In Java1 tis can be acieved troug a mecanism calledconstructors.



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Bat is a ConstructorF

Constructor is a special metod tat is invo5ed?automatically@ at te time of object creation.

Constructor is normally used for initialiAing objects witdefault values unless di=erent values are supplied.

Constructor as te same name as te class name.

Constructor cannot return values.

! class can ave more tan one constructor as long astey ave di=erent input arguments Overloading

)ere is 0O e*plicit invocation statement needed+ Bente object creation statement $new% is e*ecuted1 teconstructor metod will be e*ecuted automatically.



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#e(ning a Constructor

!ublic class Class"ame {

// <ata =ields>

// Constructor

ClassName() {

// Method Body Statements

}

//&ethods to mani!ulate data fields

}

!ddi C t t t Ci l



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!dding a Constructor to Circleclass+ /*ample

// Circle.java

class Circle {


Circle#% {

centreX ' 2;

centreY ' 2;

radius ' +2.2;

}



}

!ddi M lti l " t



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!dding a Multiple"arametersConstructor to our Circle Class

class Circle {


// Constructor

Circle#double x, double , double r%

{

centreX ' x;

centreY ' ;

radius ' r; }



}



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Constructors initialise Objects 4ecall te following OG# Code Segment+


aCircle.x ' +2.2;

aCircle. ' (2.2aCircle.r ' 3.2;

aCircle = new Circle() 3

!t creation time te center and radius

are not de(ned.

)ese values are e*plicitly set later.



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Constructors initialise Objects Bit de(ned constructor

Circle aCircle ' ne0 Circle#+2.2, (2.2, 3.2%;

aCircle ' ne0 Circle#+2.2, (2.2, 3.2%;

aCircle is created wit center $E1 HE%and radius



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Constructor )est /*ample

// CircleConstructorest.javaclass CircleConstructorest {


Circle aCircle ' ne0 Circle#+2.2, (2.2, 3.2%;

stem.out.!rintln#?CentreX'?6aCircle.-etX#%%; stem.out.!rintln#?CentreY'?6aCircle.-etY#%%;

stem.out.!rintln#45adius'46aCircle.-et5#%%;

}

}

"rogram Output @ CentreX'+2.2

CentreY'(2.2

5adius'3.2



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Multiple Constructors

Sometimes want to initialiAe in a number of di=erent ways1depending on circumstance.

)is can be supported by aving multiple constructorsaving di=erent input arguments.



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Multiple Constructors

class Circle {


// Constructor

Circle#double x, double , double r%{

centreX ' x; centreY ' ; radius ' r;

}

Circle#double r%{

centreX ' 2.2; centreY ' 2.2; radius '

r;

}

Circle#double x, double %{centreX ' x; centreY ' ; radius '

+2.2;

}





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InitialiAing wit constructors!ublic class estCircles {


Circle circle7 ' ne0 Circle#+2.2, +(.2, (2.2%;

Circle circleA ' ne0 Circle#+2.2%;

Circle circleC ' ne0 Circle#3.2,+2.2%;

}}

circle! D new Circle$E1 H1 HE%circle, D new Circle$E%

Centre D $E1E%4adiusDE

circleC D new Circle$1E%

Centre D $1E%

4adius D E

Centre D $E1H%

4adius D HE



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Metod Overloading Constructors all ave te same name. Metods are distinguised by teir signature+

name number of arguments type of arguments position of arguments

)at means1 a class can also ave multiple metods wit tesame name + metod overloading. )is is a form of polymorpism.

0ot to confuse wit method overriding $coming up%.



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"olymorpism !llows a single method or operator associated wit di=erent

meaning depending on te type of data passed to it. It can berealised troug+ Metod Overloading

#e(ning te same method wit di=erent argument types$metod overloading% polymorpism.

)e metod body can ave di=erent logic depending on tedate type of arguments.



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)e 0ew this 5eyword

class Circle {

double centreX, centreY, radius; // Constructor

Circle #double centreX, double centreY, B double radius% {

this.centreX ' centreX;

this.centreY ' centreY; this.radius ' radius;

}

// ther methods

}

this 5eyword can be used to refer to te object itself. It isgenerally used for accessing class members $from its ownmetods% wen tey ave te same name as tose passedas arguments.

Visibility Control+ #ata :iding and



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Visibility Control+ #ata :iding and/ncapsulation

Java provides control over te visibility of variables andmetods.

)is allows encapsulation, wic is safely sealing data witin

te capsule of te class.

"revents programmers from relying on details of classimplementation1 so you can update witout worry.

:elps in protecting against accidental or wrong usage.

eeps code elegant and clean $easier to maintain%.



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Visibility

area

Circle

circumference

Center $*1y%4adius r

message

Construction time message

message

Circle

Visibility Modi(ers+ "ublic



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Visibility Modi(ers+ "ublic1"rivate1 "rotected

Public 5eyword applied to a class1 ma5es it available7visible

everywere. !pplied to a metod or variable1 completelyvisible.

#efault $0o visibility modi(er is speci(ed%+ it beaves li5epublic in its pac5age and private in oter pac5ages.

Private (elds or metods for a class only visible witin tatclass. "rivate members are not visible witin subclasses1 andare not inerited.

Protected members of a class are visible witin te class1subclasses and also witin all classes tat are in te samepac5age as tat class. )ey are also visible to subclasses inoter pac5ages.



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Visibility

!ublic class Circle { !rivate double centreX, centreY, radius;

// Constructor

!ublic Circle #double x, double , double r%

{

this.x ' x;this. ' ;

this.r ' r;

}

!ublic double circumference#%{return ()*.+)r;

}

!ublic double area#% {

return *.+ ) r ) r;

}

}

centre&centre'radius

Karea$%

Kcircumferenc

Circle



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!ccessors 2 ?<etters7Setters@

!ublic class Circle { !rivate double x,,r;

//&ethods to return circumference and area

!ublic double -etX#% { return x;}

!ublic double -etY#% { return ;} !ublic double -et5#% { return r;}

!ublic void setX#double x% { this.x ' x;}

!ublic void serY#double % { this. ' ;}

!ublic void set5#double r% { this.r ' r;}

}

More on ?Visibility@ during Ineritance and "ac5age #iscussion



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Static Members

Java supports de(nition of global metods and attributes tatare common to all objects of te type of class. Suc membersare called Static members.

!n attribute or a metod can be de(ned to be static.

)is feature is useful wen we want to create a variablecommon to all instances $objects% of a class.

One of te most common e*amples of a static attribute is onetat 5eeps count of te number of objects of te class tatave been created.

0ote+ Java creates only one copy for a static variable wic

can be used even if te class is never instantiated.



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Static Variables /*ample

// Cricle.java

class Circle {

// static #class % variable

// ne for the Circle class, number of circles static int numCircles ' 2;


// Constructors and other methods

Circle#double x, double , double r%{


numCircles66;

}

}

Us ng Stat c Var a es



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Us ng Stat c Var a es /*ample

// CountCircles.javaclass CountCircles {


Circle circle7 ' ne0 Circle# +2, +(, (2%;

B // numCircles ' +

stem.out.!rintln#?"umber of Circles ' 6

Circle.numCircles %;

Circle circleA ' ne0 Circle# 3, *, +2%;

// numCircles ' (

stem.out.!rintln#?"umber of Circles ' 6

Circle.numCircles %;

}



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Static Variables /*ample

circle! D new Circle$E1 H1 HE% circle, D new Circle$1 L1 E%

numCircl

es

"umber of Circles ' +

"umber of Circles ' (

"rogram output@



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Instance Vs Static Variables

Instance variables + One copy per object. /very objectas its own instance variable.

/.g. centre&1 centre'1 radius $centre and radius inte circle%

Static variables + One copy per class. /.g. numCircles $total number of circle objects created%



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Static Metods ! class can ave metods tat are de(ned as static $e.g.1 main

metod%.

Static metods can be accessed witout using objects. !lso1tere is 0O need to create objects.

)ey are pre(*ed wit 5eyword ?static@

Static metods are generally used to group related libraryfunctions tat don9t depend on data members of its class. -ore*ample1 Mat library functions.



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Static metods restrictions Static metods can only call oter static metods.

Static metods can only access static data.

Static metods cannot refer to ?tis@ or ?super@ $more later% inanyway.



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Static Metods /*ample

// Circle.javaclass Circle {

// static #class % variable

// ne for the Circle class, number of circles

!rivate static int numCircles ' 2;

!rivate double centreX, centreY, radius;

// Constructors and other methods

!ublic Circle#double x, double , double r%{


numCircles66;}

!ublic static void !rint"umCircles#%{

stem.out.!rintln#?"umCircles'?6numCircles%;

}

}

Using Static Metods



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Using Static Metods /*ample

// CountCircles.java

class CountCircles {


Circle circle7 ' ne0 Circle# +2, +(, (2%;

Circle circleA ' ne0 Circle# 3, *, +2%;

Circle.!rint"umCircles#%;

}

}"rogram Output+"umCircles'(

, 5 t : ll B ld



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,ac5 to :elloBorldSystem invo5es static main

metodN // DelloEorld.java@ Dello Eorld !ro-ram

class DelloEorld

{

!ublic static void main#trin- ar-s[]%

{

stem.out.!rintln#?Dello Eorld%;

}

}

,ac5 to Constants



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,ac5 to Constants(nal can also be made as staticN

// Fuared"umbers.java

class Fuared"umbers{

static final int &7X:"G&AH5 ' (3;


final int &7X:"G&AH5 ' (3; int lo ' +;

int sFuared ' 2;

0hile #sFuared I' &7X:"G&AH5%{

sFuared ' lo ) lo;

stem.out.!rintln#sFuared%; lo ' lo 6 +;

}

}

}



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-inal ! java variable can be declared using te 5eyword (nal. )en

te (nal variable can be assigned only once. ! variable tat is declared as (nal and not initialiAed is called a

blan5 (nal variable. ! blan5 (nal variable forces teconstructors to initialise it.

Java classes declared as (nal cannot be e*tended. 4estricting

ineritance; Metods declared as (nal cannot be overridden. In metods

private is e>ual to (nal1 but in variables it is not.

(nal parameters 2 values of te parameters cannot becanged after initialiAation. #o a small java e*ercise to (nd outte implications of (nal parameters in metod overriding.

Java local classes can only reference local variables andparameters tat are declared as (nal.

! visible advantage of declaring a java variable as static (nal

is1 te compiled java class results in faster performance.



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"arameter passing

Metod parameters wic are objects are passed byreference.

Copy of te reference to te object is passed into metod1original value uncanged.



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"arameter passing /*ample

// 5eferenceest.java!ublic class 5eferenceest {

!ublic static void main #trin-[] ar-s%

{

Circle c+ ' ne0 Circle#3, 3, (2%;

Circle c( ' ne0 Circle#+, +, +2%;

stem.out.!rintln#?c+ 5adius ' ?

6c+.-et5#%%;

stem.out.!rintln#?c( 5adius ' ?

6c(.-et5#%%;

!arameterester#c+, c(%;

stem.out.!rintln#?c+ 5adius ' ? 6c+.-et5#%%;

stem.out.!rintln#?c( 5adius ' ? 6c(.-et5#%%;

}

........ cont

i l



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!ublic static void !arameterester#Circlecircle7,

Circle circleA%

{

circle7.set5#+3%;

circleA ' ne0 Circle#2, 2, +22%; stem.out.!rintln#?circle7 5adius'? 6

circle7.-et5#%%;

stem.out.!rintln#?circleA5adius'? 6

circleA.-et5#%%;}

}

i l



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"arameter passing /*ample "rogram Output +

c+ 5adius ' (2.2

c( 5adius ' +2.2

circle7 5adius ' +3.2circleA 5adius ' +22.2

c+ 5adius ' +3.2

c( 5adius ' +2.2

" i / l



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"arameter passing /*ampleSTEP1 – Before calling

parameterTester()

c

$11HE%

cH

$.1E%

&

circle!

&circle,

&

circle!

STEP2 – parameterTester(c1, c2)

c

$11HE%

cH

$.1E%circle!

circle,

circle!

STEP3 –circlA.setRadis(1!)

c

$11%

cH

$.1E%circle! circle,circle!

STEP" – circlB # ne$%irlce(&,&,1&&)c

$11%

cH

$.1E%circle! circle,circle!

$E.E1EE%

" t i / l



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STEP! – After Retrning from parameterTester

c

$11%

cH

$.1E%

&

circle!

&

circle,

&

circle!

# l ti 7! i ti



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#elegation7!ssociation

!bility for a class to delegate its responsibilities to anoterclass.

! way of ma5ing an object invo5e services of oter objectstroug containersip.

# l ti / l



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#elegation /*ample

"oint

*Coord

yCoord

get&Coord$%get'Coord$%

class Joint {

!rivate double xCoord;

!rivate double Coord;

// Constructor

>>>>.

!ublic double -etXCoord#%{

return xCoord;

}

!ublic double -etYCoord#%{return Coord;

}

}

# l ti / l



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#elegation /*ample

"oint

*CoordyCoord

get&Coord$%get'Coord$%

!ublic class Circle {

!rivate Joint centre;

!ublic double -etX#%{

return centre.-etXCoord#%;

} !ublic double -etY#%{

return centre.-etYCoord#%;

}

} Circle

centre

getCentre&$%getCentre'$%

Obj t Cl 7# t t



Objects Cleanup7#estructor Unli5e C and CKK1 memory deallocation is automatic in java1

don9t worry about it. no dangling pointers and no memory lea5 problem.

Java allows you to de(ne a fnalize metod1 wic is invoked $ifde(ned% just before te object destruction.

!llows recordmaintenance operations or cleaning up of anyspecial allocations made by te user.

// done 0ith this circle

!rotected void finali1e#% thro0s BHxce!tion {

Circle.numCircles ' Circle.numCircles$$

stem.out.!rintln#?number of circles@6

Circle.num:circles%;

}

Java Second Oop

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