classes and objects java

7/25/2019 Classes and Objects java

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Chapter 6 Objects and Classes

OO Programming Concepts

Creating Objects and Object Reference Variables

Differences between primitive data type and object type

Atomatic garbage collection Constrctors

!odifiers "public# privateand static$

%nstance and Class Variables and !ethods &cope of Variables

'se the this (eyword

Case &tdies "Mortgageclass and Countclass$


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OO Programming Concepts

data field )

method n

data field n

method )

An object

***

***

&tate

+ehavior

Data ,ield

radis - .

!ethod

findArea

A Circle object


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Class and Objects

circle)/ Circle

radis - 0

new Circle"$

circlen/ Circle

radis - .

new Circle"$

***

'!1 2raphical notation for classes

'!1 2raphical notation

for objects

Circle

radis/ doble

findArea"$/ doble

'!1 2raphical notation for fields

'!1 2raphical notation for methods


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Class Declaration

class Circle

{

double radius = 1.0;

double findArea()

{

return radius*radius*3.11!";

##


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Declaring Object Reference Variables

Class$a%e ob&ect$a%e;

34ample/

Circle %'Circle;


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Creating Objects

ob&ect$a%e = ne Class$a%e();

34ample/

%'Circle = ne Circle();

5he object reference is assigned to the object

reference variable*


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DeclaringCreating Objects

in a &ingle &tepClass$a%e ob&ect$a%e = ne

Class$a%e();

34ample/

Circle %'Circle = ne Circle();


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Differences between variables of

primitive Data types and object types

)

c/ Circle

radis - )

Primitive type int i - ) i

Object type Circle c c reference

Created sing

new Circle"$


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Copying Variables of Primitive

Data 5ypes and Object 5ypes

)

c)/ Circle

radis - .

Primitive type assignment

i - j

+efore/

i

0j

0

After/

i

0j

Object type assignment

c) - c0

+efore/

c)

c0

After/

c)

c0

c0/ Circle

radis - 7


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2arbage Collection

As shown in the previous

figure, after the assignment

statement c1 = c2, c1 points to

the same object referenced byc2. The object previously

referenced by c1 is no longer

useful. This object is known as

garbage. Garbage is

automatically collected by JVM.


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2arbage Collection# cont

TIP: If you know that an

object is no longer needed,

you can explicitly assign

null to a reference

variable for the object.

The Java VM will

automatically collect the

space if the object is not

referenced by any variable.


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Accessing Objects

Referencing the object8s data/

ob&ect$a%e.data

%'Circle.radius

%nvo9ing the object8s method/

ob&ect$a%e.%etod

%'Circle.findArea()


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Example 6.1 Using Objects

Objective/ Demonstrate creating objects#

accessing data# and sing methods*

TestCircleTestCircle RunRun
http://testcircle.htm/http://var/www/apps/conversion/tmp/scratch_4/TestCircle.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircle.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircle.bathttp://testcircle.htm/http://testcircle.htm/


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Constrctors

Circle(double r){

radius = r;

#

Circle()

{

radius = 1.0;

#

%'Circle = ne Circle(!.0);

Constrctors are a

special 9ind ofmethods that are

invo9ed to constrct

objects*


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Constrctors# cont*

A constructor with no parametersis referred to as a default

constructor.

Constructors must have thesame name as the class itself.

Constructors do not have a

return typenot even void.

Constructors are invoked using

the new operator when an object is

created. Constructors play the


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34ample 6*0 'sing Constrctors

Objective/ Demonstrate the role of

constrctors and se them to create

objects*

TestCircleWithConstructorsTestCircleWithConstructors RunRun
http://testcirclewithconstructors.htm/http://var/www/apps/conversion/tmp/scratch_4/TestCircleWithConstructors.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircleWithConstructors.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircleWithConstructors.bathttp://testcirclewithconstructors.htm/http://testcirclewithconstructors.htm/


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Visibility !odifiers and

Accessor !ethods+y defalt# the class# variable# or data can beaccessed by any class in the same pac9age*

public

5he class# data# or method is visible to any class in anypac9age*

private

5he data or methods can be accessed only by the declaringclass*

5he get and set methods are sed to read and modify private

properties*


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34ample 6*:

'sing the private!odifier

and Accessor !ethods

TestCircleWithAccessorsTestCircleWithAccessors RunRun

%n this e4ample# private data are sed for the radisand the accessor methods getRadis and setRadis

are provided for the clients to retrieve and modify

the radis*
http://testcirclewithaccessors.htm/http://var/www/apps/conversion/tmp/scratch_4/TestCircleWithAccessors.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircleWithAccessors.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircleWithAccessors.bathttp://testcirclewithaccessors.htm/http://testcirclewithaccessors.htm/http://testcirclewithaccessors.htm/http://testcirclewithaccessors.htm/


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Passing Objects to !ethods

Passing by vale "the vale is the reference

to the object$

34ample 6*: Passing Objects as Argments

TestPassingObjectTestPassingObject RunRun
http://testpassingobject.htm/http://testpassingobject.htm/http://var/www/apps/conversion/tmp/scratch_4/TestPassingObject.bathttp://var/www/apps/conversion/tmp/scratch_4/TestPassingObject.bathttp://var/www/apps/conversion/tmp/scratch_4/TestPassingObject.bathttp://testpassingobject.htm/http://testpassingobject.htm/


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%nstance

Variables# and !ethods

%nstance variables belong to a specific instance*

%nstance methods are invo9ed by an instance of

the class*


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Class Variables# Constants#

and !ethods

Class variables are shared by all the instances of the

class*

Class methods are not tied to a specific object*

Class constants are final variables shared by all the

instances of the class*


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and !ethods# cont*

5o declare class variables# constants# and methods#

se the static modifier*


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and !ethods# cont*

Circle;ith&taticVariable

mOfObjects"$/ int

=findArea"$/ doble

) radiscircle)/Circle


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34ample 6*.

'sing %nstance and Class Variables

and !ethod

Objective/ Demonstrate the roles of

instance and class variables and theirses* 5his e4ample adds a class variablenmOfObjects to trac9 the nmber of

Circle objects created*

Test CircleWithtatic!ariableTestCircleWithtatic!ariable RunRun
http://testcirclewithstaticvariable.htm/http://testcirclewithstaticvariable.htm/http://var/www/apps/conversion/tmp/scratch_4/TestCircleWithStaticVariable.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircleWithStaticVariable.bathttp://var/www/apps/conversion/tmp/scratch_4/TestCircleWithStaticVariable.bathttp://testcirclewithstaticvariable.htm/http://testcirclewithstaticvariable.htm/http://testcirclewithstaticvariable.htm/http://testcirclewithstaticvariable.htm/


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&cope of Variables

5he scope of instance and class variables is the

entire class* 5hey can be declared anywhere inside

a class* 5he scope of a local variable starts from its

declaration and contines to the end of the bloc9

that contains the variable* A local variable mst bedeclared before it can be sed*


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5he (eyword this

'se this to refer to the crrent object*

'se this to invo9e other constrctors of the

object*


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Array of Objects

Circle[] circleArray = newCircle[10];

An array of objects is actuallyan array of reference variables. So

invoking circleArray[1].findArea()

involves two levels of referencing

as shown in the next figure.circleArray references to the

entire array. circleArray[1]

references to a Circle object.


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Array of Objects# cont*

reference Circle object ?circleArray@?

B

circleArray

circleArray@)

circleArray@7 Circle object 7

Circle object )


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Array of Objects# cont*

34ample 6*6/ &mmariing the areas of thecircles

Demonstrate the roles of instance and class

variables and their ses* 5his e4ample addsa class variable nmOfObjects to trac9 the

nmber of Circle objects created*

TotalAreaTotalArea RunRun
http://totalarea.htm/http://var/www/apps/conversion/tmp/scratch_4/TotalArea.bathttp://var/www/apps/conversion/tmp/scratch_4/TotalArea.bathttp://var/www/apps/conversion/tmp/scratch_4/TotalArea.bathttp://totalarea.htm/http://totalarea.htm/


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Class Abstraction

Class abstraction means to separate class

implementation from the se of the class* 5he

creator of the class provides a description of theclass and let the ser 9now how the class can be

sed* 5he ser of the class does not need to

9now how the class is implemented* 5he detail

of implementation is encapslated and hidden

from the ser*


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34ample 6* 5he !ortgage Class

!ortgage

mOfEears"nmOfEears/ int$/ void

=set1oanAmont"loanAmont/ doble$/ void

=monthlyPayment"$/ doble

=totalPayment"$/ doble

Test"ortgageClassTest"ortgageClass

RunRun

"ortgage"ortgage
http://testmortgageclass.htm/http://testmortgageclass.htm/http://var/www/apps/conversion/tmp/scratch_4/TestMortgageClass.bathttp://var/www/apps/conversion/tmp/scratch_4/TestMortgageClass.bathttp://mortgage.htm/http://mortgage.htm/http://var/www/apps/conversion/tmp/scratch_4/TestMortgageClass.bathttp://mortgage.htm/http://mortgage.htm/http://testmortgageclass.htm/http://testmortgageclass.htm/


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34ample 6*F 5he CountClass

CountCount

RunRun

TestCountTestCount

Cont


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Gava AP% and Core Gava classes

&ava.lang

Contains core Gava classes# sch as nmericclasses# strings# and objects* 5his pac9age is

implicitly imported to every Gava program*

&ava.at

Contains classes for graphics*

&ava.applet

Contains classes for spporting applets*


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&ava.ioContains classes #or input an$ outputstreams an$ %les.

&ava.utilContains man& utilities' such as $ate.

&ava.netContains classes #or supportingnet(or) communications.

Gava AP% and Core Gava classes#

cont*


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&ava.at.i%age

Contains classes for managing bitmap images*

&ava.at.peer

Platform

classes and objects java

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