programming

7/17/2019 programming

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Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. A

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Chapter 9 Objects and Classes


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Motivations

After learning the preceding chapters, you are capable of

solving many programming problems using selections,loops, methods, and arrays. o!ever, these "ava features

are not sufficient for developing graphical user interfaces

and large scale soft!are systems. #uppose you !ant to

develop a graphical user interface as sho!n belo!. o! doyou program it$


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Objectives &o describe objects and classes, and use classes to model objects '(9.2).

&o use *M+ graphical notation to describe classes and objects '(9.2).

&o demonstrate ho! to define classes and create objects '(9.%). &o create objects using constructors '(9.).

&o access objects via object reference variables '(9.-).

&o define a reference variable using a reference type '(9.-.1).

&o access an objects data and methods using the object member access operator '.) '(9.-.2).

&o define data fields of reference types and assign default values for an objects data fields '(9.-.%).

&o distinguish bet!een object reference variables and primitive data type variables '(9.-.).

&o use the "ava library classes Date, Random, and Point2D'(9./).

&o distinguish bet!een instance and static variables and methods '(9.0).

&o define private data fields !ith appropriategetand setmethods '(9.).

&o encapsulate data fields to mae classes easy to maintain '(9.9).

&o develop methods !ith object arguments and differentiate bet!een primitive3type arguments andobject3type arguments '(9.14).

&o store and process objects in arrays '(9.11).

&o create immutable objects from immutable classes to protect the contents of objects '(9.12).

&o determine the scope of variables in the conte5t of a class '(9.1%).

&o use the ey!ord thisto refer to the calling object itself '(9.1).


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OO 6rogramming Concepts

Object3oriented programming 'OO6) involves

programming using objects. An objectrepresentsan entity in the real !orld that can be distinctly

identified. 7or e5ample, a student, a des, a circle,

a button, and even a loan can all be vie!ed asobjects. An object has a uni8ue identity, state, and

behaviors. &hestateof an object consists of a set of

datafields'also no!n asproperties) !ith theircurrent values. &he behaviorof an object is defined

by a set of methods.


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Objects

An object has both a state and behavior. &he state

defines the object, and the behavior defines !hat

the object does.

Class ame: Circle

;ata 7ields:radius is


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Classes

Classesare constructs that define objects of thesame type. A "ava class uses variables to define

data fields and methods to define behaviors.

Additionally, a class provides a special type ofmethods, no!n as constructors, !hich are

invoed to construct objects from the class.


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Classes

class Circle {

/** The radius of this circle */double radius = 1.0;

/** Construct a circle object */

Circle() {

}

/** Construct a circle object */

Circle(double newadius) {

radius = newadius;

}

/** eturn the area of this circle */double !et"rea() {

return radius * radius * #.1$1%&;

}

}

'ata field

ethod

Constructors


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*M+ Class ;iagram

Circle

radius: double

Circle')

Circle'ne!=adius: double)

getArea'): double

circle1: Circle

radius > 1.4

Class name

;ata fields

Constructors and

methods

circle2: Circle

radius > 2-

circle%: Circle

radius > 12-

*M+ Class ;iagram

*M+ notation

for objects


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Example: Defning Classes

and Creating Objects

Objective: ;emonstrate creating objects,

accessing data, and using methods.

TestSimpleCircleTestSimpleCircle RunRun

Animation
http://var/www/apps/conversion/tmp/scratch_1/html%2FTestSimpleCircle.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestSimpleCircle.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestSimpleCircle.bat


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Example: Defning Classes and CreatingObjects

TestTVTestTV RunRun

TVTV

TV

channel int

*olu+e,e*el int

on boolean

-T.()

-turn/n() *oid

-turn/ff() *oid

-setChannel(newChannel int) *oid

-set.olu+e(new.olu+e,e*el int) *oid

-channel01() *oid

-channel'own() *oid

-*olu+e01() *oid

-*olu+e'own() *oid

&he current channel '1 to 124) of this &?.

&he current volume level '1 to 0) of this &?.

@ndicates !hether this &? is onAoff.

Constructs a default &? object.

&urns on this &?.

&urns off this &?.

#ets a ne! channel for this &?.#ets a ne! volume level for this &?.

@ncreases the channel number by 1.

;ecreases the channel number by 1.

@ncreases the volume level by 1.

;ecreases the volume level by 1.

&he B sign indicatesa public modifier.
http://var/www/apps/conversion/tmp/scratch_1/html%2FTestTV.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestTV.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestTV.bat


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Constructors

Circle() {

}

Circle(double newRadius) {

radius = newRadius;

}

Constructors are a specialind of methods that are

invoed to construct objects.


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Constructors, cont.

A constructor !ith no parameters is referred to as ano-arg constructor.

Constructors must have the same name as the

class itself.

Constructors do not have a return typeDnot

even void.

Constructors are invoed using the ne!

operator !hen an object is created. Constructors

play the role of initialiEing objects.


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Creating Objects *sing

Constructors

new ClassName();

F5ample:new Circle();

new Circle(5.0);


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;efault Constructor

A class may be defined !ithout constructors. @nthis case, a no3arg constructor !ith an empty body

is implicitly defined in the class. &his constructor,

called a default constructor, is providedautomatically only if no constructors are explicitly

defined in the class.


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;eclaring Object =eference ?ariables

&o reference an object, assign the object to a referencevariable.

&o declare a reference variable, use the synta5:

ClassName objectRefVar;

F5ample:

Circle mCircle;


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;eclaringCreating Objects

in a #ingle #tepClass2a+e objectefar= new Class2a+e();

F5ample:

Circle +3Circle = new Circle();

Create an objectAssign object reference


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Accessing Objects Members

=eferencing the objects data: objectefar.data

e.g., +3Circle.radius

@nvoing the objects method:

objectefar.+ethod2a+e(ar!u+ents)

e.g., +3Circle.!et"rea()

i ti


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&race Code

Circle myCircle = new Circle(5.0);

Circle yorCircle = new Circle();

yorCircle.radis = !00;

;eclare myCircle

no valuemyCircle

animation

i ti


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&race Code, cont.




: Circle

radius: -.4

no valuemyCircle

Create a circle

animation

i ti


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&race Code, cont.




: Circle

radius: -.4

reference valuemyCircle

Assign objectreference to myCircle

animation

animation


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&race Code, cont.




: Circle

radius: -.4


no valueyourCircle

;eclare yourCircle

animation

animation


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&race Code, cont.




: Circle

radius: -.4


no valueyourCircle

: Circle

radius: 1.4Create a ne!

Circle object

animation

animation


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&race Code, cont.




: Circle

radius: -.4

re"erence #alemyCircle

re"erence #aleyourCircle

: Circle

radius: 1.4

Assign object

reference to yourCircle

animation

animation


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&race Code, cont.




: Circle

radius: -.4


reference valueyourCircle

: Circle

radius: 144.4Change radius in

yourCircle

animation


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Caution

=ecall that you use

Math.methodame'arguments) 'e.g., Math.po!'%, 2.-))

to invoe a method in the Math class. Can you invoe getArea') using#impleCircle.getArea')$ &he ans!er is no. All the methods used before

this chapter are static methods, !hich are defined using the staticey!ord. o!ever, getArea') is non3static. @t must be invoed from anobject using

object=ef?ar.methodame'arguments) 'e.g., myCircle.getArea')).

More e5planations !ill be given in the section on G#tatic ?ariables,Constants, and Methods.H


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=eference ;ata 7ields

&he data fields can be of reference types. 7or e5ample,

the follo!ing #tudent class contains a data field name ofthe #tring type.

!ublic class "tudent {

"trin# name; $$ name %as default &alue null

int a#e; $$ a#e %as default &alue 0

boolean is"cience'ajor; $$ is"cience'ajor %as default &alue false

c%ar #ender; $$ c %as default &alue u0000

}


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&he null ?alue

@f a data field of a reference type does notreference any object, the data field holds a

special literal value, null.


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;efault ?alue for a ;ata 7ield

&he default value of a data field is null for a referencetype, 4 for a numeric type, false for a boolean type,and IJu4444I for a char type. o!ever, "ava assignsno default value to a local variable inside a method.

!ublic class *est {

!ublic static &oid main("trin#+, ar#s) {

"tudent student = new "tudent();

"stem.out.!rintln(-name - / student.name);

"stem.out.!rintln(-a#e - / student.a#e);

"stem.out.!rintln(-is"cience'ajor - / student.is"cience'ajor);

"stem.out.!rintln(-#ender - / student.#ender);

}

}


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F5ample

!ublic class *est {

!ublic static &oid main("trin#+, ar#s) {

int ; $$ %as no default &alue "trin# ; $$ %as no default &alue

"stem.out.!rintln(- is - / );

"stem.out.!rintln(- is - / );

}

}

Compile error: variable not

initialiEed

"ava assigns no default value to a local variable inside a method.

;iff b t ? i bl f


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;ifferences bet!een ?ariables of

6rimitive ;ata &ypes and Object &ypes

16rimitive type int i > 1 i

Object type Circle c c reference

Created using ne! Circle')

c: Circle

radius > 1

C i ? i bl f 6 i i i


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Copying ?ariables of 6rimitive

;ata &ypes and Object &ypes

i

6rimitive type assignment i > j

Kefore:

1

j 2

i

After:

2

j 2

c1

Object type assignment c1 > c2

Kefore:

c2

c1

After:

c2

c1: Circle

radius > -

C2: Circle

radius > 9

c1: Circle

radius > -

C2: Circle

radius > 9


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Larbage Collection

As sho!n in the previous figure, after theassignment statement c1 > c2, c1 points to

the same object referenced by c2. &he object

previously referenced by c1 is no longerreferenced. &his object is no!n as garbage.

Larbage is automatically collected by "?M.


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Larbage Collection, cont

&@6: @f you no! that an object is no longerneeded, you can e5plicitly assign null to a

reference variable for the object. &he "?M

!ill automatically collect the space if theobject is not referenced by any variable.


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&he ;ate Class

"ava provides a system3independent encapsulation of date

and time in the java.util.;ate class. ou can use the ;ateclass to create an instance for the current date and time anduse its to#tring method to return the date and time as a string.

java.util.;ate

B;ate')

B;ate'elapse&ime: long)

Bto#tring'): #tring

Bget&ime'): long

Bset&ime'elapse&ime: long): void

Constructs a ;ate object for the current time.

Constructs a ;ate object for a given time in

milliseconds elapsed since "anuary 1, 1904, LM&.

=eturns a string representing the date and time.

=eturns the number of milliseconds since "anuary 1,

1904, LM&.

#ets a ne! elapse time in the object.

&he B sign indicates

public modifer


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&he ;ate Class F5ample

7or e5ample, the follo!ing code

jaa.util.'ate date = new jaa.util.'ate();

43ste+.out.rintln(date.to4trin!());

displays a string lie4un ar 0& 1#%01&

54T 600#.


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&he =andom Class

ou have used Math.random') to obtain a random double

value bet!een 4.4 and 1.4 'e5cluding 1.4). A more usefulrandom number generator is provided in the java.util.=andomclass.

java.util.=andomB=andom')

B=andom'seed: long)

Bne5t@nt'): int

Bne5t@nt'n: int): int

Bne5t+ong'): long

Bne5t;ouble'): double

Bne5t7loat'): float

Bne5tKoolean'): boolean

Constructs a =andom object !ith the current time as its seed.

Constructs a =andom object !ith a specified seed.

=eturns a random int value.

=eturns a random int value bet!een 4 and n 'e5clusive).

=eturns a random long value.

=eturns a random double value bet!een 4.4 and 1.4 'e5clusive).

=eturns a random float value bet!een 4.47 and 1.47 'e5clusive).

=eturns a random boolean value.


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&he =andom Class F5ample@f t!o =andom objects have the same seed, they !ill generate

identical se8uences of numbers. 7or e5ample, the follo!ing codecreates t!o =andom objects !ith the same seed %.

Random random1 = new Random(2);

"stem.out.!rint(-3rom random14 -);

for (int i = 0; i 10; i//) "stem.out.!rint(random1.net6nt(1000) / - -);

Random random7 = new Random(2);

"stem.out.!rint(-n3rom random74 -);

for (int i = 0; i 10; i//)

"stem.out.!rint(random7.net6nt(1000) / - -);

7rom random1: 0% //4 214 -1 12 242 -9 -/ -9 9/1

7rom random2: 0% //4 214 -1 12 242 -9 -/ -9 9/1


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&he Point2DClass

"ava A6@ has a conveninent Point2Dclass in the

$a#a"%.geometrypacage for representing a point in a t!o3dimensional plane.

PointDPointD RunRun

@ t
http://var/www/apps/conversion/tmp/scratch_1/html%2FTestPoint2D.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestPoint2D.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestPoint2D.bat


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@nstance

?ariables, and Methods

@nstance variables belong to a specific instance.

@nstance methods are invoed by an instance of

the class.

#t ti ? i bl C t t


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#tatic ?ariables, Constants,

and Methods

#tatic variables are shared by all the instances of the

class.

#tatic methods are not tied to a specific object.

#tatic constants are final variables shared by all the

instances of the class.

#t ti ? i bl C t t


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and Methods, cont.

&o declare static variables, constants, and methods,

use the static modifier.

#t ti ? i bl C t t


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and Methods, cont.

F5ample of


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F5ample of

*sing @nstance and Class ?ariables

and Method

Objective: ;emonstrate the roles of

instance and class variables and theiruses. &his e5ample adds a class variablenumberOfObjects to trac the number ofCircle objects created.

TestCircle!it"Static#embersTestCircle!it"Static#embers RunRun

Circle!it"Static#embersCircle!it"Static#embers

?isibility Modifiers and
http://var/www/apps/conversion/tmp/scratch_1/html%2FTestCircleWithStaticMembers.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestCircleWithStaticMembers.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestCircleWithStaticMembers.bat


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?isibility Modifiers and

AccessorMutator Methods

Ky default, the class, variable, or method can beaccessed by any class in the same pacage.

ublic

&he class, data, or method is visible to any class in anypacage.

riate

&he data or methods can be accessed only by the declaringclass.

&he get and set methods are used to read and modify private

properties.


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&he private modifier restricts access to !ithin a class, the default

modifier restricts access to !ithin a pacage, and the public

modifier enables unrestricted access.


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O&F

An object cannot access its private members, as sho!n in 'b).@t is ON, ho!ever, if the object is declared in its o!n class, as

sho!n in 'a).

hy ;ata 7ields #hould Ke


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hy ;ata 7ields #hould Ke

private$

&o protect data.

&o mae code easy to maintain.

F ample of


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F5ample of

;ata 7ield Fncapsulation

Circle!it"Pri$ateData%ieldsCircle!it"Pri$ateData%ields

RunRunTestCircle!it"Pri$ateData%ieldsTestCircle!it"Pri$ateData%ields

Circle

3radius: double

3numberOfObjects: int

BCircle')

BCircle'radius: double)

Bget=adius'): double

Bset=adius'radius: double): void

BgetumberOfObjects'): int

BgetArea'): double

&he radius of this circle 'default: 1.4).

&he number of circle objects created.

Constructs a default circle object.

Constructs a circle object !ith the specified radius.

=eturns the radius of this circle.

#ets a ne! radius for this circle.

=eturns the number of circle objects created.

=eturns the area of this circle.

&he 3 sign indicates

private modifier
http://var/www/apps/conversion/tmp/scratch_1/html%2FTestCircleWithPrivateDataFields.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestCircleWithPrivateDataFields.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestCircleWithPrivateDataFields.bat


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6assing Objects to Methods

6assing by value for primitive type value

'the value is passed to the parameter)

6assing by value for reference type value'the value is the reference to the object)

TestPassObjectTestPassObject RunRun

6 i Obj M h d
http://var/www/apps/conversion/tmp/scratch_1/html%2FTestPassObject.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestPassObject.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTestPassObject.bat


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6assing Objects to Methods, cont.


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Array of Objects

Circle78 circle"rra3 = new Circle7108;

An array of objects is actually an array of

reference variables. #o invoingcircleArrayP1Q.getArea') involves t!olevels of referencing as sho!n in the ne5t

figure. circleArray references to the entirearray. circleArrayP1Q references to aCircle object.


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Array of Objects, cont.

Circle78 circle"rra3 = new Circle7108;


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Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Arights reserved.

-%

Array of Objects, cont.

#ummariEing the areas of the circles

TotalAreaTotalArea RunRun

@ bl Obj d Cl
http://var/www/apps/conversion/tmp/scratch_1/html%2FTotalArea.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTotalArea.bathttp://var/www/apps/conversion/tmp/scratch_1/html%2FTotalArea.bat


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@mmutable Objects and Classes

@f the contents of an object cannot be changed once the object is

created, the object is called an immutable objectand its class is

called an immutable class. @f you delete the set method in the

Circle class in +isting .14, the class !ould be immutable because

radius is private and cannot be changed !ithout a set method.

A class !ith all private data fields and !ithout mutators is not

necessarily immutable. 7or e5ample, the follo!ing class #tudent

has all private data fields and no mutators, but it is mutable.

F5ample bli l 8i t%9 t {


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--

F5ample!ublic class "tudent {

!ri&ate int id;

!ri&ate 8irt%9ate birt%9ate;

!ublic "tudent(int ssn:

int ear: int mont%: int da) {

id = ssn;

birt%9ate = new 8irt%9ate(ear: mont%: da);

}

!ublic int #et6d() {

return id;

}

!ublic 8irt%9ate #et8irt%9ate() {

return birt%9ate;

}

}

!ublic class 8irt%9ate {

!ri&ate int ear;

!ri&ate int mont%;

!ri&ate int da;

!ublic 8irt%9ate(int newear:int new'ont%: int new9a) {

ear = newear;

mont% = new'ont%;

da = new9a;

}

!ublic &oid setear(int newear) { ear = newear;

}

}

!ublic class *est {

!ublic static &oid main("trin#+, ar#s) { "tudent student = new "tudent(111772222: 1

}

}

h Cl i @ bl $


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hat Class is @mmutable$

7or a class to be immutable, it must mar all data fields privateand provide no mutator methods and no accessor methods that

!ould return a reference to a mutable data field object.


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#cope of ?ariables

&he scope of instance and static variables is the

entire class. &hey can be declared any!here inside

a class.

&he scope of a local variable starts from its

declaration and continues to the end of the bloc

that contains the variable. A local variable must be

initialiEed e5plicitly before it can be used.


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&he this Ney!ord

&he this ey!ord is the name of a reference thatrefers to an object itself. One common use of the

this ey!ord is reference a classs hidden data

fields.

Another common use of the this ey!ord to

enable a constructor to invoe another

constructor of the same class.


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=eference the idden ;ata 7ields

!ublic class9 {!ri&ate inti = %;

!ri&ate static double: = 0;

&oidset(inti) {

this.i = i;

}

static &oidset where thisrefers f1

n*o:in! f6.set($%) is to eecute

t%is.i = $%> where thisrefers f6

C lli O l d d C


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Calling Overloaded Constructor

ublic class Circle {riate double radius;

ublic Circle(double radius) {

t%is.radius = radius;

}

ublic Circle() {

t%is(1.0);

}

ublic double !et"rea() {return t%is.radius * t%is.radius * ath.?;

}

} Fvery instance variable belongs to an instance represented by this,!hich is normally omitted

this must be e5plicitly used to reference the data

field radius of the object being constructed

this is used to invoe another constructor

programming

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