Introduction to Java Programming - Constructor

8/9/2019 Introduction to Java Programming - Constructor

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ConstructorConstructor


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ObjectivesObjectives

Able to define constructorAble to define constructor

Able to declare constructorAble to declare constructor

Able to create overloadingAble to create overloadingconstructorconstructor


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Purpose and Function ofPurpose and Function of

ConstructorConstructor Constructors have one purpose in life: toConstructors have one purpose in life: to

create an instance of a class. This cancreate an instance of a class. This canalso be called creating an object, as in:also be called creating an object, as in:

University unikl = new University();University unikl = new University();

The purpose of methods, by contrast, isThe purpose of methods, by contrast, ismuch more general.much more general.

A method's basic function isA method's basic function is to executeto executeJava codes.Java codes.


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Signature differencesSignature differences

Constructors and methods differ in three aspects of theConstructors and methods differ in three aspects of thesignature: modifiers, return type, and name.signature: modifiers, return type, and name.

Like methods, constructors can have any of the accessLike methods, constructors can have any of the access

modifiers: public, protected, private, or none (oftenmodifiers: public, protected, private, or none (oftencalledcalled packagepackage oror friendlyfriendly). Unlike methods,). Unlike methods,constructors can take only public access modifiers.constructors can take only public access modifiers.

Therefore, constructorsTherefore, constructors cannot be final or static.cannot be final or static.

The return types are very different too. Methods canThe return types are very different too. Methods canhave any valid return type, or no return type, in whichhave any valid return type, or no return type, in whichcase the return type is given as void. Constructors havecase the return type is given as void. Constructors haveNONO return type, not even void.return type, not even void.


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Signature differencesSignature differences

Finally, in terms of the signature, methods andFinally, in terms of the signature, methods andconstructors have different names.constructors have different names.

Constructors have the same name as theirConstructors have the same name as their

class; by convention, methods use namesclass; by convention, methods use namesother than the class name.other than the class name.

If the Java program follows normalIf the Java program follows normalconventions, methods will start with aconventions, methods will start with alowercase letter, constructors with anlowercase letter, constructors with anuppercase letter. Also, constructor names areuppercase letter. Also, constructor names areusually nouns because class names are usuallyusually nouns because class names are usuallynouns; method names usually indicate actions.nouns; method names usually indicate actions.


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Example1: DeclaringExample1: Declaring

ConstructorConstructorpublic class Universitypublic class University{{

private String institute;private String institute;

publicpublicUniversity (String input)University (String input)

{{ institute = input;institute = input;}}

}}

public class UniversityTestpublic class UniversityTest

{{ public static void main(String args[ ])public static void main(String args[ ])

{{University unikl = new University (miit");University unikl = new University (miit");

}}

}}

Constructor name

Receiving parameter

Store value of input into name

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ConstructorConstructorpublic class Transportationpublic class Transportation

{{private String name;private String name;

private int year;private int year;

private String model;private String model;

public Transportation( )public Transportation( )

{{name = Pesona";name = Pesona";

year = 2007;year = 2007;

model = Sedan1.4";model = Sedan1.4";

}}

public void DisplayInfo( )public void DisplayInfo( ){{

System.out.println("The default car name is "+name);System.out.println("The default car name is "+name);

System.out.println("The default car year is "+year);System.out.println("The default car year is "+year);

System.out.println("The default car model is "+model);System.out.println("The default car model is "+model);

}}

}}

Constructor name

No parameter

Store default value into data member

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ConstructorConstructor

public class TransportationTestpublic class TransportationTest

{{

public static void main(String[ ] args)public static void main(String[ ] args){{

Transportation car = new Transportation();Transportation car = new Transportation();

car.DisplayInfo( );car.DisplayInfo( );

}}

}}

Create an instance called car

Call DisplayInfo method


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OutputOutput

The following will be display ifThe following will be display if

you compile and run theyou compile and run the

program.program.


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ConstructorConstructorpublic class Transportation{

String name;int year;String model;

public Transportation (String CarName, int CarYear, String CarModel)

{name=CarName;year=CarYear;model=CarModel

}

public void DisplayInfo( ){

System.out.println("The default car name is "+name);System.out.println("The default car year is "+year);System.out.println("The default car model is "+model);

}

}


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ConstructorConstructorpublic class TransportationTestpublic class TransportationTest

{{

public static void main(String[ ] args)public static void main(String[ ] args)

{{

Transportation car = new Transportation(Perdana,2002,DANA1.6);Transportation car = new Transportation(Perdana,2002,DANA1.6);

car.DisplayInfo( );car.DisplayInfo( );

}}

}}


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ConstructorConstructorThe following will be displayed ifThe following will be displayed if

you compile and run theyou compile and run the

program.program.


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Check PointCheck Point

Class Bicycle has 3 data members as String id,Class Bicycle has 3 data members as String id,

String ownerName and int yearBuilt.String ownerName and int yearBuilt. Create aCreate a constructor without parameterconstructor without parameter (default(default

constructor) that will assigned a default value ofconstructor) that will assigned a default value of

001,Malek and 2006 when an instance of Bike1 is001,Malek and 2006 when an instance of Bike1 iscreated.created.

Create aCreate a constructor with parameterconstructor with parameter that will passthat will pass

default value of 001,Malek and 2006 when andefault value of 001,Malek and 2006 when an

instance of Bike1 is created through parameter.instance of Bike1 is created through parameter.

Create the test class too !. Compile and execute in yourCreate the test class too !. Compile and execute in yourvirtual compilervirtual compiler


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Write your answerWrite your answer


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Overloaded ConstructorOverloaded Constructor

Constructor has exactly the same nameConstructor has exactly the same nameas the defining class.as the defining class.

Therefore, if you provide two or moreTherefore, if you provide two or moreconstructors for the same class, theyconstructors for the same class, theyare overloaded by definition.are overloaded by definition.

Those overloaded constructors used inThose overloaded constructors used inthe same program MUST have differentthe same program MUST have differentargument lists so that compiler canargument lists so that compiler can

differentiate them.differentiate them.


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Example4: OverloadedExample4: Overloaded

ConstructorConstructorpublic classpublic class NumberNumber

{{ private doubleprivate double aa;;

private Stringprivate String bb;;

public Number ( )public Number ( ) // default constructor// default constructor

{{ aa = 0.0;= 0.0; bb = Hello;= Hello;

}}

public Number ( double m, String n )public Number ( double m, String n ) // constructor// constructor

with 2 argumentswith 2 arguments{{ aa == mm;;

bb == nn;;

}}

}}


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Example4: OverloadedExample4: Overloaded

ConstructorConstructorpublic classpublic class TestNumberTestNumber

{{

public static void main (String[ ] args)public static void main (String[ ] args)

{{

NumberNumber myObj1 =myObj1 = newnew NumberNumber (100.0,Welcome );(100.0,Welcome );

////call constructor with argumentscall constructor with arguments

NumberNumber myObj2myObj2 = new= new NumberNumber( );( ); // call default// call default

constructorconstructor}}

}} // end class// end class TestNumberTestNumber


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::Check point::::Check point::public classpublic class VV

{ private double{ private double

aa

;;

private doubleprivate double bb;;

public V ( )public V ( ){{ aa = 0.0;= 0.0;

bb = 0.0;= 0.0;

}}

public V ( double m, double n )public V ( double m, double n )

{{ aa == mm;;

bb == nn

}}

public void updateValues()public void updateValues()

{ a = a + 5;{ a = a + 5;

b = b +10;b = b +10;

}}

public void display( )public void display( )

{{

System.out.println( a + b );System.out.println( a + b );

System.out.println( b - a );System.out.println( b - a );

}}

}} // end class V// end class V

public class TestVpublic class TestV

{{

public static void main (String[ ]public static void main (String[ ]

args)args)

{{ VV xx = new V(100.0,= new V(100.0,

200.0 );200.0 );xx.updateValues();.updateValues();

xx.display();.display();

VV yy = new V( );= new V( );

yy.updateValues();.updateValues();

yy. display( );. display( );

}}

}} // end class TestV// end class TestV

What is the output for the above prograWhat is the output for the above progra


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ExerciseExerciseCreate a class ShortCourse. ShortCourse class has 4 private dataCreate a class ShortCourse. ShortCourse class has 4 private data

members, courseTitle, courseId, courseFee and courseDuration.members, courseTitle, courseId, courseFee and courseDuration.

A public method, updatedFee() will calculate theA public method, updatedFee() will calculate the

new fees of specific courses based on the following discount ratenew fees of specific courses based on the following discount rateshown below:shown below:

courseIdcourseId Discount rateDiscount rate

ICT1010ICT1010 25%25%

ICT1056ICT1056 10%10%

OthersOthers 8%8%

Another public method, displayCourseDetails() will display allAnother public method, displayCourseDetails() will display allinformation about the course.information about the course.

If the new fees is greater than RM100, acknowledge the user thatIf the new fees is greater than RM100, acknowledge the user thatthey will be given a free one-week workshop.they will be given a free one-week workshop.

Create your test class called ShortCourseTest. In this class, write aCreate your test class called ShortCourseTest. In this class, write amain() method and call updatedFee() to calculate new fees andmain() method and call updatedFee() to calculate new fees anddisplay course details by calling displayCourseDetails() .display course details by calling displayCourseDetails() .

You are to produce a program using :You are to produce a program using :

default constructor(coded)default constructor(coded)

constructor with argumentsconstructor with arguments


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Extra ReadingExtra Reading

http://java.sun.com/docs/books/tutorihttp://java.sun.com/docs/books/tutori

al/java/javaOO/constructors.htmlal/java/javaOO/constructors.html

Documents

Introduction to Java Programming - Constructor