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Constructors

When you create a new instance (a new

object) of a class using the new keyword,

a constructorfor that class is called. Constructors are similar to methods, but

with some important differences.

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Constructor name is class name. Aconstructors must have the same name as

the class its in. Default constructor. If you don't define a

constructor for a class, a defaultparameterless constructoris automaticallycreated by the compiler. The defaultconstructor calls the default parentconstructor (super()) and initializes allinstance variables to default value (zerofor numeric types, null for objectreferences, and false for booleans).

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this(...) - Calls another constructor in same

class. Often a constructor with few parameters

will call a constructor with more parameters,giving default values for the missing parameters.

Use this to call other constructors in the same

class.

super(...). Use superto call a constructor in aparent class. Calling the constructor for the

superclass must be the first statementin the

body of a constructor. If you are satisfied with

the default constructor in the superclass, there isno need to make a call to it because it will be

supplied automatically.

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Box class with Constructor

class Box

{double width;double height;double depth;

Box() {System.out.println("Costructing a Box");width=3;heigth=5;depth=6;

}

double volume(){return width*height*depth;

}}

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The usage of the Constructor

class BoxTest {public static void main(String arg[]) {Box b1=new Box();Box b2=new Box();double vol;

vol=b1.volume();System.out.println("Volume is:"+vol);

vol=b2.volume();

System.out.println("Volume is:"+vol);}}

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parameterized Constructorclass Box{double width;double height;double depth;

Box(double w,double h,double d) {width=w;heigth=h;depth=d;}

double volume(){return width*height*depth;

}}

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How to use parameterized Constructor

class BoxTest {public static void main(String arg[]) {Box b1=new Box(2,5,9);Box b2=new Box(7,10,12);double vol;

vol=b1.volume();System.out.println("Volume is:"+vol);

vol=b2.volume();System.out.println("Volume is:"+vol);}}

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Sub classing and Inheritance

Class Parent

{ }

Class Child extends Parent

{ }

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Method Overloading

class Parent

{void calc(){System.out.println("This is

Addition");}

int calc(int x){return x/2;}

Void calc(int x, int y) {System.out.println("This is

multiplying");}

}

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Method Overriding

class Parent{

void calc(){System.out.println("This is parent Addition");

}}

class Child extends Parent {void calc(){System.out.println("This is child Addition");

}public static void main(String a[]){

Parent p=new Parent();Child c=new Child();p.calc();c.calc();

}}

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Overloading Constructor

class Parent{

parent(){System.out.println("This is default

Costructor");}parent(int x){

System.out.println("Costructoroverloaded"+x);}}

class Child extends Parent {

public static void main(String a[]){Parent p1=new Parent();Parent p1=new Parent(10);Child c1=new Child();

}}

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How to invoke parent defaultConstructor by child class

class Parent

{parent(){System.out.println("This is parent Costructor");

}

parent(int x){

System.out.println("parent Costructor with args"+x);}

class Child extends Parent {

Child(int y){

System.out.println("Child Costructor with args"+y);}

public static void main(String a[]){Child c1=new Child(10);

}}

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How to use this() to invoke Constructor

class Parent

{parent(){System.out.println("This is parent Costructor");

}

parent(int x){this();System.out.println("parent Costructor with args"+x);

}

class Child extends Parent {

public static void main(String a[]){

Parent p1=new Parent(10);}

}