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Java

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Constructor in Java

Constructor is a special type of method that is used to initialize the object.

Constructor is invoked at the time of object creation. It constructs the values i.e. provides data for the object that is why it is known as constructor.


Rules for creating constructor

There are basically two rules defined for the constructor.

  • Constructor name must be same as its class name
  • Constructor must have no explicit return type

Types of constructors

There are two types of constructors:

  • default constructor (no-arg constructor)
  • parameterized constructor
Nested Class

Fig 1. Types of constructors



Default Constructor

A constructor that have no parameter is known as default constructor.

Syntax of default constructor:

<class_name>(){}

Example of default constructor


class Bike{

Bike(){System.out.println("Bike is created");}

public static void main(String args[]){
Bike b=new Bike();
}
}

Output

Bike is created

Note: If there is no constructor in a class, compiler automatically creates a default constructor.

Nested Class

Fig 2.

Example of default constructor that displays the default values


class Student{
int id;
String name;

void display(){System.out.println(id+" "+name);}

public static void main(String args[]){
Student s1=new Student();
Student s2=new Student();
s1.display();
s2.display();
}
}

Output

0 null
0 null

Explanation: In the above class,you are not creating any constructor so compiler provides you a default constructor.Here 0 and null values are provided by default constructor.



Parameterized constructor

A constructor that have parameters is known as parameterized constructor.

Why use parameterized constructor?

Parameterized constructor is used to provide different values to the distinct objects.

In this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.


class Student{
int id;
String name;

Student(int i,String n){
id = i;
name = n;
}
void display(){System.out.println(id+" "+name);}

public static void main(String args[]){
Student s1 = new Student(111,"Karan");
Student s2 = new Student(222,"Aryan");
s1.display();
s2.display();
}
}

Output

111 Karan
222 Aryan

Constructor Overloading

Constructor overloading is a technique in Java in which a class can have any number of constructors that differ in parameter lists.The compiler differentiates these constructors by taking into account the number of parameters in the list and their type.

Example of Constructor Overloading


class Student{
int id;
String name;
int age;
Student(int i,String n){
id = i;
name = n;
}
Student(int i,String n,int a){
id = i;
name = n;
age=a;
}
void display(){System.out.println(id+" "+name+" "+age);}

public static void main(String args[]){
Student s1 = new Student(111,"Karan");
Student s2 = new Student(222,"Aryan",25);
s1.display();
s2.display();
}
}

Output

111 Karan 0
222 Aryan 25
What is the difference between constructor and method ?

There are many differences between constructors and methods. They are given below.


ConstructorMethod
Constructor is used to initialize the state of an object.Method is used to expose behaviour of an object.
Constructor must not have return type.Method must have return type.
Constructor is invoked implicitly.Method is invoked explicitly.
The java compiler provides a default constructor if you don't have any constructor.Method is not provided by compiler in any case.
Constructor name must be same as the class name. Method name may or may not be same as class name.

Copying the values of one object to another like copy constructor in C++

There are many ways to copy the values of one object into another. They are:

  • By constructor
  • By assigning the values of one object into another
  • By clone() method of Object class

In this example, we are going to copy the values of one object into another using constructor.


class Student{
int id;
String name;
Student(int i,String n){
id = i;
name = n;
}

Student(Student s){
id = s.id;
name =s.name;
}
void display(){System.out.println(id+" "+name);}

public static void main(String args[]){
Student s1 = new Student(111,"Karan");
Student s2 = new Student(s1);
s1.display();
s2.display();
}
}

Output

111 Karan
111 Karan

Copying the values of one object to another without constructor

We can copy the values of one object into another by assigning the objects values to another object. In this case, there is no need to create the constructor.


class Student{
int id;
String name;
Student(int i,String n){
id = i;
name = n;
}
Student(){}
void display(){System.out.println(id+" "+name);}

public static void main(String args[]){
Student s1 = new Student(111,"Karan");
Student s2 = new Student();
s2.id=s1.id;
s2.name=s1.name;
s1.display();
s2.display();
}
}

Output

111 Karan
111 Karan


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