Constructor

Constructor

Constructor is a special type of method that is used to initialize the state of an object. Constructor is invoked at the time of object creation. It constructs the values i.e. data for the object that is why it is known as constructor.
Constructor is just like method but it does not have any explicit return type.

Rules for creating constructor
There are basically two rules defined for the constructor.
      1. Constructor name must be same as its class name
      2. Constructor must have no explicit return type

Types of constructors
  There are two types of constructors:
       1. default constructor (no-arg constructor)
       2. parameterized constructor



1) Default Constructor
              A constructor that have no parameter is known as default constructor.

Syntax of default constructor:
<class_name>(){}

Example of default constructorIn this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of
object creation.
 
//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


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

 
Que)What is the purpose of default constructor?Default constructor provides the default values to the object.
Example of default constructor that displays the default values
//example of default values provided by constructor
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 constructorA constructor that have parameter is known as parameterized constructor.

Why use parameterized constructor?Parameterized constructor is used to provide different values to the distinct objects.
 
Example of parameterized constructorIn 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 OverloadingConstructor 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
//Program 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.


Constructor

• Constructor is used to initialize the state of an object.
• Constructor must not have return type.
• Constructor is invoked implicitly.
• The java compiler provides a default constructor if you don't have any constructor. 
• Constructor name must be same as the class name.

Method

• Method is used to expose behaviour of an object.Method must have return type.
• Method is invoked explicitly.
• Method is not provided by compiler in any case. 
  
• Method name may or may not be same as class name.

Que)Does constructor return any value?Ans:yes,that is current class instance (You cannot use return type yet it returns a value).


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.
 
//Program of Copying the values of one object to anotherclass 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 constructorWe 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.
 
//Anoter program of Copying the values of one object to anotherclass Student{
int id;
String name;
Student(int i,String n){
id = i;
name = n;
}
void display(){System.out.println(id+" "+name+" "+age);}
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


Can constructor perform other tasks instead of initialization?Yes, like object creation, starting a thread, calling method etc. You can perform any operation in the
constructor as you perform in the method.