Constructor in Java

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.

    1. Constructor name must be same as its class name
    2. Constructor must have no explicit return type

1) Default Constructor

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

Syntax of default constructor:

    <class_name>(){}

Example of default constructor

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

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 like 0, null etc. depending on the type.

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.

2] 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.

Example of parameterized constructor

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