In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling the constructor, memory for the object is allocated in the memory.

It is a special type of method which is used to initialize the object.

Every time an object is created using the new() keyword, at least one constructor is called.

It calls a default constructor if there is no constructor available in the class. In such a case, the Java compiler provides a default constructor by default.

There are two types of constructors in Java: no-arg constructor, and parameterized constructor.
Note: It is called a constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because the java compiler creates a default constructor if your class doesn't have any.


Rules for creating Java constructor

There are two rules defined for the constructor.

  1. Constructor name must be the same as its class name
  2. A Constructor must have no explicit return type
  3. A Java constructor cannot be abstract, static, final, and synchronized

Types of Java constructors

There are two types of constructors in Java:

  1. Default constructor (no-arg constructor)
  2. Parameterized constructor

1. Java Default Constructor

A constructor is called a "Default Constructor" when it doesn't have any parameter.

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.

//Java Program to create and call a default constructor  
class Bike1{  
//creating a default constructor  
Bike1(){System.out.println("Bike is created");}  
//main method  
public static void main(String args[]){  
//calling a default constructor  
Bike1 b=new Bike1();  
}  
} 

Java Parameterized Constructor

A constructor which has a specific number of parameters is called a parameterized constructor.

Why use the parameterized constructor?

The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.

Example: -

//Java Program to demonstrate the use of the parameterized constructor.  
class Student4{  
    int id;  
    String name;  
    //creating a parameterized constructor  
    Student4(int i,String n){  
    id = i;  
    name = n;  
    }  
    //method to display the values  
    void display(){System.out.println(id+" "+name);}  
   
    public static void main(String args[]){  
    //creating objects and passing values  
    Student4 s1 = new Student4(111,"Karan");  
    Student4 s2 = new Student4(222,"Aryan");  
    //calling method to display the values of object  
    s1.display();  
    s2.display();  
   }  
} 

Difference between constructor and method in Java

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