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Constructor in Java

Introduction

 
In Java, a constructor initializes its object. It seems similar to a method in Java but is completely different.
 

Constructor in Java

 
Constructors are very important in Java. They are used to initialize the object in Java. They are invoked at the time of the creation of the object. Constructors are used to providing the values for the object.
 

Main Purpose of Constructor

 
Creation of the instance of a class is the sole purpose of a constructor in Java.
 

Rules for the Constructor

  • A constructor should always have the same name as of the class.
  • We cannot use static, final or abstract keyword with a constructor. 
  • We cannot override constructors.
  • A constructor cannot have any return type. 
  • Constructor name should start with a capital letter.

Invocation of Constructor

 
Constructors are invoked when the instance of the class is created.
 
Example
  1. package demo;  
  2. public class Demo {  
  3.  public Demo() {  
  4.   System.out.println("Constructor");  
  5.  }  
  6.  public static void main(String[] args) {  
  7.   Demo cr = new Demo();  
  8.  }  
  9. }   
Output
 
constructor invocation
 

Types of Constructors

 
In Java, constructors are of two types.
 
Default Constructor: A default constructor is a constructor that has no parameters.
 
Example
  1. package demo;  
  2. public class Demo {  
  3.  public Demo() {  
  4.   System.out.println("Demo constructor is created");  
  5.  }  
  6.  public static void main(String[] args) {  
  7.   Demo d = new Demo();  
  8.  }  
  9. }  
Output
default constructor
 
Parameterized Constructor: A parameterized constructor is a constructor that has parameters.
 
Example
  1. package demo;  
  2. public class Demo {  
  3.  int rollno;  
  4.  String name;  
  5.  String standard;  
  6.  Demo(int r, String n, String s) {  
  7.   rollno = r;  
  8.   name = n;  
  9.   standard = s;  
  10.  }  
  11.  void show() {  
  12.   System.out.println("rollno- " + rollno + " " + "name- " + name + " " + "standard- " + standard);  
  13.  }  
  14.  public static void main(String[] args) {  
  15.   Demo d = new Demo(11, "sam", "fifth");  
  16.   d.show();  
  17.  }  
  18. }  
Output
 
parameterized constructor

Constructor Overloading

 
We can overload constructors but cannot override them. We can create multiple constructors in a class.
 
Example
  1. package demo;  
  2. public class Demo {  
  3.  int rollno;  
  4.  String name;  
  5.  String standard;  
  6.  Demo(int r, String n, String s) {  
  7.   rollno = r;  
  8.   name = n;  
  9.   standard = s;  
  10.  }  
  11.  Demo(int r, String n) {  
  12.   rollno = r;  
  13.   name = n;  
  14.  }  
  15.  void show() {  
  16.   System.out.println("rollno- " + rollno + " " + "name- " + name + " " + "standard- " + standard);  
  17.  }  
  18.  public static void main(String[] args) {  
  19.   Demo d1 = new Demo(11, "sam", "fifth");  
  20.   Demo d2 = new Demo(12, "ram");  
  21.   d1.show();  
  22.   d2.show();  
  23.  }  
  24. }  
Output
constructor overloading
 
Difference Between Constructor and Method
  • A Constructor's name should always be the same as the name of the class, but that is not a requirement of a method.
  • A Constructor does not have a return type, it even cannot have a void. A method should always have a return type.
  • If we do not create a constructor in Java then the default constructor is automatically created in Java, but in the case of methods, there are no default methods.
  • We cannot use non-access modifiers with constructors, but with methods, we can.
  • Constructors are invoked implicitly, on the other hand, methods are invoked explicitly.

Summary

 
 This article explains constructors in Java and also the difference between constructors and methods in Java.


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