Constructor in Java

A special type of method that is used to initialize the object, is known as constructor.
Both the constructor and class name must be same.
They are invoked automatically when an object creates.
Java compiler creates a default constructor for a class when any explicit constructor of that class is not declared.

Types of Constructor

Java provides two types of constructors:
1. Default constructor
2. Parameterized constructor

Default constructor

Default constructor is also known as no-args constructor because it has no parameter.

Example: Sample program for default constructor

class DefaultCon
{
     int length = 25;
     int width = 20;
     DefaultCon()
     {
          System.out.println("Area of rectangle: "+(length*width));
     }
     public static void main(String args[])
     {
          DefaultCon emp = new DefaultCon();
     }
}

Output: Area of rectangle: 500

Parameterized Constructor

A constructor that contains argument list is known as parameterized constructor.

Example: Sample program for parameterized Constructor

class Rectangle
{
     int length;
     int width;
     Rectangle (int a, int b)
     {
          length = a ;
          width = b ;
     }
     int calculateArea()
     {
          return(length*width);
     }
     public static void main(String args[])
     {
          Rectangle rec = new Rectangle (25, 15);
          long area = rec.calculateArea();
          System.out.println("Area of Rectangle: "+area);
     }
}

Output:
Area of Rectangle: 375

Constructor Overloading

Constructor overloading means declaring multiple constructors with different parameter in the similar class.

Example: Sample program for constructor Overloading

class Employee
{
     String name;
     int age;
     String company;
     Employee (String a, int b)
     {
          name = a ;
          age = b ;
     }
     Employee (String a, int b, String c)
     {
          name = a ;
          age = b ;
          company = c;
     }
     Employee (String a, String b)
     {
          name = a ;
          company = b ;
     }
     void display()
    {
          System.out.println(name+"  "+age+"  "+company);
     }
     public static void main(String args[])
     {
          System.out.println("Name Age Company");
          Employee emp1 = new Employee("ABC", 23);
          Employee emp2 = new Employee("XYZ",30,"TutorialRide.com");
          Employee emp3 = new Employee("PQR", "CareerRide Info");
          emp1.display();
          emp2.display();
          emp3.display();
     }
}

Output:
Name  Age  Company
ABC  23  null
XYZ  30  TutorialRide.com
PQR  0  CareerRide Info