by Dinesh Thakur Category: Introduction to Java

Initializing a variable is considered very helpful while making programs. We can initialize variables of primitive types at the time of their declarations. For example:

int a = 10;

In Object Oriented Programming language (OOPL) like Java, the need of initialization of fields of a new object is even more common. We have already done this using two approaches.

In the first approach, we used a dot operator to access and assign values to the instance variables individually for each object. However, it can be a tedious job to initialize the instance variables of all the objects individually. Moreover, it does not promotes data hiding.

r1.length = 5;

r2.length = 7;

Where rl,r2 are objects of a Rectangle class.

In another approach, we made use of method setData() to assign values to fields of each object individually. But it would have to be called explicitly for each object. This would become inconvenient if the number of objects are very large.

rl.setData(5,6);//sets length and breadth of rl Rectangle object



The above two approaches does not simulate the problem properly. A better solution to the above problem is to initialize values to the object at the time of its creation in the same way as we initialize values to a variable of primitive data types. This is accomplished using a special method in Java known as constructor that enables an object to initialize itself at the time of its creation without the need to make separate call to the instance method.  

A constructor is a special method that is called whenever an object is created using the new keyword. It contains a block of statements that can be used to initialize instance variables of an object before the reference to this object is returned by new. A constructor does look and feel a lot like a method but it is different from a method generally in two ways.

A constructor always has the same name as the class whose instance members they initialize. The constructor does not have a return type, nor even void. It is because the constructor is automatically called by the compiler whenever an object of a class is created.

The syntax for constructor is as follows.



//Constructor Body


Here, the ConstructorName is same as the class name it belongs to.

The parameterList is the list of optional zero or more parameter(s) that is specified after the classname in parentheses. Each parameter specification, if any, consists of a type and a name and are separated from each other by commas.

Now let us consider a program

// use of Constructor

Class Rectangle


      int length;

      int breath;



             length = 5;

             breath = 6;


      int area()


           Int rectArea = length * breath;

           Return rectArea;



Class Constructordemo


      Public static void main(String args[])


           Rectangle firstRect = new Rectangle();

           System.out.println(“Area of Rectangle = ”+ firstrect.area());



Output: Area of rectangle =30

Explanation : In this program, when the statement

Rectangle firstRect = new Rectangle();


IS executed, the new operator creates a new but uninitialized object of the class. Then the constructor (Rectangle (») is called and the statements in its body are executed. As a result, the instance variables length and breadth of object firstRect will be initialized to integer literals 5 and 6 respectively.

Then the address of the allocated Rectangle object is returned and assigned to the reference variable firstRect. This method of initializing instance variable(s) of an object(s) using constructor is very simple and concise as there is no need to explicitly call the method for each object separately.


There are three types of Constructor as follows:-


1)Default Constructor:-  Default Constructor is also called as Empty Constructor which has no arguments andis Automatically called when we creates the object of class  but Remember name of Constructor is same as name of class.

2)Parameterized Constructor :- This is AnotherConstructor which has some Arguments and same name as class name but it uses some Arguments So For this We have to create object of Class by passing some Arguments at the time of creating object with the name of class.

3)Copy Constructor:- This is also Another type of Constructor. InConstructor object of another Constructor is passed As name Suggests you Copy means Copy values of another Class object This is used for Copying the values of class object into an another object of class So For Calling Copy Constructor We have to pass the name of object whose values we wants to Copying .




1. It is a member function whose name is same as the class. But preceded by a ~ (tilde) symbol.

2. It has no return type.

3. It cannot have parameters.

4. It is implicitly called when object is no longer required.

5. It is used to release the memory and close files and database conversions.

( No Destructor concept in java )


Note :- java does not support destructor. But it is supported by C,C++.

About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.