Constructors are used to initialize the object’s state. Like methods, a constructor also contains collection of statements(i.e. instructions) that are executed at time of Object creation.

Need of Constructor
Think of a Box. If we talk about a box class then it will have some class variables (say length, breadth, and height). But when it comes to creating its object(i.e Box will now exist in computer’s memory), then can a box be there with no value defined for its dimensions. The answer is no.
So constructors are used to assign values to the class variables at the time of object creation, either explicitly done by the programmer or by Java itself (default constructor).

When is a Constructor called ?
Each time an object is created using new() keyword at least one constructor (it could be default constructor) is invoked to assign initial values to the data members of the same class.

A constructor is invoked at the time of object or instance creation. For Example:

class Prutor
{
.......

// A Constructor
new Prutor() {}

.......
}

// We can create an object of the above class
// using the below statement. This statement
// calls above constructor.
Prutor obj = new Prutor();

Rules for writing Constructor:

• Constructor(s) of a class must have same name as the class name in which it resides.
• A constructor in Java can not be abstract, final, static and Synchronized.
• Access modifiers can be used in constructor declaration to control its access i.e which other class can call the constructor.

Types of constructor

There are two type of constructor in Java:

• No-argument constructor: A constructor that has no parameter is known as default constructor. If we don’t define a constructor in a class, then compiler creates default constructor(with no arguments) for the class. And if we write a constructor with arguments or no-arguments then the compiler does not create a default constructor.
  Default constructor provides the default values to the object like 0, null, etc. depending on the type.

// Java Program to illustrate calling a
// no-argument constructor
import java.io.*;

class Prutor
{
int num;
String name;

// this would be invoked while an object
// of that class is created.
Prutor()
{
System.out.println("Constructor called");
}
}

class P_AI
{
public static void main (String[] args)
{
// this would invoke default constructor.
Prutor prutor1 = new Prutor();

// Default constructor provides the default
// values to the object like 0, null
System.out.println(prutor1.name);
System.out.println(prutor1.num);
}
}
Output :

Constructor called
null
0

• Parameterized Constructor: A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with your own values, then use a parameterized constructor.

// Java Program to illustrate calling of
// parameterized constructor.
import java.io.*;

class Prutor
{
// data members of the class.
String name;
int id;

// constructor would initialize data members
// with the values of passed arguments while
// object of that class created.
Prutor(String name, int id)
{
this.name = name;
this.id = id;
}
}

class P_AI
{
public static void main (String[] args)
{
// this would invoke the parameterized constructor.
Prutor prutor1 = new Prutor("adam", 1);
System.out.println("PrutorName :" + prutor1.name +
" and PrutorId :" + prutor1.id);
}
}
Output:

PrutorName :adam and PrutorId :1
Does constructor return any value?

There are no “return value” statements in constructor, but constructor returns current class instance. We can write ‘return’ inside a constructor.

Constructor Overloading

Like methods, we can overload constructors for creating objects in different ways. Compiler differentiates constructors on the basis of numbers of parameters, types of the parameters and order of the parameters.

// Java Program to illustrate constructor overloading
// using same task (addition operation ) for different
// types of arguments.
import java.io.*;

class Prutor
{
// constructor with one argument
Prutor(String name)
{
System.out.println("Constructor with one " +
"argument - String : " + name);
}

// constructor with two arguments
Prutor(String name, int age)
{

System.out.println("Constructor with two arguments : " +
" String and Integer : " + name + " "+ age);

}

// Constructor with one argument but with different
// type than previous..
Prutor(long id)
{
System.out.println("Constructor with one argument : " +
"Long : " + id);
}
}

class P_AI
{
public static void main(String[] args)
{
// Creating the objects of the class named 'Prutor'
// by passing different arguments

// Invoke the constructor with one argument of
// type 'String'.
Prutor prutor2 = new Prutor("rajeev");

// Invoke the constructor with two arguments
Prutor prutor3 = new Prutor("Suresh", 26);

// Invoke the constructor with one argument of
// type 'Long'.
Prutor prutor4 = new Prutor(436256897);
}
}
Output:

Constructor with one argument - String : rajeev
Constructor with two arguments - String and Integer : Suresh 26
Constructor with one argument - Long : 436256897

How constructors are different from methods in Java?

• Constructor(s) must have the same name as the class within which it defined while it is not necessary for the method in java.
• Constructor(s) do not return any type while method(s) have the return type or void if does not return any value.
  Constructor is called only once at the time of Object creation while method(s) can be called any numbers of time.