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Java Constructor

what is constructor?


A constructor is special method used to initialize objects.
  • The constructor is called when an object of a class created.
  • constructor is name must match the class name.
  • it cannot have a return type(like void).
  • constructors can not inherited by subclass, but the constructor of the superclass can be invoked from the subclass. 
  • All classes have constructors by default: if you do not create a class constructor yourself, Java creates one for you. 
  • Constructors are called only once at the time of Object creation while method(s) can be called any number of times. 
  • Constructor in Java can not be abstract, final, static, or Synchronized.

Types of constructors in Java 
Default constructor 
Parameterised Constructor 
Copy constructor


 ➤Default constructor 

  • A constructor that has no parameters is known as default the constructor. 
  • if we create a default constructor with no arguments, the compiler does not create a default constructor


➤Parameterised constructor 

  • A constructor that has parameters is known as parameterized constructor. 

 1) Example of parameterised constructor 
 public class A
 {
 int x; 
 public A(int y)
 {
 x = y;
 }
 public static void main(String[] args) 
{ 
A obj = new A(5);
 System.out.println(obj.x);
 }
 } 

 2)Example of parameterised constructor (using "this" keyword)
 public class A 
{ 
 int x; 
 public A(int x) 
{ this.x=x; 
 }
 //constructor 
 public static void main(String[] args) 
{
 A obj = new A(5);
 System.out.println(obj.x);
 }
 }

 ➤Copy constructor 

  •  Like C++, Java also supports a copy constructor. But, unlike C++, Java doesn’t create a default copy constructor if you don’t write your own.

NOTE : 

  •  If constructors are inherited (class is inheriting other classes) then always first parent class's constructors invoke first one by one then child classes. 
  • Java does not call a parameterized constructor implicitly, you need to explicitly invoke parameterised constructor.
  • In Java, when you create an object of a child class, the default constructor of the parent class is implicitly called. If the parent class has a parameterized constructor and no default constructor is explicitly defined, you need to ensure that the child class constructor explicitly calls the appropriate parent constructor using the super keyword
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