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Can we create private constructor in java

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instanceof java

Posts: 576
Nickname: instanceof
Registered: Jan, 2015

instanceof java is a java related one.
Can we create private constructor in java Posted: Feb 25, 2016 2:21 AM
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1.Can a constructor in Java be private?
  • Yes we can declare private constructor in java.
  • If we declare constructor as private we can not able to create object of the class.
  • In singleton design pattern we use this private constructor. 

private constructor in java example program


 2.In what scenarios we will use private constructor in java.

  •  Singleton Design pattern
  • It wont allow class to be sub classed.
  • It wont allow to create object outside the class.
  • If All Constant methods is there in our class we can use private constructor.
  • If all methods are static then we can use private constructor.



 3.What will happen if we extends a class which is having private constructor.

  •  If we try to extend a class which is having private constructor compile time error will come.
  • Implicit super constructor is not visible for default constructor. Must define an explicit constructor

private constructor in java


Singleton Design pattern:


  1. package com.privateConstructorSingleTon;
  2.  
  3. public class SingletonClass {

  5. private static SingletonClass object;
  6.  
  7. private SingletonClass ()
  8. {
  9.         System.out.println("Singleton(): Private constructor invoked");
  10. }
  11.  
  12. public static SingletonClass getInstance()
  13. {
  14.  
  15. if (object == null)
  16. {
  17.  
  18. System.out.println("getInstance(): First time getInstance was called and object created !");
  19. object = new SingletonClass ();
  20.  
  21.  }
  22.  
  23. return object;
  24.  
  25. }
  26.  
  27. }
     

  1. package instanceofjava;
  2.  
  3. public class SingletonObjectDemo {

  5. public static void main(String args[]) {
  6.  
  7.      SingletonClass s1= SingletonClass .getInstance();
  8.      SingletonClass s2= SingletonClass .getInstance();
  9.      System.out.println(s1.hashCode());
  10.      System.out.println(s2.hashCode());
  11.  
  12. }
  13. }

Output:

  1. getInstance(): First time getInstance was called and object created !
  2. Singleton(): Private constructor invoked
  3. 655022016
  4. 655022016

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