Q) What is a “singleton class” in java? How to make a java class a singleton?

A)

->only one instance of a java class is available throughout the application, that class is known as singleton.

Ex:

//singleton.java

class productDAO

{

private static productDAO dao=new productDAO();------2

privateproductDAO()---------------------------------1

       {

              System.out.println("productDAO class object is created");

       }

public static productDAO getproductDAO()-------------3

       {

              return dao;

       }

}

class singleton {

       public static void main(String[] args) {

              productDAO b1=productDAO.getproductDAO();

              System.out.println(01);

              productDAO b2= productDAO.getproductDAO();

              System.out.println(02);

       }

}

Output:
productDAO class object is created

1

2

Note:

In the above example, productDAO is made singleton. We did the fallowing to make a class singleton.

1)      private accessibility made is given to the contributor of the class.

2)      Created object of the class with in the class itself, its reference is made static.

3)      Through a user defined static method, the object reference is given to the callers.

Additional information:

->whenever constructor declared private that class object can create inside that class only. Any no. of times we can create inside that class, but not in outside of the class.

->so, when the constructor is declared private we prevent the outside object creation of that class.

->once the constructor is declared as private then that class object can’t create outside of that class.

->singleton class should not contain more than one object. I.e. in entire project only one object is created, but anywhere this object reference is used.