What is Singleton Class ?

Singleton Class

The Singleton's purpose is to control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.

Since there is only one Singleton instance, any instance fields of a Singleton will occur only once per class, just like static fields. Singletons often control access to resources such as database connections or sockets. For example, if you have a license for only one connection for your database or your JDBC driver has trouble with multithreading, the Singleton makes sure that only one connection is made or that only one thread can access the connection at a time.

Implementing Singletons:

The easiest implementation consists of a private constructor and a field to hold its result, and a static accessor method with a name like getInstance().

The private field can be assigned from within a static initializer block or, more simply, using an initializer. The getInstance( ) method (which must be public) then simply returns this instance:

// File Name: Singleton.java

public class Singleton {

   private static Singleton singleton = new Singleton( );

   /* A private Constructor prevents any other class from instantiating.   */

   private Singleton(){ }

      /* Static 'instance' method */

   public static Singleton getInstance( ) {

      return singleton;

   }

   /* Other methods protected by singleton-ness */

   protected static void demoMethod( ) {

      System.out.println("demoMethod for singleton");

   }

}

// File Name: SingletonDemo.java

public class SingletonDemo {

   public static void main(String[] args) {

      Singleton tmp = Singleton.getInstance( );

      tmp.demoMethod( );

   }

}

This would produce following result:

demoMethod for singleton