Should the Singleton instance be a final variable?

[Eric Kizaki]

I am doing coding exercises on Bert Bates's OCP Java SE 6 Programmer Practice Exams book. One of the questions is to write a singleton class (pg. 150).
Should the singleton instance be a final variable?

/*
5.e write a singleton class (pg.150)

DOES THE SINGLETON HAVE TO BE FINAL?  IS THERE A WAY TO SET THAT INSTANCE ON THE HEAP TO NULL?
NOT THAT I CAN THINK OF unless the singleton class itself kills itself or is GC.
*/

class Singleton{

	//should it be final? not in this example!!
	private static Singleton oneInstance;

	int num;

	private Singleton(){
		System.out.println("Making new singleton");
		num = 5;
	}

	public static synchronized Singleton getInstance(){
		if(oneInstance == null){
			
			oneInstance = new Singleton();
		}
		return oneInstance;
	}

	public static void killYourself(){
		oneInstance = null;
	}
}

public class SingletonTest{

	public static void main(String[] args){
		Singleton one = Singleton.getInstance();
		one.num = 6;
		System.out.println(one.num);  //6

		one = null;
		Singleton two = Singleton.getInstance();
		System.out.println(two.num); //6!

		two.num = 9;
		one = Singleton.getInstance();
		System.out.println(one.num + " " + two.num);//9 9

		one = null;
		two = null;

		one = Singleton.getInstance();
		System.out.println(one.num);//9

		//vey contrived example!
		Singleton.killYourself();
		two = Singleton.getInstance();
		System.out.println(two.num); //5!
	}	
}

/*
output:
Making new singleton
6
6
9 9
9
Making new singleton
5
*/

[Paul Clapham]

Did you try changing the declaration for that variable to include "final"? What happened then?

[Eric Kizaki]

I had to rewrite a lot of it. Now I think it won't be changed.

/*
5.e write a singleton class (pg.150)

DOES THE SINGLETON HAVE TO BE FINAL?  IS THERE A WAY TO SET THAT INSTANCE ON THE HEAP TO NULL?
NOT THAT I CAN THINK OF unless the singleton class itself kills itself or is GC.
*/

class Singleton{

	//should it be final? not in this example!!
	private static final Singleton oneInstance = new Singleton();

	int num;

	private Singleton(){
		System.out.println("Making new singleton");
		num = 5;
	}

	public static synchronized Singleton getInstance(){
		//if(oneInstance == null){
			
		//	oneInstance = new Singleton();
		//}
		return oneInstance;
	}

	//public static void killYourself(){
	//	oneInstance = null;
	//}
}

public class SingletonTest{

	public static void main(String[] args){
		Singleton one = Singleton.getInstance();
		one.num = 6;
		System.out.println(one.num);  //6

		one = null;
		Singleton two = Singleton.getInstance();
		System.out.println(two.num); //6!

		two.num = 9;
		one = Singleton.getInstance();
		System.out.println(one.num + " " + two.num);//9 9

		one = null;
		two = null;

		one = Singleton.getInstance();
		System.out.println(one.num);//9

		//vey contrived example!
		//Singleton.killYourself();
		//two = Singleton.getInstance();
		//System.out.println(two.num); //5!
	}	
}

/*
output:
Making new singleton
6
6
9 9
9
*/

[Javin Paul]

Hi Dude,

I don't think there is a need to make single instance as final but while writing Singleton class its worth noting to implement double checking of Singleton instance. e..g

public static Singleton getInstance(){
  if (instance == null)  {
    synchronized(Singleton.class) {  
      if (instance == null)          
        instance = new Singleton(); 
    }
  }
  return instance;
}

Thanks
Javin

[Vijitha Kumara]

And welcome to CodeRanch, Eric Kizaki