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about Singleton Pattern

siva prakash
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Joined: Aug 24, 2004
Posts: 80
hi friends ,


can any tell when ( ie at what situation ) one can use Sigleton Pattern in designing a class , what are its advantages and disavantages



hoping for reply ,


thanks,

bye
siva
Nikhil Vasaikar
Ranch Hand

Joined: Aug 18, 2004
Posts: 56
Hi Siva,

A Singleton is used when you want to make sure that there is only one instance of your class present with a global point of access. The same instance is used to serve diferent requests. Please refer to the example below.



The Output -
Creating Instance...
Single@3ac748
Reusing Instance
Single@3ac748

Notice that the constructor of class Single is private. Also it provides a public static method to create an object of class Single. Singletons can be used for ConcreteFactories which return a family of objects.
The pattern makes it easy to change your mind and allow more than one instance of the Singleton class. Moreover, you can use the same approach to control the number of instances that the application uses. Only the operation that grants access to the Singleton instance i.e the static method used in our example needs to change.
[ August 24, 2004: Message edited by: Nikhil Vasaikar ]
steve souza
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Joined: Jun 26, 2002
Posts: 861
I'd also add that lazy initialization is overused in the singleton pattern and I would opt for initializing the static instance directly unless you have a compelling reason not to.


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Stan James
(instanceof Sidekick)
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Joined: Jan 29, 2003
Posts: 8791
I've just been frustrated by a vendor package with a few static and singleton classes that were not designed for customization. The getInstance method on the class may burn you one day, too. Consider a nice utilitiy package you pick up somewhere with SingletonClass.getInstance(). After a while you decide you'd like to subclass SingletonClass with CustomSingleton and override a couple methods. But many places in your code you have calls to SingletonClass.getInstance(). Ouch.

If Singleton still seems to be your answer, maybe it should implement an interface so clients can be coded:

SingletonInterface x = ConfigurableFactory.getSingleton();

and some configuration file

singletonclass = com.mycompany.myproject.CustomSingleton


A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi
Maulin Vasavada
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Joined: Nov 04, 2001
Posts: 1871
Hi Stan,

Thats exactly what I do.

I create Service Interface, Impl as Singleton and use Factory to specify that ServiceNameX has ImplX or ImplY or something...

Thanks
Maulin
 
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subject: about Singleton Pattern