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I am sorry if this is a duplicate or too elementary, but how do I make a singleton class that can be subclassed?
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Unlike static classes, Singleton classes can be inherited, can have base class, can be serialized and can implement interfaces.
Subclassing a Singleton class may be tricky, since a subclass object cannot be created unless the superclass object has not yet been created. You may want to extend the Singleton class to allow not just a single instance, but some small fixed maximum number of instances.
A singleton class has a private constructor which is not accessible to any other class inside the same package or outside. Hence it cannot be sub classed from any other class.
Steve Yegge has an amusing article about singletons that mentions subclassing in this quote:
Then there's the subclassing thing. It's almost impossible to subclass a Singleton, and if you manage it, then you shouldn't have been using a Singleton in the first place. You don't even want to go there. I've walked roads that I dare not recount. Just pretend you can't do it, and you'll save yourself amazing amounts of pain.
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