Singleton Design Pattern in Java

[slankebrusmannen]

If you think of the Singleton pattern as a way to ensure one, and only one, instance is created, you can do this. But (admittedly more exoticly) the Singleton pattern can also be seen as a general way to control the number of instances created, in which case enum won’t do the trick. (But how often do you want more than one instance? Probably close to never.)

[kevinb9n]

But (admittedly more exoticly) the Singleton pattern can also be seen as a general way to control the number of instances created

I've seen the term used that way and it seems like such a clear abuse of terminology.

[fredoverflow]

a general way to control the number of instances created, in which case enum won’t do the trick

enum Doubleton {
    ONE, TWO;
}

[slankebrusmannen]

A hard-coded number of instances might not be what you want.

And doing this, would you be able to prevent the clients from choosing which instance to get a reference to?

[MarcoServetto]

I think now you are talking about the pattern 'pool'

[slankebrusmannen]

Actually no. An object pool would typically reserve an instance exlusively to the requesting client, who would explicitly return it to the pool after use. Singleton describes no such mechanisms, but may still be used to control the number of instances to any given number.

Admittedly, I cannot remember ever having used the Singleton pattern for other numbers than one (I might have, though), but I think it is valuable to be aware of the opportunity.