When should you use the singleton pattern instead of a static class? [closed]

Answer 1

Use the singleton pattern when you need to compute something at runtime that you would compute at compile time if you could, like lookup tables.

Answer 2

Singletons are more flexible, which can be useful in cases where you want the Instance method to return different concrete subclasses of the Singleton's type based on some context.

Answer 3

I think a place where Singleton will make more sense than the static class is when you have to construct a pool of costly resources (like database connections). You would not be interested in creating the pool if noone ever uses them (static class will means that you do the costly work when class is loaded).

Answer 4

A singleton is also a good idea if you want to force efficient caching of data. for example, I have a class that looks up definitions in an xml document. Since parsing the document can take a while, I set up a cache of definitions (I use SoftReferences to avoid outOfmemeoryErrors). If the desired definition is not in the cache I do the expensive xml parsing. Otherwise I return a copy from the cache. Since having multiple caches would mean I still might have to load the same definition multiple times, I need to have a static cache. I choose to implement this class as a singleton so that I could write the class using only normal (non-static) data members. This allows me to still create an istantiation of the class should I need it for some reason (serialization, unit testing, etc.)