Question

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Difference between static class and singleton pattern?

I was wondering,

Would a class such as Java’s Math class, where all methods are static be considered a singleton? Or does a singleton have to have an instance, eg: Math.getInstance().abs(...) to qualify as a singleton?

Thanks

Answer 1

Having just static methods in a class does not qualify it being a Singleton, as you can still make as many instances of that class, if you have a public constructor in it.

For a class to qualify as Singleton, it should have private constructor, so that it can’t be instantiated from outside the class, and have a static factory that returns the same instance everytime invoked.

If you really mean static class, then first of all, you can’t have your top-level class as static. You can only have static nested class, in which case you don’t need to create any instance of that class, but you can and you can create multiple instances and hence it as not Singleton.

Also, the class you mentioned – java.lang.Math, is not a static class. You should see the documentation of that.