Is a static class a singleton?
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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
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.
Static classes in Java are just nested classes which aren't inner classes. (They're not like static classes in C#, for example.) They can still have instance methods, state etc - and there can be multiple instances.
java.lang.Math is not a static class.
And no, a class which never has an instance is not a singleton. The important difference is that a singleton can implement an interface (or even derive from an abstract class) whereas if you never create an instance of a class, any instance methods are pointless.
A class that is applied Singleton Pattern has one or none instance at any time on a JVM. That's why it's called single-ton. Having static or non-static members has no relationship with being singleton or non-singleton.
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