In Java,C++,Objective C, C#, and all other similar OOP languages,
Why don't we use a class with static methods and vars instead of going through a singleton syntax having to deal with various scenarios ?
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First of all, let me say that I'm not a fan of either of these ideas -- global variables create a bevvy of problems regardless of whether they come in design pattern packaging or not.
That said, a singleton is (generally) externally instance based with the exception of its static Instance() method, meaning that you can participate to some degree in runtime polymorphism. In other words, a singleton can implement an interface, inherit from some other class, or have another class inherit from it (I seriously don't recommend this last one). Static methods are entirely procedural in nature so you can't do any of these object-oriented things with them.
The main advantage of singleton, as far as I'm concerned, is that it's somewhat easier to factor toward a better design if you're starting with a singleton than if you're starting with pure static classes that house state. With the latter approach, your application architecture is about as flexible as a diamond and with singletons, its as flexible as, say, steel. The singleton is slightly less bad, from a flexibility standpoint, and also slightly easier to fix.
There is another pattern, called monostate, in which all instances share internal static state but expose only instance methods. If you've just gotta have your global variables, this is a nice way to prevent them from creating the hidden dependencies between your classes that tend to make singletons and static classes with state such a maintenance headache.
For me the big difference is that a singleton is far more flexible.
I can use the
If I want to do this with a class that only has static methods I have to refactor the whole application.
BTW I also use sometimes classes with static methods if I'm sure that I don't need the above features.