language agnostic - Is using a lot of static methods a bad thing?

Question

I tend to declare as static all the methods in a class when that class doesn't require to keep track of internal states. For example, if I need to transform A into B and don't rely on some internal state C that may vary, I create a static transform. If there is an internal state C that I want to be able to adjust, then I add a constructor to set C and don't use a static transform.

I read various recommendations (including on StackOverflow) NOT to overuse static methods but I still fail to understand what it wrong with the rule of thumb above.

Is that a reasonable approach or not?

Answer 1

There are two kinds of common static methods:

• A "safe" static method will always give the same output for the same inputs. It modifies no globals and doesn't call any "unsafe" static methods of any class. Essentially, you are using a limited sort of functional programming -- don't be afraid of these, they're fine.
• An "unsafe" static method mutates global state, or proxies to a global object, or some other non-testable behavior. These are throwbacks to procedural programming and should be refactored if at all possible.

There are a few common uses of "unsafe" statics -- for example, in the Singleton pattern -- but be aware that despite any pretty names you call them, you're just mutating global variables. Think carefully before using unsafe statics.