Why aren't static methods considered good OO practice?
I'm reading Programming Scala. At the beginning of chapter 4, the author comments that Java supports static methods, which are "not-so-pure OO concepts." Why is this so?
Answer
Object-orientation is about three things:
- messaging,
- local retention and protection and hiding of state-process, and
- extreme late-binding of all things.
Of those three, the most important one is messaging.
Static methods violate at least messaging and late-binding.
The idea of messaging means that in OO, computation is performed by networks of self-contained objects which send messages to each other. Sending a message is the only way of communication/computation.
Static methods don't do that. They aren't associated with any object. They really aren't methods at all, according to the usual definition. They are really just procedures. There's pretty much no difference between a Java static method Foo.bar and a BASIC subroutine FOO_BAR.
As for late-binding: a more modern name for that is dynamic dispatch. Static methods violate that, too, in fact, it's even in their very name: static methods.
Static methods break some very nice properties of object-orientation. For example, object-oriented systems are automatically capability-safe with objects acting as capabilities. Static methods (or really any statics, be that static state or static methods) break that property.
You can also execute every object in parallel in its own process, since they only communicate via messaging, thus providing some trivial concurrency. (Like Actors, basically, which shouldn't be too surprising, since Carl Hewitt created the Actor Model based on Smalltalk-71, and Alan Kay created Smalltalk-71 partially based on PLANNER, which in turn was created by Carl Hewitt. The close relationship between actors and objects is far from coincidental, in fact, they are essentially one and the same.) Again, statics (both static methods, and especially static state) break that nice property.