by Miško Hevery Guide to Testability , wrote to telling me there is nothing wrong with static methods. After all what can be easier to test than Math.abs() ! And Math.abs() is static method! If abs() was on instance method, one would have to instantiate the object first, and that may prove to be a problem. (See how to think about the new operator , and class does real work )wire the dependencies with mocks/friendlies which replace the real dependencies. With procedural programing there is nothing to "wire" since there are no objects, the code and data are separate.main() method it will be harder and harder to set up the state in your test and harder to assert things. Many things will become impossible to assert. Your tests will get progressively larger. Once you reach the main() method you no longer have a unit-test (as your unit is the whole application) you now have a scenario test. Imagine that the application you are trying to test is a word processor. There is not much you can assert from the main method. global state is bad and how it makes your application hard to understand . If your application has no global state than all of the input for your static method must come from its arguments. Chances are very good that you can move the method as an instance method to one of the method's arguments. (As in method(a,b) becomes a.method(b) .) Once you move it you realized that that is where the method should have been to begin with. The use of static methods becomes even worse problem when the static methods start accessing the global state of the application. What about methods which take no arguments? Well, either methodX() returns a constant in which case there is nothing to test; it accesses global state, which is bad; or it is a factory.new operator is called we can not override the method with a sub-class. A caller of such a static factory is permanently bound to the concrete classes which the static factory method produced. In other words the damage of the static method is far beyond the static method itself. Butting object graph wiring and construction code into static method is extra bad, since object graph wiring is how we isolate things for testing.Math.abs(-5) goes, I think Java got it wrong. I really want to write -5.abs() . Ruby got that one right.
Misko,
ReplyDeleteThis is a great writeup. I've been looking for a good explanation of why statics almost always lead to "badness".
The slow evolution of "simple statics" to "bad statics" is hard for people to spot. It's sort of like a glacier moving. But eventually you blink and take a fresh look and realize a LOT has changed.
Thanks!
Patrick
too bad x.x
ReplyDeleteI thought 'twas a satiric moment. (indeed it was the best one)
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ReplyDelete"If your application has no global state than all of the input for your static method must come from its arguments. Chances are very good that you can move the method as an instance method to one of the method's arguments. (As in method(a,b) becomes a.method(b).) Once you move it you realized that that is where the method should have been to begin with."
ReplyDeleteThat's the money quote right there. Excellent point!