Thursday, January 15, 2009

Google Programmer Explains Why Static Methods Are Bad For Java Unit Testing

This article made me question my frequent use of static utility methods:

http://misko.hevery.com/2008/12/15/static-methods-are-death-to-testability/

Here are a couple of the excerpts I liked from the article:

The basic issue with static methods is they are procedural code. I have no idea how to unit-test procedural code. Unit-testing assumes that I can instantiate a piece of my application in isolation. During the instantiation I 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.

Lets do a mental exercise. Suppose your application has nothing but static methods. (Yes, code like that is possible to write, it is called procedural programming.) Now imagine the call graph of that application. If you try to execute a leaf method, you will have no issue setting up its state, and asserting all of the corner cases. The reason is that a leaf method makes no further calls. As you move further away from the leaves and closer to the root 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.

The Sunburned Surveyor

2 comments:

Larry D. Becker said...

I seems like more of a critique of procedural programming than static methods. No one wants to go back to the bad old days, but the excesses of object oriented programming are just as bad. Everyone has seen cases of "over design" of classes where they attempt to put in every possible feature. I suspect the answer, as usual, lies somewhere in the middle. Static methods have their place.

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