How to develop complex methods with TDD

Question

A few weeks ago I started my first project with TDD. Up to now, I have only read one book about it.

My main concern: How to write tests for complex methods/classes. I wrote a class that calculates a binomial distribution. Thus, a method of this class takes n, k, and p as input, and calculates the resp. probability. (In fact it does a bit more, that's why I had to write it myself, but let's stick to this description of the class, for ease of the argument.)

What I did to test this method is: copying some tables with different n I found in the web into my code, picking randomly an entry in this table, feeded the resp. values for n, k, and p into my function, and looked whether the result was near the value in the table. I repeat this a number of times for every table.

This all works well now, but after writing the test, I had to tank for a few hours to really code the functionality. From reading the book, I had the impression that I should not code longer than a few minutes, until the test shows green again. What did I do wrong here? Of course I have broken this task down in a lot of methods, but they are all private.

A related question: Was it a bad idea to pick randomly numbers from the table? In case of an error, I will display the random-seed used by this run, so that I can reproduce the bug.

Answer 1

I don't agree with people saying that it's ok to test private code, even if you make them into separate classes. You should test entry points to your application (or your library, if it's a library you're coding). When you test private code, you limit your re-factoring possibilities for later (because refactoring your privates classes mean refactoring your test code, which you should refrain doing). If you end up re-using this private code elsewhere, then sure, create separate classes and test them, but until you do, assume that You Ain't Gonna Need It.

To answer your question, I think that yes, in some cases, it's not a "2 minutes until you go green" situation. In those cases, I think it's ok for the tests to take a long time to go green. But most situations are "2 minutes until you go green" situations. In your case (I don't know squat about binomial distribution), you wrote you have 3 arguments, n, k and p. If you keep k and p constant, is your function any simpler to implement? If yes, you should start by creating tests that always have constant k and p. When your tests pass, introduce a new value for k, and then for p.