Random data in Unit Tests?

There's a compromise. Your coworker is actually onto something, but I think he's doing it wrong. I'm not sure that totally random testing is very useful, but it's certainly not invalid.

A program (or unit) specification is a hypothesis that there exists some program that meets it. The program itself is then evidence of that hypothesis. What unit testing ought to be is an attempt to provide counter-evidence to refute that the program works according to the spec.

Now, you can write the unit tests by hand, but it really is a mechanical task. It can be automated. All you have to do is write the spec, and a machine can generate lots and lots of unit tests that try to break your code.

I don't know what language you're using, but see here:

Java http://functionaljava.org/

Scala (or Java) http://github.com/rickynils/scalacheck

Haskell http://www.cs.chalmers.se/~rjmh/QuickCheck/

.NET: http://blogs.msdn.com/dsyme/archive/2008/08/09/fscheck-0-2.aspx

These tools will take your well-formed spec as input and automatically generate as many unit tests as you want, with automatically generated data. They use "shrinking" strategies (which you can tweak) to find the simplest possible test case to break your code and to make sure it covers the edge cases well.

Happy testing!

This kind of testing is called a Monkey test. When done right, it can smoke out bugs from the really dark corners.

To address your concerns about reproducibility: the right way to approach this, is to record the failed test entries, generate a unit test, which probes for the entire family of the specific bug; and include in the unit test the one specific input (from the random data) which caused the initial failure.

There is a half-way house here which has some use, which is to seed your PRNG with a constant. That allows you to generate 'random' data which is repeatable.

Personally I do think there are places where (constant) random data is useful in testing - after you think you've done all your carefully-thought-out corners, using stimuli from a PRNG can sometimes find other things.