Much of the time, the test-driven development yahoogroup is pretty quiet, but it has recently awakened from winter hibernation.  The question “Is it OK to add code to a class only to improve its testability?” stirred up a wide-ranging discussion that brought in the topic of what constitutes good design.  “Uncle Bob” Martin drew a bold line in the sand with his comment,

One reasonable definition of good design is testability.  It is hard to imagine a software system that is both testable and poorly designed.  It is also hard to imagine a software system that is well designed but also untestable.

I greatly sympathize with this statement, though I wouldn’t go quite that far.  I don’t think it is so hard to imagine code that is testable, but poorly designed.  For a trivial counter-case, there could be rampant duplication of testable code.  I would call that poorly designed, but it doesn’t affect it’s testability.  Therefore I would soften Uncle Bob’s definition to “One reasonable component of the definition of good design is testability.”

To me, the notion of “testable code” is the same thing that “testable circuit” was back when I worked on a custom integrated circuit.  Mostly, that depends on the ability to put the circuit or code into a known state, exercise it, and see the interactions with its collaborators and its resulting state. Read More