Unit Testing or Functional Testing?

Question

I have recently heard of Functional Testing over Unit Testing.

I understand that Unit Testing tests each of the possibilities of a given piece of code from its most atomic form. But what about Functional Testing?

This sounds to me like only testing if the code works, but is it as reliable as Unit Testing?

I've been told there was two school of thoughts for the matter. Certains would prefer Unit Testing, others Functional Testing.

Is there any good resources, links, books, any references or one of you all who can explain and elighten my path on the subject?

Thanks!

Answer 1

Jason's answer is correct. Different types of tests have different purposes, and can be layered for best results (good design, meeting specifications, reduced defects).

• Unit testing = drives design (with Test-Driven Development, or TDD)
• Integration testing = do all the pieces work together
• Customer acceptance testing = does it meet the customer's requirements
• Manual testing = often covers the UI; dedicated testers can find what automation misses
• Load testing = how well does the system perform with realistic amounts of data

There is some overlap between these categories; unit tests can specify behavior, for instance.

And there are others; for more than most people care to know, see Software Testing.

One point people missed is that unit testing is testing pieces of code in isolation. Good unit tests don't hit the database, for instance. This has two advantages: it makes the tests run fast so you'll run them more often, and it forces you to write loosely coupled classes (better design).

You asked for resources; I recommend Roy Osherove's book The Art of Unit Testing with Examples in .NET. While no book is perfect, this one gives many excellent pointers on writing good tests.

EDIT: And for writing tests against existing software, nothing beats Michael Feathers' book Working Effectively with Legacy Code.