Why are functional tests not enough? What do unit tests offer?

Question

I just had a conversation with my lead developer who disagreed that unit tests are all that necessary or important. In his view, functional tests with a high enough code cov

Answer 1

If you use a pure Extreme Programing / Agile Development methodology the Unit tests are always required as they are the requirements for development.

In pure XP/Agile one makes all requirements based on the tests which are going to be performed to the application

• Functional tests - Generate functional requirements.
• Unit tests - Generate functions or object requirements.

Other than that Unit testing can be used to keep a persistent track of function requirements.

i.e. If you need to change the working way of a function but the input fields and output keep untouched. Then unit testing is the best way to keep tracking of possible problems as you only need to run the tests.