Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming (resit) / 8 / a / Solution

Test-driven development uses a repeating sequence: write a small test for the next required behaviour, observe it fail for the expected reason, add enough implementation to make it pass, and refactor while keeping the tests passing. The initial failure checks that the test can actually detect the absent behaviour. This differs from writing a whole implementation first and only then constructing tests for it; it also uses the tests to guide interface and design decisions rather than only to inspect a finished program.^[ref]

A mixture of test-selection styles is possible; the defining test-first step is normally behaviour-driven. Choosing inputs and expected results from a public contract is black-box testing with respect to the implementation, even if the developer writes the test. Knowing the internal branches can then motivate white-box tests for boundary paths or exceptional cases. TDD specifies the order of development activities, not a rule forbidding either source of test cases. It therefore does not guarantee branch coverage, and passing the growing test suite is not a proof of correctness for untested inputs.