Measuring what was exercised.
WHAT COVERAGE MEASURES
The proportion of code executed during tests.
WHAT TYPES EXIST
Line coverage Branch coverage Condition coverage Path coverage
WHAT BRANCH COVERAGE ADDS OVER LINE
Whether both outcomes of each decision were taken.
WHY THAT MATTERS
Full line coverage can miss half of every condition.
WHAT COVERAGE DOES NOT MEASURE
Whether the tests assert anything meaningful Whether the right things were tested Whether behaviour is correct
WHAT PROVES THAT
A test suite with no assertions can reach high coverage.
WHAT TO USE COVERAGE FOR
Finding untested areas.
WHAT NOT TO USE IT FOR
A target to be hit.
WHY TARGETS CORRUPT IT
They produce tests written to touch code rather than verify behaviour.
WHAT A REASONABLE APPROACH IS
Review uncovered areas, and decide deliberately whether they warrant tests.
WHAT MUTATION TESTING DOES
Changes the code deliberately and checks whether tests fail.
WHAT IT REVEALS
Tests that execute code without verifying it.
WHY IT IS A BETTER MEASURE
It assesses assertion quality, not execution.
WHAT IT COSTS
Substantial running time.
WHAT TO APPLY IT TO
Critical code, periodically.
