Muutostestejä

Muutostestejä, in English often referred to as change tests or mutation tests, are a software testing technique used to improve the quality of existing tests. The core idea is to introduce small, deliberate errors, or "mutations," into the source code of the program under test. These mutations are typically simple modifications, such as changing an operator (e.g., `>` to `>=`), negating a condition, or deleting a statement.

The purpose of a change test is to determine whether the existing test suite is effective at detecting these introduced faults. If a mutation is made and none of the existing tests fail, it indicates a potential weakness in the test suite. The test suite might not be comprehensive enough, or certain code paths might not be adequately covered.

When a change test is performed, a mutation operator modifies the source code, creating a "mutant" program. Then, the original test suite is run against this mutant. If a test fails, the mutation is considered "killed," meaning the test suite successfully identified the introduced error. If all tests pass for a mutant, that mutation is said to have "survived." Surviving mutations highlight areas where the test suite needs improvement.

Change testing is a valuable tool for assessing test suite robustness and identifying gaps in test coverage. By systematically introducing faults and verifying test suite detection capabilities, developers can gain confidence in the reliability of their tests and, by extension, the quality of their software. It's a technique that complements traditional testing methods by focusing on the "fault-detection effectiveness" of the tests themselves.