Can you explain what is meant by mutation testing method?

Mutation testing is a method of assessing the quality of tests by making small changes (mutations) to the source code that mimic errors. The goal is to check whether the existing tests can detect these changes.

If the tests do not detect the mutation (i.e., do not fail), it means the coverage or quality of the tests is insufficient. If the tests 'kill' the mutant (the test fails), it indicates the tests are effective.

An example of mutation: replacing the == operator with !=, changing a constant, removing a method call.

This helps improve tests by making them more sensitive to errors.