A method of software testing that involves modifying the source code or compiled intermediate code (Java: byte code, .NET: IL). Test suites that do not fail under each of these "mutations" is considered to be inadequate.
Mutation testing is a method of testing the adequacy of test suites. By applying mutations to the code under test, the effectiveness of a test suite can be judged. If the test suite fails under this mutation, it is effective against it.
Different mutation types are used: Ma, Offutt and Kwon describe intra-method, inter-method, intra-class and inter-class mutations. The simplest to implement is intra-method mutation, for which some of the mutations that may be used are:
- Statement deletion.
- Replace each boolean subexpression with true and false.
- Replace each arithmetic operation with another one, e.g. + with *, - and /.
- Replace each boolean relation with another one, e.g. > with >=, == and <=.
- Replace each variable with another variable declared in the same scope (variable types should be the same). (source: Wikipedia)
