Mutant Selection Strategies in Mutation Testing

Mutation Testing offers a powerful approach to assessing unit test set quality; however, software developers are often reluctant to embrace the technique because of the tremendous number of mutants it generates, including redundant and equivalent mutants. Researchers have sought strategies to reduce the number of mutants without reducing effectiveness, and also ways to select more effective mutants, but no strategy has performed better than random mutant selection. Equivalent mutants, which cannot be killed, make achieving mutation adequacy difficult, so most research is conducted with the assumption that unkilled mutants are equivalent. Using 15 java.lang classes that are known to have truly mutation adequate test sets, this research demonstrates that even when the number of equivalent mutants is drastically reduced, they remain a tester's largest problem, and that apart from their presence achieving mutation adequacy is relatively easy. It also assesses a variety of mutant selection strategies and demonstrates that even with mutation adequate test sets, none perform as well as random mutant selection.