01.mutation-testing.md

Mutation Testing

• In scala we can use Stryker4s as mutation testing library :
  • We need to add a project/plugins.sbt file
  • Add the following line in it

addSbtPlugin("io.stryker-mutator" % "sbt-stryker4s" % stryker4sVersion)

• After that we can run sbt stryker
  • It mutates your code and run mutation coverage analysis
• Check the report inside target/stryker4s-report

• We can see that we have a mutation score of 88.89%
  • Let's analyze which mutant survived

MissingExchangeException

Stryker was able to create a mutant as we don't verify the exception message. The process replaced the exception message by an empty string and our test still passed.

It proves our test isn't reliable. Testing the message does provide value as it contain business information.

Let's fix the corresponding test

test("Throws a MissingExchangeRateException in case of missing exchange rates") {
    val exception = intercept[MissingExchangeRateException](bank.convert(10, EUR, KRW))
    assert(exception.getMessage == "EUR->KRW")
}

When implemented, verify your test pass and run the stryker analysis again

Mutants eradicated!

• Congratulations you have killed your first mutant !!!

• Why you should focus on good assertion?
  • Take a look at the Test Desiderata