How to Test Shared Behaviour with JUnit Contracts

Duncan McGregor demonstrates how to use JUnit contract tests in Kotlin to ensure that test doubles and real implementations behave identically.

Now that we have created a fake implementation of the external ValueElf service, to stand in for the real thing in our tests, it would be nice to know that they behave the same way. We can increase our confidence by running the same tests against the test double and the real article. So we extract an abstract class with the tests, with setup delegated to concrete subclasses, one for the real implementation and one for our fake. I used to call the superclass AbstractFooTest, but as usual, cleverer people learned me to use more simple words - in England we say FooContract. This is Part 44 of an exploration of where a Test Driven Development implementation of the Gilded Rose stock control system might take us in Kotlin. You can see the whole series as a playlist youtube.com/playlist?list=PL1ssMPpyqociJNwykAOB9_KEZVW7BW7m2 and the code on GitHub github.com/dmcg/gilded-rose-tdd If you like this, you’ll probably like the book Java to Kotlin, A Refactoring Guidebook java-to-kotlin.dev.