Java to Kotlin

Chapter 17. Mocks to Maps

Mocks are a common technique to decouple object-oriented code from its production dependencies. Are better solutions available in Kotlin?

This is a short bonus chapter, following on from Chapter 16. In that chapter, we saw that our tests used mocks because they had to implement two multimethod interfaces, even though most of those methods were not used. We left the refactoring, having replaced dependencies on multimethod interfaces with a dependency on just the two operations that were actually required to perform the task. The tests, though, still mock the whole interface, and then pass a reference to the required methods to the subject under test (Recommendations):

public class RecommendationsTests {

    private final DistanceCalculator distanceCalculator =
        mock(DistanceCalculator.class);
    private final FeaturedDestinations featuredDestinations =
        mock(FeaturedDestinations.class);
    private final Recommendations recommendations = new Recommendations(
        featuredDestinations::findCloseTo,
        distanceCalculator::distanceInMetersBetween
    );
    ...
}

Example 17.1 [interfaces-to-funs.7:src/test/java/travelator/recommendations/RecommendationsTests.java] (diff)

The tests abstract the mocking behind methods givenFeaturedDestinationsFor and givenADistanceBetween:

@Test
public void returns_recommendations_for_single_location() {
    givenFeaturedDestinationsFor(paris,
        List.of(
            eiffelTower,
            louvre
        ));
    givenADistanceBetween(paris, eiffelTower, 5000);
    givenADistanceBetween(paris ...