Mockito and Hamcrest: how to verify invocation of Collection argument?

Question

I'm running into a generics problem with Mockito and Hamcrest.

Please assume the following interface:

public interface Service {
    void perform(Collection<String> elements);
}

And the following test snippet:

Service service = mock(Service.class);

// ... perform business logic

verify(service).perform(Matchers.argThat(contains("a", "b")));

So I want to verify that my business logic actually called the service with a collection that contains "a" and "b" in that order.

However, the return type of contains(...) is Matcher<Iterable<? extends E>>, so Matchers.argThat(...) returns Iterable<String> in my case, which naturally does not apply to the required Collection<String>.

I know that I could use an argument captor as proposed in Hamcrest hasItem and Mockito verify inconsistency, but I would very much like not to.

Any suggestions! Thanks!

Answer 1

You can just write

verify(service).perform((Collection<String>) Matchers.argThat(contains("a", "b")));

From the compiler's point of view, this is casting an Iterable<String> to a Collection<String> which is fine, because the latter is a subtype of the former. At run time, argThat will return null, so that can be passed to perform without a ClassCastException. The important point about it is that the matcher gets onto Mockito's internal structure of arguments for verification, which is what argThat does.

Answer 2

If you get stuck in situations like these, remember that you can write a very small reusable adapter.

verify(service).perform(argThat(isACollectionThat(contains("foo", "bar"))));

private static <T> Matcher<Collection<T>> isACollectionThat(
    final Matcher<Iterable<? extends T>> matcher) {
  return new BaseMatcher<Collection<T>>() {
    @Override public boolean matches(Object item) {
      return matcher.matches(item);
    }

    @Override public void describeTo(Description description) {
      matcher.describeTo(description);
    }
  };
}

Note that David's solution above, with casting, is the shortest right answer.

Answer 3

As an alternative one could change the approach to ArgumentCaptor:

@SuppressWarnings("unchecked") // needed because of `List<String>.class` is not a thing
// suppression can be worked around by using @Captor on a field
ArgumentCaptor<List<String>> captor = ArgumentCaptor.forClass(List.class);

verify(service).perform(captor.capture());
assertThat(captor.getValue(), contains("a", "b"));

Notice, that as a side effect this decouples the verification from the Hamcrest library, and allows you to use any other library (e.g. Truth):

assertThat(captor.getValue()).containsExactly("a", "b");

Answer 4

Why not just verify with the expected arguments, assuming the list only contains the two items, e.g.:

final List<String> expected = Lists.newArrayList("a", "b");
verify(service).perform(expected);

Whilst I agree with Eugen in principle, I think that relying on equals for String comparison is acceptable... besides, the contains matcher uses equals for comparison anyway.

Answer 5

You could have your own java.util.Collection implementation and override the equals method like below.

public interface Service {
    void perform(Collection<String> elements);
}

@Test
public void testName() throws Exception {
    Service service = mock(Service.class);
    service.perform(new HashSet<String>(Arrays.asList("a","b")));
    Mockito.verify(service).perform(Matchers.eq(new CollectionVerifier<String>(Arrays.asList("a","b"))));
}

public class CollectionVerifier<E> extends ArrayList<E> {

    public CollectionVerifier() {

    }

    public CollectionVerifier(final Collection<? extends E> c) {
        super(c);
    }

    @Override
    public boolean equals(final Object o) {
        if (o instanceof Collection<?>) {
            Collection<?> other = (Collection<?>) o;
                return this.size() == other.size() && this.containsAll(other);
        }
        return false;
    }

}

Answer 6

You can put your own lambda as an ArgumentMatcher

when(myClass.myMethod(argThat(arg -> arg.containsAll(asList(1,2))))
    .thenReturn(...);