Why can BDDMockito not resolve types in this case? - java-8

Question

Consider DynamoDB's QueryApi. Through a series of (unfortunate?) hoops,
ItemCollection<QueryOutcome>>
ends up being equivalent to
Iterable<Item>
I know this because I can do:
public PuppyDog getPuppy(final String personGuid, final String name) {
final QuerySpec spec = new QuerySpec()
.withKeyConditionExpression("#d = :guid and #n = :name")
.withNameMap(new NameMap().with("#d", "guid").with("#n", "name"))
.withValueMap(new ValueMap().withString(":guid", personGuid).withString(":name", name));
return getDog(index.query(spec));
}
private PuppyDog getDog(final Iterable<Item> itemCollection) {
// http://stackoverflow.com/questions/23932061/convert-iterable-to-stream-using-java-8-jdk
return StreamSupport.stream(itemCollection.spliterator(), false)
.map(this::createDogFor)
// it would be a little weird to find more than 1, but not sure what to do if so.
.findAny().orElse(new PuppyDog());
}
But when I try to write tests in Mockito using BDDMockito:
#Test
public void canGetPuppyDogByPersonGuidAndName() {
final PuppyDog dawg = getPuppyDog();
final ArgumentCaptor<QuerySpec> captor = ArgumentCaptor.forClass(QuerySpec.class);
final ItemCollection<QueryOutcome> items = mock(ItemCollection.class);
given(query.query(captor.capture())).willReturn(items);
}
The compiler complains when I try to make items an Iterable.
Why dis?

Answer 1

Dis not because of BDDMockito. Dis because ItemCollection<QueryOutcome> simply can't be cast safely into
Iterable<Item>. It can be cast into Iterable<QueryOutcome> or even Iterable<? extends Item>, but not Iterable<Item>.
Otherwise you could do this:
final ItemCollection<QueryOutcome> items = mock(ItemCollection.class);
Collection<Item> yourItemCollection = items;
yourItemCollection.add(itemThatIsNotAQueryOutcome); // violating safety of items
See also:
Is List<Dog> a subclass of List<Animal>? Why aren't Java's generics implicitly polymorphic?
Why are arrays covariant but generics are invariant?