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Mockito error with method that returns Optional<T>

I have an interface with the following method

public interface IRemoteStore {

    <T> Optional<T> get(String cacheName, String key, String ... rest);

}

The instance of the class implementing the interface is called remoteStore.

When I mock this with mockito and use the method when:

Mockito.when(remoteStore.get("a", "b")).thenReturn("lol");

I get the error:

Cannot resolved the method thenReturn(java.lang.String)

I thought it has to do with the fact that get returns an instance of the Optional class so I tried this:

Mockito.<Optional<String>>when(remoteStore.get("cache-name", "cache-key")).thenReturn
        (Optional.of("lol"));

But, I get this error instead:

when (Optional<String>) in Mockito cannot be applied to (Optional<Object>).

The only time it worked was with this:

String returnCacheValueString = "lol";
Optional<Object> returnCacheValue = Optional.of((Object) returnCacheValueString);
Mockito.<Optional<Object>>when(remotestore.get("cache-name", "cache-key")).thenReturn(returnCacheValue);

But above returns an instance of Optional<Object> and not Optional<String>.

Why couldn't I just return an instance of Optional<String> directly? If I could, how should I go about doing that?

Solution

  • Mocks that return have the expectation that the return type matches the mocked object's return type.

    Here's the mistake:

    Mockito.when(remoteStore.get("a", "b")).thenReturn("lol");

    "lol" isn't an Optional<String>, so it won't accept that as a valid return value.

    The reason it worked when you did

    Optional<Object> returnCacheValue = Optional.of((Object) returnCacheValueString);
Mockito.<Optional<Object>>when(remotestore.get("cache-name", "cache-key")).thenReturn(returnCacheValue);

    is due to returnCacheValue being an Optional.

    This is easy to fix: just change it to an Optional.of("lol") instead.

    Mockito.when(remoteStore.get("a", "b")).thenReturn(Optional.of("lol"));

    You can also do away with the type witnesses as well. The result above will be inferred to be Optional<String>.