Writing unit tests for Java 8 streams

Question

I have a list and I'm streaming this list to get some filtered data as:

List<Future<Accommodation>> submittedRequestList = 
    list.stream().filter(Objects::nonNull)
                 .map(config -> taskExecutorService.submit(() -> requestHandler
                 .handle(jobId, config))).collect(Collectors.toList());

When I wrote tests, I tried to return some data using a when() :

List<Future<Accommodation>> submittedRequestList = mock(LinkedList.class);
when(list.stream().filter(Objects::nonNull)
                  .map(config -> executorService.submit(() -> requestHandler
                            .handle(JOB_ID, config))).collect(Collectors.toList())).thenReturn(submittedRequestList);

I'm getting org.mockito.exceptions.misusing.WrongTypeOfReturnValue: LinkedList$$EnhancerByMockitoWithCGLIB$$716dd84d cannot be returned by submit() error. How may I resolve this error by using a correct when() ?

Answer 1

You can only mock single method calls, not entire fluent interface cascades.

Eg, you could do

Stream<Future> fs = mock(Stream.class);
when(requestList.stream()).thenReturn(fs);
Stream<Future> filtered = mock(Stream.class);
when(fs.filter(Objects::nonNull).thenReturn(filtered);

and so on.

IMO it's really not worth mocking the whole thing, just verify that all filters were called and check the contents of the result list.