How to return a list when unit testing DynamoDB PaginatedQueryList

Question

I have a query statement which returns a PaginatedQueryList and I assign it to a List. Then I operate on the list.

List<MyClass> entries= mapper.query(MyClass.class, queryExpression);
//Logic to test
Map<String, Map> map = entries.stream()
                        .collect(Collectors.toMap(MyClass::getA, MyClass::getB));
....
....

I tried to test it like

when(mapper.query(eq(MyClass.class), any(DynamoDBQueryExpression.class)))
                              .thenReturn((PaginatedQueryList<MyClass>) list);

where list is List<MyClass>(ArrayList) that I construct myself and want to pass it to the main program to test the main logic.

But it throws

java.lang.ClassCastException: java.util.ArrayList cannot be cast to com.amazonaws.services.dynamodbv2.datamodeling.PaginatedQueryList

How can I send the list when it executes mappper.query(..).

The motive is to send the list that I constructed to entries when executing mapper.query(..).

NOTE:

I also tried sending a mocked PaginatedQueryList but when it does entries.stream() I want to send the actual stream. I tried like

when(paginatedQueryList.stream()).thenReturn(Stream.of(list.get(0), list.get(1)));

It throws

java.lang.IllegalStateException: stream has already been operated upon or closed at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:229)

NOTE 2:

I use entries.stream() twice in my logic to test

Answer 1

Mockito has some powerful tools for this.

List<MyClass> list = new ArrayList<>();
when(mapper.query(eq(MyClass.class), any(DynamoDBQueryExpression.class)))
    .thenReturn(mock(PaginatedQueryList.class, withSettings().defaultAnswer(new ForwardsInvocations(list))));

This will return a mocked PaginatedQueryList which forwards all method calls to list.

Answer 2

I was able to solve the problem by sending mocked PaginatedQueryList and then send list.stream() twice (for each access of entries.stream()).

when(paginatedQueryList.stream()).thenReturn(list.stream()).thenReturn(list.stream());

Any better approaches are welcome.

Answer 3

My answer is a generic solution for mocking PaginatedQueryList in tests (might be useful for googlers, since there's not much answers for this topic).

For my test this worked:

    // given
    List<UserDevice> expectedDevices = Arrays.asList(generateUserDevice(USER_ID, DEVICE_ID), generateUserDevice(USER_ID, "deviceId2"));

    PaginatedQueryList listMock = mock(PaginatedQueryList.class);
    when(listMock.listIterator()).thenReturn(expectedDevices.listIterator());
    when(mapper.query(eq(UserDevice.class), any(DynamoDBQueryExpression.class))).thenReturn(listMock);

    // when
    List<UserDevice> devices = dao.findAll(USER_ID);

    // then
    assertEquals(expectedDevices, devices);

so, as you can see, I mocked the PaginatedQueryList and mocked its listIterator() method.