return iterator from thenReturn mockito

Question

Hi I have a class A implementing Iterable. I have this code to stub an object of this class:

First way:

A source = mock(A.class);
final List<String> documents = getList();
when(source.iterator()).thenAnswer(new Answer<Iterator<String>>() {
    @Override
    public Iterator<String> answer(final InvocationOnMock invocation) throws Throwable {
        return documents.iterator();
    }
});

Second way:

A source = mock(A.class);
final List<String> documents = getList();
when(source.iterator()).thenReturn(getList().iterator());

The first way returns a new Iterator on every call to source.iterator() but the second one returns the same Iterator object. So if I use second way to stub the source object, I get an empty iterator in subsequent calls to source.iterator().

Why is this happening? Shouldn't Mock return a new Iterator on every call to source.iterator()?

Answer 1

Inlining code can sometimes be confusing. Let's unwrap the second block of code:

A source = mock(A.class);
final List<String> documents = getList();
final Iterator<String> iter = getList().iterator()
when(source.iterator()).thenReturn(iter);

Now, it's easier to understand. The same iter object is always returned, and hence it retains it's state. The fact that before the change above mockito had a function call in the brackets is inconsequential - mockito has an object passed to it, which it stores internally, and returns when required. It cannot "remember" that this object was produced by calling a function unless it has the actual code to execute it - for example, like you did in the first snippet.