Java Enumerating list in mockito's thenReturn

Question

Is there a way to enumerate the items in a list within mockito's thenReturn function so I return each item in a list. So far I've done this:

List<Foo> returns = new ArrayList<Foo>();
//populate returns list

Mockito.when( /* some function is called */ ).thenReturn(returns.get(0), returns.get(1), returns.get(2), returns.get(3));

This works exactly how I want it to. Each time the function is called, it returns a different object from the list, e.g get(1), get(2) etc.

But I want to simplify this and make it more dynamic to any size list in case I have a list with size say 100. I tried something like this:

Mockito.when( /* some function is called */ ).thenReturn(
    for(Foo foo : returns) {
        return foo;
    }
);

I've tried this as well:

Mockito.when(service.findFinancialInstrumentById(eq(1L))).thenReturn(
    for (int i=0; i<returns.size(); i++) {
        returns.get(i);
    }
);

But this doesn't work....so how do I enumerate this list within the thenReturn....I've come across other methods to like then or answerbut I'm not sure which one works best in this scenario.

Answer 1

The thenReturn() method signature is

thenReturn(T value, T... values)

So it takes an instance of T, followed by a vararg T..., which is syntactic sugar for an array. So you can use

when(foo.bar()).thenReturn(list.get(0), 
                           list.subList(1, list.size()).toArray(new Foo[]{}));

But a cleaner solution would be to create an Answer implementation that takes a List as argument and answers the next element of the list every time it's used. And then use

when(foo.bar()).thenAnswer(new SequenceAnswer<>(list));

For example:

private static class SequenceAnswer<T> implements Answer<T> {

    private Iterator<T> resultIterator;

    // the last element is always returned once the iterator is exhausted, as with thenReturn()
    private T last;

    public SequenceAnswer(List<T> results) {
        this.resultIterator = results.iterator();
        this.last = results.get(results.size() - 1);
    }

    @Override
    public T answer(InvocationOnMock invocation) throws Throwable {
        if (resultIterator.hasNext()) {
            return resultIterator.next();
        }
        return last;
    }
}

Answer 2

Another way of doing it (but personally, I prefer JB Nizet SequenceAnswer idea), would be something like this...

OngoingStubbing stubbing = Mockito.when(...);
for(Object obj : list) {
    stubbing = stubbing.thenReturn(obj);
}