Mockito: mocking an arraylist that will be looped in a for loop

Question

I have a method under test that contains the following snippet:

private void buildChainCode(List<TracedPath> lines){
    for(TracedPath path : lines){
        /.../
    }
}

My unit test code looks like this:

public class ChainCodeUnitTest extends TestCase {

    private @Mock List<TracedPath> listOfPaths;
    private @Mock TracedPath tracedPath;

    protected void setUp() throws Exception {
        super.setUp();
        MockitoAnnotations.initMocks(this);
    }

    public void testGetCode() {
        when(listOfPaths.get(anyInt())).thenReturn(tracedPath);

        ChainCode cc = new ChainCode();
        cc.getCode(listOfPaths);

        /.../
    }
}

The problem is, that while running the test, the test code never enters the for loop. What when conditions should I specify, so that the for loop would be entered? Currently I have specified when(listOfPaths.get(anyInt())).thenReturn(tracedPath), but I guess it is never used.

score 66 · Accepted Answer · answered Aug 28 '13 at 09:19

66

Your problem is that when you use a collection in a for-each loop, its iterator() method gets called; and you haven't stubbed that particular method.

Instead of mocking the list, I strongly recommend you just pass a real list, where the elements are just your mocked TracedPath, as many times as you want it. Something like

listOfPaths = Arrays.asList(mockTracedPath, mockTracedPath, mockTracedPath);

answered Aug 28 '13 at 09:19

Dawood ibn Kareem

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what if you do not know the elements that will be added to the list? – BlueShark Dec 24 '14 at 14:29
1

You choose the elements, depending on what you're trying to test. – Dawood ibn Kareem Dec 25 '14 at 00:25
2

The listOfPaths must be then set explicit to the class under test and can't be annotated with @Mock. – Cengiz Feb 17 '15 at 14:27
what if the list is supposed to be empty? – Shilan Nov 29 '16 at 10:02
1

@Shilan then just use an empty list! – Dawood ibn Kareem Nov 29 '16 at 18:17
If list needs to be empty then just new ArrayList() – Stoycho Andreev Sep 01 '17 at 12:03
@Cengiz what can be done if the list is autowired using spring framework and there is no setter in the class? – user3153014 Feb 22 '18 at 09:24
@user3153014 Create setter and if that is not possible use reflection. – Cengiz Feb 22 '18 at 15:41

score 7 · Answer 2 · answered Aug 28 '13 at 09:23

7

The Java For-Each loop used inside your buildChainCode method isn't calling get(), as you already figured out - it's using the iterator() method defined in Collection<E>, which List<E> extends.

I really wouldn't recommend mocking the list. There's no advantage to it, unless you absolutely must check that the list was iterated through, and even then, it's better to assert on or verify the results of your class's behaviour given certain inputs.

Pass some implementation of List<E> like LinkedList<E> with tracedPath in it.

answered Aug 28 '13 at 09:23

Chris Mantle

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Thank you for a good explanation. I would mark it as an answer but a very similar one was submitted just 3 minutes before yours. – Rebane Lumes Aug 28 '13 at 09:35
Thank you for your comment. I got called away to fix something before I could finish my post - c'est la vie :) David's is a good answer. – Chris Mantle Aug 28 '13 at 09:39

Mockito: mocking an arraylist that will be looped in a for loop

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