Mocking a List and attempting to iterate over it

Question

Currently using Mockito to Test a method from one of my classes. My class contains a List, and the method takes in an object of the same class. The issue is when I attempt to it

Answer 1

There are some fundamental issues why your example not does work and throws a NullPointerException.

1. The call to add() on the mocked list effectively doesn't do anything. All void methods on mocks are "no-ops" by default
2. Iterating over a list using for-each syntax calls Collection.iterator() under the hood. This returns null, because you've not setup mockito to return anything else.

Instead, I would not mock the list and instead pass an actual list. Arrays.asList() is convenient for testing.

@Before
public void setUp(){
    MockitoAnnotations.initMocks(this);
    shipment = new Shipment(1, Arrays.asList(mockOrder1, mockOrder2));
    shipment2 = new Shipment(2, Arrays.asList(mockOrder3));
}

If you're determined to mock a list then you'll have to mock its behaviour, i.e. making the add() actually store something and .iterator() return an iterator. This can be done rather painfully as follows. I've only included this to demonstrate the principle.

@Mock
private List mockedList;

@Before
public void init() {
    MockitoAnnotations.initMocks(this);

    List realList = new ArrayList<>();
    doAnswer(new Answer() {
        @Override
        public String answer(InvocationOnMock invocation) throws Throwable {
            realList.add(invocation.getArgumentAt(0, String.class));
            return null;
        }

    }).when(mockedList).add(any());

    when(mockedList.iterator()).thenAnswer(new Answer>() {

        @Override
        public Iterator answer(InvocationOnMock invocation) throws Throwable {
            return realList.iterator();
        }
    });

    mockedList.add("bar");
    mockedList.add("baz");
}

@Test
public void iterateOverMockedList() {
    for (String each : mockedList) {
        System.out.println(each);
    }
}