how to mock method which invoke collection method and this collection has not setter?

Question

I have following class:

public class UsersHolder {
    private Set<User> users = new HashSet<>();

    public void addUser(User user) {
        users.add(user);
    }
    ...
}

users have not setter method

Is there way to test addUser method?

Answer 1

There's a way.

@RunWith(MockitoJUnitRunner.class)
public class UsersHolderTest {
    @InjectMocks private UsersHolder usersHolder;
    @Mock private Set<User> users;

    @Test
    public void addUser_shouldAddUser() {
        // given
        User user = new User();
        // when
        usersHolder.addUser(user);
        // then
        Mockito.verify(usersHolder).add(user);
    }
}

That being said I wouldn't test such a simple logic. Moreover in this case a set becomes a class dependency that should come from outside (usually collections are treated as data holders a managed inside the class).