Right way of testing method that depends on other method calls

Question

Hi have read a lot about this but can't come to a conclusion about the best way to test a method that is dependent on other method call results to perform its actions.

Some of the questions I've read include:

• Testing methods that depend on each other
• Unit testing a method that calls other methods
• Unit testing a method that calls another method

Some of the answers sugest that we should only test the methods that perform only one action and then test the method that call this methods for conditional behaviuour (for example, verifying if a given method was called or not) and that's fine, I get it, but I'm struggling with other scenario.

I have a service with a REST api.

The controller has a create method that receives a DTO and calls the Service class create method with this argument (DTO).

I'm trying to practice TDD and for this I use this project I'm building without a database.

The code is as follows:

@Service
public class EntityService implements FilteringInterface {

    private MemoryDatabase db = MemoryDatabase.getInstance();

    //Create method called from controller: receives DTO to create a new 
    //Entity after validating that it's name and code parameters are unique
    public EntityDTO create(EntityDTO dto) throws Exception {

        validateUniqueFields(dto);
       
        Entity entity = Entity.toEntity(dto, "id1"); //maps DTO to Entity object
        db.add(entity);

        return new EntityDTO.Builder(entity);//maps entity to DTO
    }

    public void validateUniqueFields(EntityDTO dto) throws Exception {
        Set<Entity> foundEntities = filterEntityByNameOrCode(dto.getName(),
                dto.getCode(), db.getEntities());

        if (!foundEntities.isEmpty()) {
            throw new Exception("Already exists");
        }
    }
}

This is the interface with methods reused by other service classes:

public interface FilteringInterface {

    default Set<Entity> filterEntityByNameOrCode(String name, String code, Set<Entity> list) {
        return list.stream().filter(e -> e.getSiteId().equals(siteId)
                && (e.getName().equals(name)
                || e.getCode().equals(code))).collect(Collectors.toSet());
    }

    default Optional<Entity> filterEntityById(String id, Set<Entity> list) {
        return list.stream().filter(e -> e.getId().equals(id)).findAny();
    };
}

So, I'm testing this service class and I need to test the create() method because it can have different behaviors:

1. If the received DTO has a name that already exists on the list of entities -> throws Exception
2. If the received DTO has a code that already exists on the list of entities -> throws Exception
3. If the received DTO has a name and a code that already exists on the list of entities -> throws Exception
4. If name and code are different, than everything is ok, and creates the entity -> adds the entity to the existing list - > converts the entity to DTO and retrieves it.

Problem: To test any of the scenarios, suppose, scenario 1: I need to make the filterEntityByNameOrCode() method return a list with an Entity that has the same name as the Entity I'm trying to create. This method is called inside validateUniqueFields() method.

Problem is: I can't call mockito when() for any of this methods because, for that, I would have to mock the service class, which is the class that I'm testing and, thus, it's wrong approach. I've also read that using Spy for this is also wrong approach. So, where thus that leaves me?

Also: if this code is not the correct aprocah, and thats why it can't be correctly tested, than, whats should the correct approach be?

This service will have other methods (delete, update, etc.). All of this methods will make use of the FilteringInterface as well, so I will have the same problems. What is the correct way of testing a service class?

Answer 1

I would apply an DI pattern in your service, in order to mock and control the db variable.

@Service
public class EntityService implements FilteringInterface {

    private Persistence db;

    public EntityService(Persistence db) {
        this.db = db;
    }
}

After that, you will be able to add entities to Set accordingly to your scenarios

@ExtendWith(MockitoExtension.class)
class EntityServiceTest {

    @Mock
    private Persistence persistence;

    @InjectMocks
    private EntityService entityService;

    @BeforeEach
    void before() {
        final Set<Entity> existentEntity = Set.of(new Entity(1L,1L, "name", "code"));
        when(persistence.getEntities()).thenReturn(existentEntity);
    }

    @Test
    void shouldThrowWhenNameAlreadyExists() {
        final EntityDTO dto = new EntityDTO(1L, "name", "anything");
        assertThrows(RuntimeException.class, () -> entityService.create(dto));
    }

    @Test
    void shouldThrowWhenCodeAlreadyExists() {
        final EntityDTO dto = new EntityDTO(1L, "anything", "code");
        assertThrows(RuntimeException.class, () -> entityService.create(dto));
    }

    @Test
    void shouldThrowWhenNameAndCodeAlreadyExists() {
        final EntityDTO dto = new EntityDTO(1L, "name", "code");
        assertThrows(RuntimeException.class, () -> entityService.create(dto));
    }

    @Test
    void shouldNotThrowWhenUnique() {
        final EntityDTO dto = new EntityDTO(1L, "diff", "diff");
        final EntityDTO entityDTO = entityService.create(dto);
        assertNotNull(entityDTO);
    }
}