Isolating Dependencies with Test Doubles: Fakes

Introduction to Fakes in TDD

Welcome to our lesson on using Fakes as test doubles in Test Driven Development (TDD) with Java, JUnit, and Mockito . In this lesson, you'll explore how fakes can streamline your testing by simulating real-world components. Our journey so far has exposed you to various test doubles like dummies, stubs, spies, and mocks. Now, we'll dive into fakes, which enable you to create realistic implementations that mirror complex dependencies, making your tests more robust and reliable. As always, we'll practice the TDD cycle: Red, Green, Refactor, as we see how fakes fit into our testing strategy.

Code Example and Walkthrough: Implementing an In-memory Fake Repository

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Java

public class InMemoryUserRepository implements IUserRepository {
    private final Map<String, User> users = new ConcurrentHashMap<>();
    private int currentId = 1;

    private synchronized String generateId() {
        return String.valueOf(currentId++);
    }

    @Override
    public User create(User userData) {
        User user = new User();
        user.setId(generateId());
        user.setEmail(userData.getEmail());
        user.setName(userData.getName());
        user.setCreatedAt(LocalDateTime.now());
        users.put(user.getId(), user);
        return user;
    }

    @Override
    public User findById(String id) {
        return users.get(id);
    }

    @Override
    public User findByEmail(String email) {
        return users.values().stream()
                .filter(u -> u.getEmail().equalsIgnoreCase(email))
                .findFirst()
                .orElse(null);
    }

    @Override
    public User update(String id, User data) {
        User existing = users.get(id);
        if (existing == null) {
            return null;
        }

        if (data.getEmail() != null) {
            existing.setEmail(data.getEmail());
        }
        if (data.getName() != null) {
            existing.setName(data.getName());
        }
        // Prevent modification of id and createdAt
        users.put(id, existing);
        return existing;
    }

    @Override
    public boolean delete(String id) {
        return users.remove(id) != null;
    }

    @Override
    public List<User> findAll() {
        return new ArrayList<>(users.values());
    }

    @Override
    public void clear() {
        users.clear();
        currentId = 1;
    }
}

Java

public class UserServiceTest {
    private UserService userService;
    private InMemoryUserRepository userRepository;

    @BeforeEach
    public void setUp() {
        // Arrange
        userRepository = new InMemoryUserRepository();
        userService = new UserService(userRepository);
    }
    
    @Test
    public void testRegisterUserCreatesNewUserSuccessfully() {
        // Act
        User user = userService.registerUser("test@example.com", "Test User");
        
        // Assert
        assertEquals("test@example.com", user.getEmail());
        assertEquals("Test User", user.getName());
        assertNotNull(user.getId());
        assertNotNull(user.getCreatedAt());
        
        List<User> repositoryUsers = userRepository.findAll();
        assertEquals(1, repositoryUsers.size());
        assertEquals("Test User", repositoryUsers.get(0).getName());
    }
}

public class UserServiceTest {
    private UserService userService;
    private InMemoryUserRepository userRepository;

    @BeforeEach
    public void setUp() {
        // Arrange
        userRepository = new InMemoryUserRepository();
        userService = new UserService(userRepository);
    }
    
    @Test
    public void testRegisterUserCreatesNewUserSuccessfully() {
        // Act
        User user = userService.registerUser("test@example.com", "Test User");
        
        // Assert
        assertEquals("test@example.com", user.getEmail());
        assertEquals("Test User", user.getName());
        assertNotNull(user.getId());
        assertNotNull(user.getCreatedAt());
        
        List<User> repositoryUsers = userRepository.findAll();
        assertEquals(1, repositoryUsers.size());
        assertEquals("Test User", repositoryUsers.get(0).getName());
    }
}

Java

public class UserService {
    private final IUserRepository repository;

    public UserService(IUserRepository repository) {
        this.repository = repository;
    }

    public User registerUser(String email, String name) {
        return this.repository.create(new User(email, name));
    }
}

public class UserService {
    private final IUserRepository repository;

    public UserService(IUserRepository repository) {
        this.repository = repository;
    }

    public User registerUser(String email, String name) {
        return this.repository.create(new User(email, name));
    }
}

Java

public class UserServiceTest {
    private UserService userService;
    private InMemoryUserRepository userRepository;

    @BeforeEach
    public void setUp() {
        // Arrange
        userRepository = new InMemoryUserRepository();
        userService = new UserService(userRepository);
    }
    
    @Test
    public void testRegisterUserCreatesNewUserSuccessfully() {
        // Act
        User user = userService.registerUser("test@example.com", "Test User");
        
        // Assert
        assertEquals("test@example.com", user.getEmail());
        assertEquals("Test User", user.getName());
        assertNotNull(user.getId());
        assertNotNull(user.getCreatedAt());
        
        List<User> repositoryUsers = userRepository.findAll();
        assertEquals(1, repositoryUsers.size());
        assertEquals("Test User", repositoryUsers.get(0).getName());
    }
}

Java

public class UserService {
    private final IUserRepository repository;

    public UserService(IUserRepository repository) {
        this.repository = repository;
    }

    public User registerUser(String email, String name) {
        return this.repository.create(new User(email, name));
    }
}

Introduction to Fakes in TDD

Code Example and Walkthrough: Implementing an In-memory Fake Repository

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Building Tests Using the Fake Repository

Review, Summary, and Preparation for Practice Exercises