Use a fake object to test user service without real database

Preconditions (2)

Step 1: Create a fake UserRepository that returns a fixed User for id=1

Step 2: Inject the fake UserRepository into UserService

Step 3: Call UserService.getUserName(1)

Step 4: Verify the returned user name matches the fake user's name

✅ Expected Result: UserService returns the name of the fake user without accessing a real database

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the returned user name equals the expected fake user name

Best Practices:

Use a simple fake class implementing the interface

Avoid using real database or external dependencies

Keep test isolated and fast

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

// User entity
record User(int id, String name) {}

// UserRepository interface
interface UserRepository {
    User findUserById(int id);
}

// UserService uses UserRepository
class UserService {
    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public String getUserName(int id) {
        User user = userRepository.findUserById(id);
        return user != null ? user.name() : null;
    }
}

// Fake UserRepository for testing
class FakeUserRepository implements UserRepository {
    @Override
    public User findUserById(int id) {
        if (id == 1) {
            return new User(1, "FakeUser");
        }
        return null;
    }
}

public class UserServiceTest {

    @Test
    void testGetUserNameReturnsFakeUserName() {
        UserRepository fakeRepo = new FakeUserRepository();
        UserService userService = new UserService(fakeRepo);

        String userName = userService.getUserName(1);

        assertEquals("FakeUser", userName, "UserService should return the fake user's name");
    }
}

This test uses a fake object to replace the real UserRepository. The FakeUserRepository class implements the interface and returns a fixed user when asked for id 1. This avoids any real database calls.

The UserService is created with the fake repository. When getUserName(1) is called, it returns the name from the fake user. The test asserts this name matches the expected "FakeUser" string.

This approach keeps the test fast, isolated, and simple. It shows how to use fake objects to test logic without external dependencies.

Common Mistakes - 3 Pitfalls

Using the real database in the test instead of a fake object

Not implementing the interface methods in the fake object

Hardcoding test data inside the service instead of using a fake

Bonus Challenge

Now add data-driven testing with 3 different user IDs and expected names

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