Stub Object in Unit testing with Java

In unit testing with Java, a stub object is a simplified implementation of a class or an interface that is used to isolate the behavior of the code being tested. Stubs are commonly used to simulate the behavior of external dependencies or components, allowing you to focus on testing specific units of code without worrying about external factors.

Key Characteristics of a Stub:

Simplified Behavior: Stubs often provide hardcoded responses to method calls.
No Real Logic: Unlike mocks, stubs typically don’t involve intricate behavior or logic.
Passive Object: They are used to supply inputs or simulate the state required for testing.

When to Use Stubs:

When you need to isolate the code under test from external dependencies (like databases, APIs, or other services).
When testing behavior that relies on specific return values or conditions from other components.

Example: Stub Implementation in Java

Here is an example of how to use a stub object in unit testing:

Scenario:

Suppose you have a UserService class that relies on a UserRepository interface to fetch user details.

// Interface
public interface UserRepository {
    User findUserById(String id);
}

// Implementation of Service
public class UserService {
    private final UserRepository userRepository;

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

    public String getUserName(String id) {
        User user = userRepository.findUserById(id);
        return user != null ? user.getName() : "User not found";
    }
}

// Model Class
public class User {
    private final String id;
    private final String name;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

Stub for `UserRepository`:

You can create a stub for UserRepository in your test class.

// Stub Implementation
public class UserRepositoryStub implements UserRepository {
    @Override
    public User findUserById(String id) {
        if ("123".equals(id)) {
            return new User("123", "Alice");
        }
        return null;
    }
}

Test Case Using Stub:

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

public class UserServiceTest {
    @Test
    public void testGetUserName() {
        // Arrange: Use the stub
        UserRepository userRepositoryStub = new UserRepositoryStub();
        UserService userService = new UserService(userRepositoryStub);

        // Act: Call the method
        String userName = userService.getUserName("123");

        // Assert: Verify the result
        assertEquals("Alice", userName);

        // Test for a non-existent user
        assertEquals("User not found", userService.getUserName("999"));
    }
}

Advantages of Using Stubs:

Simplifies Testing: Focuses only on the unit under test by removing external complexities.
Improves Speed: Avoids delays caused by real components (e.g., database or network calls).
Reliability: Provides predictable outputs for test cases.

Considerations:

Stubs are less flexible compared to mocks because they provide static responses.
For more dynamic and complex behavior testing, you might use a mocking framework like Mockito.

Would you like an example using Mockito for dynamic stubbing?