What is the difference between using 'let' and 'also' keywords in programming?

Proper dependency injection (Dependency Injection, DI) in Java services is usually achieved through constructor injection. This approach is considered preferable because:

• Dependencies are explicitly visible in the constructor, which improves readability and testability.
• Objects are created in a correct state, without the possibility of forgetting to initialize dependencies.
• It is easy to use with DI frameworks, such as Spring.

Example of constructor injection with Spring:

@Service
public class UserService {
    private final UserRepository userRepository;

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

    // service methods
}

Alternatives are setter injection or field injection, but they are less preferred due to potential issues with immutability and testing.

It is also important to adhere to the Dependency Inversion Principle (DIP) and use interfaces for dependencies to increase flexibility and modularity of the code.