Why is it necessary to use a constructor for dependency injection into an object?

Using a constructor for dependency injection allows explicitly passing all necessary dependencies to an object at the time of its creation. This provides several advantages:

• Explicit dependencies: it's immediately clear what the class depends on.
• Immutability: dependencies can be made final, increasing safety and predictability.
• Simplified testing: it's easy to substitute mocks or stubs when creating the object.
• No hidden dependencies: there's no need to look for dependencies inside methods.

Example:

public class UserService {
    private final UserRepository userRepository;

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

    public void createUser(String name) {
        userRepository.save(new User(name));
    }
}

Here, UserService does not create UserRepository itself but receives it from outside through the constructor, which simplifies dependency management and testing.