Why is the constructor used when injecting dependencies into an object?

Dependency injection through the constructor provides several important advantages:

• Immutability of the object: All dependencies are passed during object creation, allowing fields to be final and ensuring the object is fully initialized.
• Explicit dependencies: The constructor clearly shows which components the class depends on, improving code readability and maintainability.
• Simplified testing: Mocks or stubs can be easily substituted during testing by passing them into the constructor.
• No state after creation: The object cannot end up in an inconsistent state since all dependencies are set immediately.

Example:

public class UserService {
    private final UserRepository userRepository;

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

    public void registerUser(User user) {
        userRepository.save(user);
    }
}

Here, UserService explicitly requires UserRepository through the constructor, making the dependency transparent and mandatory.