Problem Statement:

You are tasked with designing a user management system in a Spring Boot application. The system should adhere to the Single Responsibility Principle (SRP) from the SOLID principles. SRP states that a class should have only one reason to change, meaning that it should have only one responsibility.

Solution:

1. Create a User Entity:

   public class User {
       private String username;
       private String password;
   
       // Constructors, getters, and setters
   }

2. Create a UserRepository:

   import java.util.Optional;
   
   public interface UserRepository {
       Optional findByUsername(String username);
       void save(User user);
       // Additional methods for user management
   }

   import org.springframework.stereotype.Repository;
   
   import java.util.HashMap;
   import java.util.Map;
   import java.util.Optional;
   
   @Repository
   public class UserRepositoryImpl implements UserRepository {
       private final Map users = new HashMap<>();
   
       @Override
       public Optional findByUsername(String username) {
           return Optional.ofNullable(users.get(username));
       }
   
       @Override
       public void save(User user) {
           users.put(user.getUsername(), user);
       }
   }

3. Create a UserService:

   import java.util.Optional;
   
   public interface UserService {
       Optional findUserByUsername(String username);
       void registerUser(User user);
       // Additional methods for user service
   }

   import org.springframework.beans.factory.annotation.Autowired;
   import org.springframework.stereotype.Service;
   
   import java.util.Optional;
   
   @Service
   public class UserServiceImpl implements UserService {
       private final UserRepository userRepository;
   
       @Autowired
       public UserServiceImpl(UserRepository userRepository) {
           this.userRepository = userRepository;
       }
   
       @Override
       public Optional findUserByUsername(String username) {
           return userRepository.findByUsername(username);
       }
   
       @Override
       public void registerUser(User user) {
           userRepository.save(user);
       }
   }

4. Example Usage:

   import org.springframework.beans.factory.annotation.Autowired;
   import org.springframework.web.bind.annotation.*;
   
   import java.util.Optional;
   
   @RestController
   @RequestMapping("/users")
   public class MainController {
       private final UserService userService;
   
       @Autowired
       public MainController(UserService userService) {
           this.userService = userService;
       }
   
       @GetMapping("/{username}")
       public String getUser(@PathVariable String username) {
           Optional user = userService.findUserByUsername(username);
           return user.map(value -> "User found: " + value.getUsername()).orElse("User not found");
       }
   
       @PostMapping("/register")
       public String registerUser(@RequestBody User user) {
           userService.registerUser(user);
           return "User registered successfully";
       }
   }