I am a bit confused with Controller Service Dao implementation . I created a dummy User servlet along with Model controller dao and a service , though it will work , but is it the right approach. I am still confused about the purpose of service class

//user registration Service //add user , delete user @WebServlet("/useraction") class UserController {     UserService userService = new userService();         int id = request.getParameter("id");      userService.userAction(int id,String action);     //minimum code for sake of question   }  class UserService {     UserDao userDao = UserDaoImpl.getInstance();      void userAction(int id,String action)       switch(action){        case "add" :          userDao.addUser(id);          break;        case "delete" :          userDao.deleteUser(id);          break;        default :          throw IllegalArgumentException();            }     } }  interface UserDao {     void deleteUser(int id);     void addUser(int id); }  class UserDaoImpl implements UserDao {     private static UserDaoImpl userDaoImpl = new UserDaoImpl();      public static UserDaoImpl getInstance(){         if(userDaoImpl == null)             userDaoImpl = new UserDaoImpl();         return userDaoImpl;     }      void deleteUser(int id){         //some validations and db activity     }      void addUser(int id){         //some validations and db activity     }  }  //Model class User {     private int id;     private String name;     private String email;     //a lot more parameters } 

Questions i want to ask :

• Is this the right approach for Controller dao service pattern?
• Purpose of service class , is it for redirection only to dao or for validations also
• Do we need to use inheritance for Dao classes ?

Thanks in Advance