Online Examination System Class Diagram

classDiagram
  class User {
    <<abstract>>
    +int id
    +String name
    +String email
    +login(String password) bool
  }
  class Student {
    +String rollNumber
    +startAttempt(Exam) Attempt
  }
  class Examiner {
    +createExam(String title) Exam
  }
  class Exam {
    +String title
    +DateTime startTime
    +int durationMin
    +totalMarks() int
  }
  class Question {
    <<abstract>>
    +String text
    +int marks
    +grade(Answer) decimal
  }
  class MultipleChoiceQuestion {
    +List~String~ options
    +int correctIndex
    +grade(Answer) decimal
  }
  class DescriptiveQuestion {
    +int wordLimit
    +grade(Answer) decimal
  }
  class Attempt {
    +DateTime submittedAt
    +submit() void
  }
  class Answer {
    +String response
  }
  class Grader {
    +grade(Attempt) Result
  }
  class Result {
    +decimal score
    +String grade
  }
  User <|-- Student
  User <|-- Examiner
  Exam "1" *-- "many" Question
  Question <|-- MultipleChoiceQuestion
  Question <|-- DescriptiveQuestion
  Student "1" --> "many" Attempt
  Attempt --> Exam
  Attempt "1" o-- "many" Answer
  Answer --> Question
  Grader ..> Result : creates