UML practice · The whole hotel, every relationship.

A small hotel takes bookings. This is the diagram an interviewer would ask you to draw, and it uses every relationship the lesson teaches. Before you code, read each line aloud: what is it, and what does it look like in code?

%% title: The hotel
classDiagram
  class Hotel {
    +name str
    +rooms list
    +guests list
    +bookings list
    +register(guest)
    +book(room_number, guest, nights) Booking
    +check_out(booking)
  }
  class Room {
    +number int
    +rate int
    +nightly_rate(policy) int
  }
  class Suite {
    +nightly_rate(policy) int
  }
  class Guest {
    +name str
  }
  class Booking {
    +nights int
    +cost(policy) int
  }
  class PricingPolicy {
    <<interface>>
    +apply(rate) int
  }
  class FlatPricing
  class WeekendPricing {
    +percent int
  }
  Hotel "1" *-- "1..*" Room : rooms
  Hotel "1" *-- "*" Booking : bookings
  Hotel "*" o-- "*" Guest : guests
  Room <|-- Suite
  Booking "*" --> "1" Room : of
  Booking "*" --> "1" Guest : for
  Booking ..> PricingPolicy : priced by
  Room ..> PricingPolicy : priced by
  PricingPolicy <|.. FlatPricing
  PricingPolicy <|.. WeekendPricing

Build this API (names are Python's; JavaScript and TypeScript use camelCase where they differ: nightlyRate, checkOut, roomNumber):

• PricingPolicy is the interface: apply(rate) is abstract. Building it, or an implementation without apply, raises TypeError (JavaScript: Error containing interface). FlatPricing() returns the rate unchanged. WeekendPricing(percent) returns rate * (100 + percent) // 100 (whole rupees).
• Room(number, rate) with .number, .rate. nightly_rate(policy) is policy.apply(self.rate). Suite(number, rate) is a Room and adds 1000 to the policy's price for the room: policy.apply(rate) + 1000.
• Guest(name) with .name.
• Booking(room, guest, nights) refers to the very room and guest it is given. nights is 1 to 30: otherwise ValueError (Error containing nights). cost(policy) is nights * room.nightly_rate(policy); the policy is only a parameter.
• Hotel(name, room_specs): room_specs is a list of (number, rate, is_suite). The hotel creates a Room for each (a Suite when is_suite is true) in .rooms. A hotel needs at least one room (1..*): an empty list raises ValueError (Error containing at least one room). .guests and .bookings start empty.
• Hotel.register(guest) keeps the very guest it is given, never a copy (aggregation).
• Hotel.book(room_number, guest, nights) creates a Booking for the room with that number, keeps it in .bookings and returns it. It raises ValueError for an unknown room (Error containing no room), for a guest who is not registered (Error containing not registered), and for a room that already has a booking (Error containing occupied). A refused call changes nothing.
• Hotel.check_out(booking) removes the booking, freeing the room. A booking that is not in .bookings raises ValueError (Error containing not an active booking).