@startuml
class User {
    + name : string
}

User "1" --> "*" User: friends
User "1" --> "*" Notification: notifications
User "1" --> "*" Ask: friendRequests
class Notification {
    - text : string
}

interface INotifier {
 + notify() : void
}

INotifier --|> Observer

abstract class UserManager {
    - currentUser : User
    + deleteFriend(userId : int) : void
    + addFriend(userId : int) : void
    + respondToFriendRequest(requestId : int, choice : bool) : void
    + getFriends(userId : int) : User[]
}

class Ask {
    - fromUser : int
    - toUser : int
}

Ask --|> Subject

abstract class Subject {
    + attach(o : Observer) : void
    + detach(o : Observer) : void
    + notify() : void
}

Subject "1" --> "*" Observer
interface Observer {
    + update() : void
}

UserManager ..> User
UserManager o-- IUserRepository
UserManager o-- INotifier

interface IUserRepository {
    + findByUsername(username : string) : User
    + addUser(user : User) : bool
}

IUserRepository ..> User
@enduml