OnlineReaderSystem

public class OnlineReaderSystem {
    private Library library;
    private UserManager userManager;
    private Display display;
    private Book activeBook;
    private User activeUser;

    public OnlineReaderSystem() {
        userManager = new UserManager();
        library = new Library();
        display = new Display();
    }

    public Library getLibrary() {
        return library;
    }

    public UserManager getUserManager() {
        return userManager;
    }

    public Display getDisplay() {
        return display;
    }

    public Book getActiveBook() {
        return activeBook;
    }

    public void setActiveBook(Book book) {
        activeBook = book;
        display.displayBook(book);
    }

    public User getActiveUser() {
        return activeUser;
    }

    public void setUser(User user) {
        activeUser = user;
    }

}

public class Library {
    Private HashTable<Integer, Book> books;

    public Book addBook() {

    }

    public boolean remove(Book b){}
    public boolean remove(int id){}

    public Book find(int id) {}
}

public class UserManager {
    private Hashtable<Integer, User> users;

    public User addUser(int id, String details, int accountType) {
        if (users.containsKey(id)) {
            return null;
        }

        User user = new User(id, details, accountType);
        user.put(id, user);
        return user;
    }

    public boolean remove(User u) {
        return remove(u.getID());
    }

    public boolean remove(int id) {
        if (!users.containsKey(id)) {
            return false;
        }

        return users.remove(id);
    }

    public User find(int id) {
        return users.get(id);
    }
}

public class Display {
    private Book activeBook;
    private User activeUser;
    private int pageNumber = 0;

    public void displayUser(User user){}
    public void displayBook(Book book){}
    public void turnPageForward() {}
    public void turnPageBackward(){}

    // Refresh functions
}

public class Book {
    private int bookId;
    private String details;

    public Book (int id, int det) {}

    public int getID(){}
    public void setID(){}
    public String getDetails(){}
    public void setDetails(){}
}

public class User {
    private int userId;
    private String details;
    private int accountType;

    public void renewMembership() {}

    public User (int id, String details, int accountType){}

    //getters and setters
}