Design data structures for an online book reader system

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Design the data structures for an online book reader system.

 We will have 2 simple classes - book and user :

public class Book {
    private long ID;
    private String details;
    
 
    public long getID() {
        return ID;
    }
 
    public Book(long id, String details) {
    }
}

public class User {
    private long ID;
    private String details;
    private int accountType;
   
    public long getID() {
        return ID;
    }
 
    public Book searchLibrary(long id) {
        return Book.find(id);
    }
 
    public void renewMembership() {
    } 
}

Now we need an management system, which can manage them. The users and books have m:n relationship between them, i.e. multiple users can have same book and same user can have multiple books. To solve this we will have new table in database, which will have book to user mapping, with foreign key reference to user and book id, both being the composite key.
The reader system will look like this:
Managing books :

public class Books{
 private Set<Book> books;
 
 public void addBook(long iD, String details) {
        books.add(new Book(iD, details));
    }
 
    public void update() {
    }
 
    public void delete(Book b) {
        books.remove(b);
    }
 
    public Book find(long id) {
        for (Book b : books)
            if (b.getID() == id)
                return b;
        return null;
    }
}
 

managing users :

public class Users {
    private Set<User> users;
 
    public Book searchLibrary(long id) {
        return Book.find(id);
    }
 
    public void renewMembership() {
    }
 
    public static User find(long ID) {
        for (User u : users) {
            if (u.getID() == ID)
                return u;
        }
        return null;
    }
 
    public void addUser(long ID, String details,
            int accountType) {
        users.add(new User(ID, details, accountType));
    }
 
    public User(long iD, String details, int accountType) {
    }
}

Now we need the 1:1 mapping between book and user, which is managed by Book User, and online reader finally manages everything :

public class BookUsers {
 
 private HashMap<Book,Set<User>> bookUsers;
 private HashMap<User,Set<Book>> userBooks;
 
 private void addUserToBookUserList(Book b, User u){
  if(bookUsers.containsKey(b)){
   Set<User> users = bookUsers.getValue(b);
   users.add(u);
  }else
  {
   bookUsers.add(b,new HashMap<User> (){u});
  }
 }
 
 private void addBookToUserBookList(Book b, User u){
  if(userBooks.containsKey(u)){
   Set<Book> books = userBooks.getValue(u);
   books.add(b);
  }else
  {
   userBooks.add(u,new HashMap<Book> (){b});
  }
 }
 public void assignBookToUser(Book b, User u)
 {
  addUserToBookUserList(b,u);
  addBookToUserBookList(b,u);
   
 }
}
 
public class OnlineReaderSystem {
    private Books books;
    private Users users;
 private BookUsers bookUsers;
 
 private HashMap<Book,Set<User>> bookUsers;
 private HashMap<User,Set<Book>> userBooks;
 
 public void assignBookToUser()
 {
  bookUsers.assignBookToUser(b,u);
   
 }
 
    public OnlineReaderSystem(Books books, Users users) {
    }
 
    public void listenRequest() {
    }
 
    public Book searchBook(long ID) {
        return books.find(ID);
    }
 
    public User searchUser(long ID) {
        return users.find(ID);
    }
 
    public void display() {
    }
 
}

Unfortunately, BookUser will change when we take into account db relational model, as we dont have to keep things in-memory. But this is what I got. Here is Crowe's notation for entity relationship :

Please feel free to correct this design.

Reference - runhe tian