public int getBookRequestSize(Book book) {
   int numberOfBookRequest = 0;
   if (borrowedBooks.containsKey(book.getIsbnNumber()))
     numberOfBookRequest = borrowedBooks.get(book.getIsbnNumber()).getSizeOfQueue();
   return numberOfBookRequest;
 }
 private boolean isBookBorrowed(Book book) {
   return borrowedBooks.containsKey(book.getIsbnNumber());
 }