public void removeBook(Integer bookID) {
        Book bookValues = bookMap.get(bookID);
        if (bookValues != null) {
            if (borrowersMap.get(bookValues) == null) {
                bookMap.remove(bookID);
                copiesMap.put(bookValues, copiesMap.get(bookValues) - 1);
            } else {
                System.out.println("Can't remove that book right now, it's lent");
            }
        } else {
            System.out.println("Invalid ID number");

        }
    }