private int uniqueID;
    private Map<Integer, Book> bookMap = new HashMap<>();
    private Map<Book, Integer> copiesMap = new HashMap<>();
    private Map<Book, List<String>> borrowersMap = new HashMap<>();

    public void addBook(Book newAddbook) {
        Integer id = ++uniqueID;
        int numberOfCopies = 1;
        for (Book book : bookMap.values()) {
            if (book.equals(newAddbook)) {
            newAddbook.setId(id);
            numberOfCopies++;
            bookMap.put(id, newAddbook);
            copiesMap.put(newAddbook, numberOfCopies);
            return;
            }
        }
        newAddbook.setId(id);
        bookMap.put(id, newAddbook);
        copiesMap.put(newAddbook, numberOfCopies);
    }