private int uniqueID;
    private int numberOfCopies;
    private Map<Integer, Book> bookMap = new HashMap<>();
    private List<Book> copy = new ArrayList<>(bookMap.values());

    public void addBook(String title, String year, String author) {
        uniqueID++;
        if (bookMap.isEmpty() == false) {
            for (Book book : copy) {
                if (book.getTitle().equals(title) && (book.getYear().equals(year))
                        && (book.getAuthor().equals(author))) {
                    book.addCopy();
                } else {
                    Book newBook = new Book(title, year, author, uniqueID, numberOfCopies);
                    bookMap.put(uniqueID, newBook);
                }
            }
        } else {
            Book newBook = new Book(title, year, author, uniqueID, numberOfCopies);
            bookMap.put(uniqueID, newBook);
        }
    }