i was told to make an Books Library . i end up with that Code lines . i hope some one could help me to make it better then it is right now , this is mine Code lines of 2 method that i need to improve :



Code:
HashMap<Long,Book> books = new HashMap<>();




@Override
public BooksReturnCode addBookItem(Book book) {
    if(book.getPickPeriod()<3)
        return BooksReturnCode.PICK_PERIOD_LESS_MIN;
    if(book.getPickPeriod()>30)
        return BooksReturnCode.PICK_PERIOD_GREATER_MAX;
    return books.putIfAbsent(book.getIsbn(), book)==null?
            BooksReturnCode.OK:BooksReturnCode.BOOK_ITEM_EXIST;
}





@Override
public BooksReturnCode addBookExemplars(long isbn, int amount) {



    if(books.containsKey(isbn)) {
            amount++;
            return BooksReturnCode.OK;
        }
    return BooksReturnCode.NO_BOOK_ITEM;
}