exception - Grails - 无法在空对象上调用方法 getAt()

Question

尝试将书籍附加到书籍的类(class)列表时出现错误。关于为什么或我可以做些什么来解决它的任何想法？

        //For each book in a class...
        for (int k = 0; k < rows.size(); k++) {
            Book book = new Book()

            //Assign the values to a new book object
            book.id            = rows[k].getProperty("ISBN")
            book.title         = rows[k].getProperty("title")
            book.author        = rows[k].getProperty("author")
            book.required      = rows[k].getProperty("required_optional")
            book.purchaseType  = rows[k].getProperty("rental_purchase")
            //book.purchasePrice = rows[k].getProperty("purchase_price")
            //book.rentalPrice   = rows[k].getProperty("rental_fee")

            //Append the book to the books list object in the particular class
            classes[i].books[k + 1] << book
        }

Answer 1

不是通过 [] 访问，而是使用 getAt，然后 ? 运算符将起作用:

classes?.getAt(i)?.books?.getAt(k+1) << book

或

classes?.getAt(i)?.books[k+1] << book