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 
        } 

请您参考如下方法:

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