将对象添加到列表时出现 NullPointerException [重复]

Question

我非常需要帮助。在尝试了 4-5 个小时后，我认为我迷路了。我知道还有 2 个其他线程具有相同的标题，但没有帮助。 T.T

我不知道如何解决它，我现在快疯了。所以我希望有人可以帮助我解决这个问题。

如果有人可以帮我解决它并向我解释我到底做错了什么，这样我以后就不会再犯了，我会非常高兴。

我有 3 个类，我试图让 OrderMapper 中的 Main 函数创建一个购物车，也就是一个 CustomerOrder 对象。然后通过列表的添加函数将 OrderDetails 对象添加到它的列表中，但我不断收到 NullPointerException。

OrderDetails 类：

package model.domain;

public class OrderDetails {
    private int id;
    private int bookid;
    private int amount;

    public OrderDetails() {
    }

    public OrderDetails(int id, int bookid, int amount) {
        this.id = id;
        this.bookid = bookid;
        this.amount = amount;
    }

    public OrderDetails(int bookid, int amount) {
        this.bookid = bookid;
        this.amount = amount;
    }

    public int getId() {
        return id;
    }

    public int getBookid() {
        return bookid;
    }

    public void setBookid(int bookid) {
        this.bookid = bookid;
    }

    public int getAmount() {
        return amount;
    }

    public void setAmount(int amount) {
        this.amount = amount;
    }

}

CustomerOrder 类：

package model.domain;

import java.util.Date;
import java.util.List;

public class CustomerOrder {
    private int id;
    private int customerid;
    private List<OrderDetails> orderDetails;

    public CustomerOrder() {
    }

    public CustomerOrder(int id, int customerid, List<OrderDetails> orderDetails) {
        this.id = id;
        this.customerid = customerid;
        this.orderDetails = orderDetails;
    }

    public CustomerOrder(int customerid, List<OrderDetails> orderDetails) {
        this.customerid = customerid;
        this.orderDetails = orderDetails;
    }

    public int getId() {
        return id;
    }

    public int getCustomerid() {
        return customerid;
    }

    public void setCustomer(int customerid) {
        this.customerid = customerid;
    }

    public List<OrderDetails> getOrderDetails() {
        return orderDetails;
    }

    public void setOrderDetails(List<OrderDetails> orderDetails, OrderDetails od) {
        orderDetails.add(od);
        this.orderDetails = orderDetails;
    }

    public void addDetails(int bookid, int amount){
        OrderDetails od = new OrderDetails(); 
        od.setBookid(bookid);
        od.setAmount(amount);
        this.orderDetails.add(od);
    }



//    public void addDetails(OrderDetails od){
//        this.orderDetails.add(od);
//    }


    //addOrderline /addDetails
    //
            //od.setOne(this.ono)
            //orderdetails.add(ad)

    //arraylist<orderdetail> getDetails(){
    //    return  orderDetails;
    //}

}

OrderMapper 类：

package model.domain;

import java.util.ArrayList;
import java.util.List;

public class OrderMapper {
    public CustomerOrder createShopcart(int customerid, List<OrderDetails> orderDetails){
        return (new CustomerOrder(customerid, orderDetails));
    }

    public static void main(String[] args) {
            OrderMapper om = new OrderMapper();

            OrderDetails ed = new OrderDetails(1, 2);

            //List<OrderDetails> orderDetails = null;
//         //   orderDetails = ed;
            //orderDetails.add(0, ed));

            CustomerOrder shopcart = new CustomerOrder();

            System.out.println(shopcart.getOrderDetails());

            shopcart.setCustomer(1);
            shopcart.addDetails(1, 3);
            shopcart.addDetails(3, 2);
            shopcart.addDetails(2, 2);

            System.out.println(shopcart.getOrderDetails());

//            CustomerOrder shopcart = om.createShopcart(1, null);
//            System.out.println(shopcart.getOrderDetails());   
//            OrderDetails od = new OrderDetails(1, 2);

//            shopcart.setOrderDetails(null, od);

//            System.out.println(shopcart.getOrderDetails());
//            shopcart.setCustomer(1);
//            shopcart.addDetails(1, 1);

    }

}

我得到的错误是，我知道第一个空值来自我设置的空值，但随后我添加它并得到 NullPointerException。我尝试了很多方法，但没有任何帮助：

null
Exception in thread "main" java.lang.NullPointerException
    at model.domain.CustomerOrder.addDetails(CustomerOrder.java:68)
    at model.dataaccess.OrderMapper.main(OrderMapper.java:155)

Answer 1

你永远不会给你的对象一个列表......因此当你调用 shopcart.addDetails(1, 1);你会得到一个空指针异常，因为你的购物车对象永远不会被一个有效的列表对象实例化。

CustomerOrder shopcart = new CustomerOrder(); //no list given

尝试使用您创建的函数调用 createShopcart(int customerid, List orderDetails) 像这样

CustomerOrder shopcart = new CustomerOrder(0, new ArrayList<OrderDetails>());