将对象添加到列表时获取NullPointerException [重复]

Question

这个问题在这里已有答案：

• What is a NullPointerException, and how do I fix it? 12个答案

我非常需要帮助。尝试4-5个小时后，我发现自己迷路了。我知道有2个其他线程具有相同的标题，但那些没有帮助。 T.T

我不知道如何解决它，我现在疯了。所以我希望有人可以帮我解决这个问题。

如果有人可以帮我解决这个问题并向我解释我做错了什么，所以我将来不再这样做，我会很高兴。

我有3个类，我正在尝试使用OrderMapper中的Main函数制作一个shopcart，也就是CustomerOrder对象。然后通过列表的添加函数将OrderDetails对象添加到它的List中，但我一直得到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null来自我设置的null，但后来我添加了它，它得到了NullPointerException。我尝试过很多东西，但没有人帮忙：

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）;您将获得空指针异常，因为您的shopcart对象永远不会使用有效的列表对象进行实例化。

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

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

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