学习中遇到的HashMap问题

2019-01-11 08:36:09来源：博客园阅读 ()

前言

今天在老师教Session的时候，利用了Session可持久化保存服务器端的特性尝试做了一下用HashMap嵌套的购物车(没有将购物车的商品信息保存在数据库中)，之所以做的这么麻烦是为了巩固之前学习的Map知识和锻炼逻辑能力，正好也在其中遇到了一个关于HashMap 的问题，在此做个小小的记录，方便日后查看。

问题

服务器端保存购物车商品信息用的是HashMap嵌套，内层HashMap存放的是商品和该商品的数量，内层的HashMap中只存放一组键值对，外层HashMap存放的是商品和该商品总价，根据页面传过来的商品id在数据库中获取到商品的信息,。再遍历HashMap根据id判断是否已经存在该商品，再针对不同情况进行处理，所遇到的问题是购物车存在页面穿进来的该商品，那么我如果修改内层Map的Integer(数量)，再修改外层HashMap的Value(该商品的总价),就会出现空指针异常，解决方案是先将商品总价保存起来，把内层HashMap从外层HashMap中remove掉，再修改商品数量，再将修改数量后的内层HashMap添加到外层HashMap中，代码如下:

<%@ page import="java.util.List" pageEncoding="utf-8" %>
<%@ page import="model.Shop" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>购物</title>
    <script src="js/jquery-3.3.1.min.js"></script>
</head>
<body>
<form action="shopcart" id="form">
    <input type="hidden" id="count" name="count" value="">
    <input type="hidden" id="id" name="goodid" value="">
    <table>
        <tr>
            <th>店铺码</th>
            <th>水果名</th>
            <th>价格</th>
            <th>类别</th>
            <th>操作</th>
        </tr>
        <%
            List<Shop> list = (List<Shop>) request.getAttribute("shoplist");
            for (int i=0;i<list.size();i++){
                Shop shop = list.get(i);
        %>
        <tr>
            <td><%=shop.getCode()%></td>
            <td><%=shop.getName()%></td>
            <td><%=shop.getPrice()%></td>
            <td><%=shop.getType()%></td>
            <td>
                <%--<a href="shopcart?goodid="--%>
                <a href="javascript:void(0)" onclick="addToCart(<%=shop.getSid()%>)">添加到购物车</a>
            </td>
        </tr>
        <%}%>
    </table>
</form>
</body>
<script>
    function addToCart(id) {
        var count = prompt('添加数量是多少个');
        $("#id").val(id);
        $("#count").val(count);
        if (confirm("确认添加?")) {
            $("#form").submit();
        }
    }
</script>
</html>

商品选择jsp

<%@ page import="model.Shop" %>
<%@ page import="java.util.Map" %>
<%@ page import="java.util.Set" %><%--
  Created by IntelliJ IDEA.
  User: asus
  Date: 2019/1/10
  Time: 23:40
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    Map<Map<Shop,Integer>,Integer> cart = (Map<Map<Shop, Integer>, Integer>) session.getAttribute("shopcart");
    int sum = (int) session.getAttribute("sum");
%>
<table>
    <tr>
        <th>水果名</th>
        <th>价格</th>
        <th>类别</th>
        <th>数量/个</th>
        <th>小计/元</th>
    </tr>
    <%
        Set<Map<Shop, Integer>> set = cart.keySet();
        for (Map<Shop, Integer> shopIntegerMap : set) {
            Shop shop = shopIntegerMap.keySet().iterator().next();
    %>
    <tr>
        <td><%=shop.getName()%></td>
        <td><%=shop.getPrice()%></td>
        <td><%=shop.getType()%></td>
        <td>&nbsp;&nbsp;<%=shopIntegerMap.get(shop)%></td>
        <td><%=cart.get(shopIntegerMap)%></td>
    </tr>
    <%}%>
</table>
<br>
<th>合计：</th><%=sum%> 元
</body>
</html>

购物车页面

出错代码待添加

//此处写的是Shop是因为数据表中给定的表名是shop
package model;

/**
 * TODO
 *
 * @Author Whyat
 * @Date 2019/1/9 17:10
 */
public class Shop {
    private int sid,price;
    private String code,name,type;

    public Shop() {
    }

    public Shop(int price, String code, String name, String type) {
        this.price = price;
        this.code = code;
        this.name = name;
        this.type = type;
    }

    public Shop(int sid, int price, String code, String name, String type) {
        this.sid = sid;
        this.price = price;
        this.code = code;
        this.name = name;
        this.type = type;
    }

    public int getSid() {
        return sid;
    }

    public Shop setSid(int sid) {
        this.sid = sid;
        return this;
    }

    public int getPrice() {
        return price;
    }

    public Shop setPrice(int price) {
        this.price = price;
        return this;
    }

    public String getCode() {
        return code;
    }

    public Shop setCode(String code) {
        this.code = code;
        return this;
    }

    public String getName() {
        return name;
    }

    public Shop setName(String name) {
        this.name = name;
        return this;
    }

    public String getType() {
        return type;
    }

    public Shop setType(String type) {
        this.type = type;
        return this;
    }

    @Override
    public String toString() {
        return "Shop{" +
                "sid=" + sid +
                ", price=" + price +
                ", code='" + code + '\'' +
                ", name='" + name + '\'' +
                ", type='" + type + '\'' +
                '}';
    }
}

商品Bean

 1 package servlet;
 2 
 3 import model.Shop;
 4 import service.ShopService;
 5 import service.impl.ShopServiceImpl;
 6 
 7 import javax.servlet.ServletException;
 8 import javax.servlet.annotation.WebServlet;
 9 import javax.servlet.http.HttpServlet;
10 import javax.servlet.http.HttpServletRequest;
11 import javax.servlet.http.HttpServletResponse;
12 import javax.servlet.http.HttpSession;
13 import java.io.IOException;
14 import java.util.HashMap;
15 import java.util.List;
16 import java.util.Map;
17 import java.util.Set;
18 
19 /**
20  * TODO
21  *
22  * @Author Whyat
23  * @Date 2019/1/10 14:50
24  */
25 @WebServlet("/shopcart")
26 public class ShopCart extends HttpServlet {
27     ShopService shopService = new ShopServiceImpl();
28 
29     @Override
30     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
31         int sid = Integer.parseInt(req.getParameter("goodid"));
32         int count = Integer.parseInt(req.getParameter("count"));
33         HttpSession session = req.getSession();
34         session.setMaxInactiveInterval(60);
35         Shop shop = shopService.getShopBySid(sid);
36         boolean flag = true;
37         Integer sum = (Integer) session.getAttribute("sum");
38         if (sum == null) {
39             sum = 0;
40         }
41         Map<Map<Shop, Integer>, Integer> cart = (Map<Map<Shop, Integer>, Integer>) session.getAttribute("shopcart");
42         if (cart == null) {
43             cart = new HashMap<Map<Shop, Integer>, Integer>();
44         }
45         Set<Map<Shop, Integer>> mapset = cart.keySet();
46         for (Map<Shop, Integer> shopIntMap : mapset) {
47             Map<Shop,Integer> xmap = shopIntMap;
48             Set<Shop> shopSet = shopIntMap.keySet();
49             Shop shopInSet = shopSet.iterator().next();
50             if (shopInSet.getSid() == sid) {
51                 //保存<商品,总价>的大map的总价
52                 int oringinGoodSum = cart.get(shopIntMap);
53                 //去掉<商品,总价>的大map里的<商品,数量>小map
54                 cart.remove(shopIntMap);
55                 //修改<商品,数量>小map的数量
56                 shopIntMap.put(shopInSet, shopIntMap.get(shopInSet) + count);
57                 //再将修改商品数量后的<商品,数量>小map添加到<商品,总价>的大map中
58                 cart.put(shopIntMap, oringinGoodSum + shopInSet.getPrice() * count);
59                 int singleSum = cart.get(shopIntMap);
60                 int singleCount = shopIntMap.get(shopInSet);
61                 sum += shop.getPrice() * count;
62                 flag = false;
63                 break;
64             }
65         }
66         if (flag) {
67             Map<Shop, Integer> newShopMap = new HashMap<>();
68             newShopMap.put(shop, count);
69             cart.put(newShopMap, shop.getPrice() * count);
70             sum += shop.getPrice() * count;
71         }
72         session.setAttribute("shopcart", cart);
73         session.setAttribute("sum", sum);
74         req.getRequestDispatcher("cart.jsp").forward(req, resp);
75 
76      }
77 
78     @Override
79     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
80         //响应头部格式设置
81         resp.setCharacterEncoding("utf-8");
82         resp.setContentType("text/plain");
83 
84         List<Shop> ul = new ShopServiceImpl().getAllShopInfo();
85         req.setAttribute("shoplist", ul);
86         req.getRequestDispatcher("shopping.jsp").forward(req, resp);
87     }
88 }

修改后的Servlet

结论

在Key嵌套HashMap的HashMap，如果修改已经存放的Key的内容时，再用修改后的外层Key去获取外层HashMap的Value，是会报空指针异常的。但是如果不是HashMap嵌套，这样做是不会出异常，以上结论仅根据做的简单的测试得出的，如有错误，望不吝赐教。

package servlet;

import model.Good;
import model.Shop;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

/**
 * TODO
 *
 * @Author Whyat
 * @Date 2019/1/10 22:05
 */
public class test {
    public static void main(String[] args) {
        //test1();
        test2();
    }

    /**
     *  修改map外面的key的内容内层的key的内容也会改变，
     *  再用修改后的key去获得之前的value是可以的,
     *  而且可以覆盖之前的键值对
     */
    private static void test1() {
        Map<Shop, Integer> map = new HashMap<>();
        Shop shop = new Shop(2, 3, "code", "name", "type");
        map.put(shop, 3);
        shop.setSid(4);
        int i = map.get(shop);
        Set<Shop> set = map.keySet();
        for (Shop shop1 : set) {
            System.out.println(shop1);
        }
        map.put(shop, 4);
        System.out.println("...");
    }

    /**
     * 修改外层的key，根据key获取外层的value会出现空指针异常
     */
    private static void test2() {
        Map<Map<Good,Integer>, Integer> bigMap = new HashMap<>();
        Map<Good,Integer> smallMap = new HashMap<>();
        Good good = new Good(1, "name", "class", 10, "code");

        smallMap.put(good, 5);
        bigMap.put(smallMap, 100);
        //修改了内层小map的内容
        smallMap.put(good, 6);
        //大map再根据小map获取之前大map的value报空指针异常
        int i = bigMap.get(smallMap);
        bigMap.put(smallMap, i + 10);
        System.out.println("...");
    }
}