學習中遇到的HashMap問題_ZenDei技術網路在線

前言今天在老師教Session的時候，利用了Session可持久化保存伺服器端的特性嘗試做了一下用HashMap嵌套的購物車(沒有將購物車的商品信息保存在資料庫中)，之所以做的這麼麻煩是為了鞏固之前學習的Map知識和鍛煉邏輯能力，正好也在其中遇到了一個關於HashMap 的問題，在此做個小小的記錄 ...

前言

今天在老師教Session的時候，利用了Session可持久化保存伺服器端的特性嘗試做了一下用HashMap嵌套的購物車(沒有將購物車的商品信息保存在資料庫中)，之所以做的這麼麻煩是為了鞏固之前學習的Map知識和鍛煉邏輯能力，正好也在其中遇到了一個關於HashMap 的問題，在此做個小小的記錄，方便日後查看。

問題

伺服器端保存購物車商品信息用的是HashMap嵌套，內層HashMap存放的是商品和該商品的數量，內層的HashMap中只存放一組鍵值對，外層HashMap存放的是商品和該商品總價，根據頁面傳過來的商品id在資料庫中獲取到商品的信息,。再遍歷HashMap根據id判斷是否已經存在該商品，再針對不同情況進行處理，所遇到的問題是購物車存在頁面穿進來的該商品，那麼我如果修改內層Map的Integer(數量)，再修改外層HashMap的Value(該商品的總價),就會出現空指針異常，解決方案是先將商品總價保存起來，把內層HashMap從外層HashMap中remove掉，再修改商品數量，再將修改數量後的內層HashMap添加到外層HashMap中，代碼如下:

<%@ page import="java.util.List" pageEncoding="utf-8" %>
<%@ page import="model.Shop" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>購物</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>店鋪碼</th>
            <th>水果名</th>
            <th>價格</th>
            <th>類別</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()%>)">添加到購物車</a>
            </td>
        </tr>
        <%}%>
    </table>
</form>
</body>
<script>
    function addToCart(id) {
        var count = prompt('添加數量是多少個');
        $("#id").val(id);
        $("#count").val(count);
        if (confirm("確認添加?")) {
            $("#form").submit();
        }
    }
</script>
</html>

商品選擇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>價格</th>
        <th>類別</th>
        <th>數量/個</th>
        <th>小計/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>合計：</th><%=sum%> 元
</body>
</html>

購物車頁面

出錯代碼待添加

//此處寫的是Shop是因為數據表中給定的表名是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,總價>的大map的總價
52                 int oringinGoodSum = cart.get(shopIntMap);
53                 //去掉<商品,總價>的大map里的<商品,數量>小map
54                 cart.remove(shopIntMap);
55                 //修改<商品,數量>小map的數量
56                 shopIntMap.put(shopInSet, shopIntMap.get(shopInSet) + count);
57                 //再將修改商品數量後的<商品,數量>小map添加到<商品,總價>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         //響應頭部格式設置
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 }

修改後的Servlet

結論

在Key嵌套HashMap的HashMap，如果修改已經存放的Key的內容時，再用修改後的外層Key去獲取外層HashMap的Value，是會報空指針異常的。但是如果不是HashMap嵌套，這樣做是不會出異常，以上結論僅根據做的簡單的測試得出的，如有錯誤，望不吝賜教。

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的內容內層的key的內容也會改變，
     *  再用修改後的key去獲得之前的value是可以的,
     *  而且可以覆蓋之前的鍵值對
     */
    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("...");
    }

    /**
     * 修改外層的key，根據key獲取外層的value會出現空指針異常
     */
    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);
        //修改了內層小map的內容
        smallMap.put(good, 6);
        //大map再根據小map獲取之前大map的value報空指針異常
        int i = bigMap.get(smallMap);
        bigMap.put(smallMap, i + 10);
        System.out.println("...");
    }
}

測試