Java程序Map.get返回null触发空指针异常修复方案

程序运行抛出的异常如下：

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Map.equals(Object)" because the return value of "java.util.Map.get(Object)" is null
at week21.03.Search2(03.java:69)
at week21.03.main(03.java:48)

抛出空指针的直接原因是Search2方法的Map操作逻辑完全错误，具体问题点如下：

• 外层Final是Map<String,Map<String,String>>类型，它的key是存储时传入的公司名称（字符串类型），不存在整数类型的key。原代码用for循环遍历整数i，再调用Final.get(i)永远拿不到对应值，只会返回null。
• 原代码直接在外层Map上调用Final.get("Name")/Final.get("Mail")这类方法完全错误："Name"、"Mail"是内层每个名片Map的key，外层Map根本没有这些key，调用get必然返回null，后续直接调用equals()方法就触发了空指针。
• 代码还存在其他隐藏bug：
  • 创建了两个Scanner对象读取同一个标准输入流，且调用nextInt()后没有处理残留的换行符，会导致后续nextLine()读取到空字符串，输入逻辑错位
  • 存储手机号时内层Map用的key是"Number"，搜索时却写的"Phone number"，key不匹配会导致手机号永远搜不到
  • Search方法没有判断公司名不存在的场景，用户输入不存在的公司名时同样会触发空指针
  • 定义的data1类全程没有被使用，属于冗余代码

按以下步骤修改即可解决问题：

• 重写Search2的遍历逻辑：HashMap没有整数索引，要遍历外层Map的entrySet拿到每一条公司-名片的键值对，再从内层名片Map中匹配字段
• 统一字段key的命名，保证存储和搜索时的key完全一致
• 所有Map取值后先做非空判断，key不存在时给出提示，不要直接调用对象方法
• 只保留一个Scanner实例，每次调用nextInt()后追加一次nextLine()吞掉输入流里残留的换行符，避免输入错位
• 删除未使用的冗余data1类

package week21;

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class _03_ {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int o1 = 0;
        Map<String, Map<String, String>> finalMap = new HashMap<>();
        while (o1 != 3) {
            System.out.println("What would you like to do:");
            System.out.println("1. Add");
            System.out.println("2. Search");
            System.out.println("3. Exit");
            o1 = scanner.nextInt();
            scanner.nextLine(); // 吞掉nextInt残留的换行
            if(o1 == 1){
                Map<String,String> save = new HashMap<>();
                System.out.println("Name of owner:");
                String name = scanner.nextLine();
                save.put("Name", name);
                System.out.println("Email:");
                String mail = scanner.nextLine();
                save.put("Mail", mail);
                System.out.println("Phone number:");
                String nr = scanner.nextLine();
                save.put("Number", nr);
                System.out.println("Address:");
                String address = scanner.nextLine();
                save.put("Address", address);
                System.out.println("Name of company:");
                String cName = scanner.nextLine();
                finalMap.put(cName, save);
            } else if (o1 == 2) {
                System.out.println("What would you like to do:");
                System.out.println("1. A piece of information");
                System.out.println("2. Everything about a company");
                System.out.println("3. Everything");
                int o = scanner.nextInt();
                scanner.nextLine();
                if (o == 1) {
                    searchSingleField(finalMap, scanner);
                } else if (o == 2) {
                    searchByKeyword(finalMap, scanner);
                } else if (o == 3) {
                    searchAll(finalMap);
                }
            }
        }
        scanner.close();
    }

    public static void searchSingleField (Map<String, Map<String, String>> finalMap, Scanner scanner){
        System.out.println("Name of the company");
        String companyName = scanner.nextLine();
        Map<String, String> card = finalMap.get(companyName);
        if (card == null) {
            System.out.println("Company not exist");
            return;
        }
        System.out.println("What information would you like: (Name/Mail/Number/Address)");
        String field = scanner.nextLine();
        String value = card.get(field);
        System.out.println(value == null ? "Field not exist" : value);
    }

    public static void searchByKeyword (Map<String, Map<String, String>> finalMap, Scanner scanner){
        System.out.println("What do you know:");
        String keyword = scanner.nextLine();
        int matchCount = 0;
        // 正确遍历外层Map的所有条目
        for (Map.Entry<String, Map<String, String>> entry : finalMap.entrySet()) {
            String companyName = entry.getKey();
            Map<String, String> card = entry.getValue();
            // 匹配内层Map的所有字段
            if (keyword.equals(card.get("Name"))
                    || keyword.equals(card.get("Mail"))
                    || keyword.equals(card.get("Address"))
                    || keyword.equals(card.get("Number"))) {
                System.out.println("Company: " + companyName + ", Info: " + card);
                matchCount++;
            }
        }
        if(matchCount == 0){
            System.out.println("It doesn't exist");
        }
    }

    public static void searchAll (Map<String, Map<String, String>> finalMap){
        for (Map.Entry<String, Map<String, String>> entry : finalMap.entrySet()) {
            System.out.println("Company: " + entry.getKey() + ", Info: " + entry.getValue());
        }
    }
}

内容的提问来源于stack exchange，提问作者Computerdude123