如何在文本文件中保存HashMap？

import java.io.*;
import java.util.HashMap;
import com.google.gson.Gson;

public class HashMapFile {
    public static void main(String[] args) {
        // 创建一个HashMap对象
        HashMap<String, String> hashMap = new HashMap<>();
        hashMap.put("key1", "value1");
        hashMap.put("key2", "value2");
        hashMap.put("key3", "value3");

        // 将HashMap转换为字符串
        Gson gson = new Gson();
        String jsonString = gson.toJson(hashMap);

        // 将字符串保存到文本文件
        try {
            FileWriter fileWriter = new FileWriter("hashmap.txt");
            BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
            bufferedWriter.write(jsonString);
            bufferedWriter.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 从文本文件中读取字符串
        String readString = "";
        try {
            FileReader fileReader = new FileReader("hashmap.txt");
            BufferedReader bufferedReader = new BufferedReader(fileReader);
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                readString += line;
            }
            bufferedReader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 将字符串转换为HashMap
        HashMap<String, String> restoredHashMap = gson.fromJson(readString, HashMap.class);

        // 打印恢复后的HashMap
        for (String key : restoredHashMap.keySet()) {
            System.out.println("Key: " + key + ", Value: " + restoredHashMap.get(key));
        }
    }
}