Java Gson JSON：序列化包含列表的 Z497031794414A552435F901B15 <map.entry<...,...> &gt; </map.entry<...,...>

Question

我目前正在嘗試序列化並正確反序列化一個 Object，其中包含一個使用 googles Gson 的帶有元組的列表。 我在這里和這里發現了類似的問題，但我無法調整他們的解決方案來解決我的問題，因為我的列表在 Object 中。

這里有一個簡單的例子：

import com.google.gson.*;
import java.util.*;


public class SerializeTheCity  {

    public static void main(String[] args) {

        HashMap <String, Integer> cityMap = new HashMap <>();
        cityMap.put("Street A", 4);
        cityMap.put("Street B", 3);
        cityMap.put("Street C", 7);
        cityMap.put("Street D", 8);
        cityMap.put("Street E", 9);

        City someCity = new City();

        someCity.streets= new ArrayList<>();
        someCity.streets.addAll(cityMap.entrySet());
        System.out.println(someCity.streets.get(1).getValue()); //works fine, how do I serialize it?

        Gson gson = new Gson();


        String saveCity = gson.toJson(someCity);
        System.out.println(saveCity); //does not work (empty List)


        // here I tried to use a solution [link 1] that worked for a similar question.
        Gson gson2 = new Gson();
        JsonElement jsonTree = gson2.toJsonTree(cityMap, Map.class);
        JsonObject jsonObject = new JsonObject();
        jsonObject.add("city", jsonTree);
        System.out.println("city = "+jsonObject); //how do I deserialize this into an object of city?


       City thisCity = gson.fromJson(jsonObject, City.class);
       System.out.println("streets = "+thisCity.streets); // doesnt work

       //works like in [link 1]. But its not a city-object.
       HashMap <String, Integer> thisStreets = gson.fromJson(jsonObject.get("city"), HashMap.class); 
       System.out.println("this streets = "+thisStreets);
       System.out.println("is this street A?"+thisStreets.get("Street A"));
         // this works, but I would like having the streets
        //in a city object (I could build a new city object like "newCity.streets=thisStreets",
        //but perhaps you know a smarter solution)
    }
}
class City { 
    List<Map.Entry<String,Integer>> streets; //Street, HouseNumber
}

謝謝您的幫助。

Answer 1

然后你需要創建新城市並設置街道。 因為基本上當你反序列化的時候，它不對應你的 City object 數據結構。

City city = new City();
city.streets = new ArrayList();
city.streets.addAll(thisStreets);

Answer 2

當您重新排列streets持有的類型時，您只是混淆了 GSON。 您已經創建了 HashMap，但隨后將街道鍵入為 Map.Entry。

如果您只是簡單地將streets輸入為 HashMap，它的原生類型，那么一切正常。

兩個變化：

1. 使用List.of(cityMap); 使用 HashMap 列表設置streets
2. 將streets的聲明更改為List<HashMap<String,Integer>> streets;

這兩個簡單的更改為您解決了它：

public class Main {

  public static void main(String[] args) {

    HashMap < String, Integer > cityMap = new HashMap < > ();
    cityMap.put("Street A", 4);
    cityMap.put("Street B", 3);
    cityMap.put("Street C", 7);
    cityMap.put("Street D", 8);
    cityMap.put("Street E", 9);

    City someCity = new City();
    // Don't do this
    // someCity.streets = new ArrayList<>>();
    // someCity.streets.addAll(cityMap.entrySet());

    // Do this instead
    someCity.streets = List.of(cityMap);
    System.out.printf("Streets in City Object: %s%n", someCity.streets);

    Gson gson = new Gson();
    String saveCity = gson.toJson(someCity);
    System.out.printf("Serialized City (JSON): %s%n", saveCity);

    City otherCity = gson.fromJson(saveCity, City.class);
    System.out.printf("Streets in DESERIALIZED City Object: %s%n", someCity.streets);


  }
}
class City {
  // Use a HashMap
  List < HashMap < String, Integer >> streets;
}

產生這個 output：

Streets in City Object: [{Street E=9, Street D=8, Street C=7, Street B=3, Street A=4}]
Serialized City (JSON): {"streets":[{"Street E":9,"Street D":8,"Street C":7,"Street B":3,"Street A":4}]}
Streets in DESERIALIZED City Object: [{Street E=9, Street D=8, Street C=7, Street B=3, Street A=4}]