Java Gson JSON: 序列化一个包含列表的对象<Map.Entry<...,...>>

Question

我目前正在尝试使用 googles Gson 序列化和正确反序列化一个包含元组列表的对象。我在这里发现了类似的问题 here and 但我无法针对我的问题调整他们的解决方案，因为我的列表在一个对象中。

这里有一个简单的例子：

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
}

感谢您的帮助。

Answer 1

然后你需要创建新城市并设置街道。因为基本上你反序列化的时候，是不对应你的City对象数据结构的。

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

Answer 2

当您重新排列 streets 持有的类型时，您只是混淆了 GSON。您已经创建了一个 HashMap，但随后将 streets 键入为 Map.Entry.

如果您简单地输入 streets 作为 HashMap，它的原生类型，那么一切正常。

两个变化：

使用 List.of(cityMap); 设置 streets 为 HashMap
将 streets 的声明更改为 List<HashMap<String,Integer>> streets;

这两个简单的更改为您解决了问题：

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;
}

产生这个输出：

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}]

Java Gson JSON: 序列化一个包含列表的对象<Map.Entry<...,...>>

Java Gson JSON: Serializing an Object which contains a List<Map.Entry<...,...>>

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