Java Gson JSON: Serializing an Object which contains a List>

Question

I am currently trying serialze and correctly deserialize an Object containing a List with tuples using googles Gson. I have found similar questions here here and here but I wasnt able to adjust their solutions to my problem, because my List is in an Object.

Here a quick example:

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
}

Thank you for your help.

Map.Entry instances are supposed to be entries for Map. They are available via iterator. So if you want to serialize/deserialize you need to iterate entry by entry. — memoricab, May 20 '21 at 14:02

score 0 · Answer 1 · answered May 20 '21 at 14:10

0

Then you need to create new city and set streets. Because basically when you deserialize, it does not correspond to your City object data structure.

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

answered May 20 '21 at 14:10

memoricab

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1
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score 0 · Accepted Answer · answered May 20 '21 at 15:21

You've simply confused GSON when you rearranged the type that streets holds. You've created a HashMap, but then type streets as a Map.Entry.

If you simply type streets as a HashMap, its native type, then everything works just fine.

Two changes:

use List.of(cityMap); to set streets with a List of HashMap
change the declaration of streets to be List<HashMap<String,Integer>> streets;

Those two simple changes fix it for you:

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;
}

Produces this 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}]

Your solution works. Thank you! So a simple advice would be to use Map.Entry only for lokal iterations over Map-entries and not for setting List-values? — Luca Herrmann, May 20 '21 at 16:29

Java Gson JSON: Serializing an Object which contains a List>

2 Answers2