Creating nested JSON objects with different formats in Java with GSON

Question

I'm 100% certain this has been asked a million times already, but I'm really unsure how to properly approach this. I haven't done a lot with JSON or serializing it yet.

Basically, this is what I want to create using GSON:

{
    "wrapper" : [
        {
            "content": "loremipsum",
            "positions": [0,3]
        },
        {
            "content": "foobar",
            "positions": [7]
        },
        {
            "content": "helloworld"
        }
    ]
}

Breaking it down, we've got a field for an array, containing objects which in themselves contains two fields, one of which maps to a string and the other to yet another array that can contain an unknown amount of integers or can be missing entirely.

I can't even begin to imagine how to get this result with GSON. So far my idea would be to have everything be in a Map<String, List<Map<String, Object>>> beast and convert it, but that Object bothers me because it could either be a String or a List in this particular case. There could be casts, but that sounds like a stupidy complex thing for something that would look easier if I even just manually typed it in a String.format() or similar.

Isn't there a simpler way of handling this stuff?

you can just use this two pojo: ´class Wrapper { public String content; public List<Integer> positions}´ and ´class Response { public List<Wrapper> wrapper}´ — db80
– db80, Commented Sep 23, 2019 at 19:35

Michał Krzywański · Accepted Answer · 2019-09-23 20:16:40Z

4

I would go with creating some POJO class for your Data :

class MyData {
    private List<Data> wrapper;

    //getters setters constructors
}

class Data {
    private String content;
    private List<Integer> positions;

    //getters setters constructors
}

And then for deserializing it :

Gson gson = new Gson();
String json = //json here
MyData myData = gson.fromJson(myJson, MyData.class);

And for serializing :

MyData myData = ...
String json = gson.toJson(myData);

Another way could be parsing this structure using JsonParser and access its elements :

JsonParser jsonParser = new JsonParser();
JsonElement jsonElement = jsonParser.parse(json);
JsonElement wrapperElement = jsonElement.getAsJsonObject().get("wrapper"); //access wrapper
JsonArray array = wrapperElement.getAsJsonArray(); // access array

edited Sep 23, 2019 at 20:16

answered Sep 23, 2019 at 19:38

Michał Krzywański

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2 Comments

Selbi Over a year ago

That's a lot of boilerplate... I don't need JSON anywhere else in the program, not sure if that's the best approach here. But it should work for a start!

Michał Krzywański Over a year ago

It all depends on what you want to achieve. For me working with POJO models in easier. Another approach would be to use JsonElement objects and traverse it that way.

asfmlr · Accepted Answer · 2019-09-23 19:39:18Z

0

you can use gson JsonElement as the contianer object that will contain JsonElements like JsonObject or JsonArray

https://static.javadoc.io/com.google.code.gson/gson/2.6.2/com/google/gson/JsonElement.html

answered Sep 23, 2019 at 19:39

asfmlr

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Comments

Eugen · Accepted Answer · 2019-09-23 19:52:11Z

0

It is better to use Pojo for formatting.

class Wrapper{
 private List<Data> data;
 // geters, seters
}
class Data{
 private String content;
 private List<Integer> positions;
 // geters, seters
}

For deserialization/serealization you can use Jackson

ObjectMapper mapper = new ObjectMapper();
Wrapper wriper = mapper.readValue(dataString, Wrapper.class);
String jsonString = mapper.writeValueAsString(wriper);

or Gson

Gson gson = new Gson();
Wrapper wriper = gson.fromJson(dataString, Wrapper.class);
String json = gson.toJson(wriper);

edited Sep 23, 2019 at 19:52

answered Sep 23, 2019 at 19:38

Eugen

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Comments

Craig D · Accepted Answer · 2022-05-26 04:05:50Z

0

Without POJO, this seems to solve the nested JSON solution using Gson

package com.test;

import com.google.gson.JsonObject;

public class Main {

    public static void main(String[] args) {
        
        JsonObject ex3 = new JsonObject();
        ex3.addProperty("example3", 30);
        
        JsonObject ex2 = new JsonObject();  
        ex2.add("example2", ex3.getAsJsonObject());
        
        JsonObject ex1 = new JsonObject();  
        ex1.add("example1", ex2.getAsJsonObject());
        
        System.out.println(ex1.toString());
    }       
}

The output is:

{"example1":{"example2":{"example3":30}}}

answered May 26, 2022 at 4:05

Craig D

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