How do you set a value to null with com.google.gson.JsonObject in java?

Question

How do you set a value to null with com.google.gson.JsonObject in java? I can certainly read if a value is null by using isNull - but it seems like when I put null, it just ignores it:

JSONObject jsonObj = new JSONObject();
jsonobjToEra.addProperty("sun", Double.parseDouble(val));
jsonobjToEra.addProperty("mon", 22.333);
jsonobjToEra.addProperty("val", null); //ignoring it

Please note that I am using com.google.gson.JsonObject which is different from similar questions like How do you set a value to null with org.json.JSONObject in java? in other posts in this SO.

The JSON:

[
  {
    "sun": 1122435343453,
    "mon": 1460538600000,
    "val": 45.900001525878906
  },
  {
    "sun": 1460538600000,
    "mon": 1460538660000,
    "val": 45.900001525878906
  }
  ]

Answer 1

You can use com.google.gson.JsonNull.INSTANCE like this:

JsonObject jsonObj = ...
jsonObj.add("val", JsonNull.INSTANCE); 

Or if you want to complety remove value from serialization

jsonObj.remove("val");

Demo:

String json = "{\"a\": 1, \"b\": 1}";
JsonObject jsonObject = new Gson().fromJson(json, JsonObject.class);

System.out.println(jsonObject); // {"a":1,"b":1}

jsonObject.addProperty("a", 2);
jsonObject.add("b", JsonNull.INSTANCE);
System.out.println(jsonObject); // {"a":2,"b":null}

jsonObject.remove("a");
System.out.println(jsonObject); // {"b":null}

Answer 2

You need to use serializeNulls() of GsonBuilder. It enables Gson to serialize null fields.

EDIT

Usage of GsonBuilder

Gson gson = new GsonBuilder()
     .serializeNulls()
     .create();

Gson is made for converting Java objects to/from JSON. So you need to make a Data Structure or a class so that you can make an object of that and convert the whole object to json with null.

See this example

public class Country {

    String name;
    int population;
    private List<String> listOfStates;

    //getter and setter methods

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPopulation() {
        return population;
    }

    public void setPopulation(int population) {
        this.population = population;
    }

    public List<String> getListOfStates() {
        return listOfStates;
    }

    public void setListOfStates(List<String> listOfStates) {
        this.listOfStates = listOfStates;
    }

}

And when you use it:

Country countryObj=new Country();
  countryObj.setName("India");
  countryObj.setPopulation(1000000);
  List<String> listOfStates=new ArrayList<String>();
  listOfStates.add("Madhya Pradesh");
  listOfStates.add("Maharastra");
  listOfStates.add("Rajasthan");
  
  countryObj.setListOfStates(listOfStates);
  Gson gson = new GsonBuilder()
     .serializeNulls()
     .create();
  
  // convert java object to JSON format,
  // and returned as JSON formatted string
  String json = gson.toJson(countryObj);