How to serialize a JSON with `null` values in Kotlin using the Gson Library

serialize-nulls.kt

/**
 * the first example uses Kotlin's data classes to define the structure
 * this is the suitable approach if the classes are already defined
 */

data class UserData(
  var user_id: String? = null,
  var user_name: String? = null
) {}

class MainActivity : AppCompatActivity() {
  val gson = GsonBuilder().serializeNulls().create();

  // serializeNulls() method will ensure that the properties with null values
  // are included in the serialized JSON String
  
  val jsonStr = gson.toJson(UserData("user_123456", null)).toString();
  Log.i("json", jsonStr);
  // {"user_id":"user_123456","user_name":null}
}




/**
 * the second example uses Gson's JsonObject class. this should be your go-to
 * approach is you are building the object on-the-fly
 */

class MainActivity : AppCompatActivity() {
  val gson = GsonBuilder().serializeNulls().create();

  // serializeNulls() method will ensure that the properties with null values
  // are included in the serialized JSON String
  
  val jsonObject = JsonObject();

  jsonObject.add("user_name", JsonNull.INSTANCE);
  jsonObject.addProperty("user_id", "user_123456");

  val jsonStr = gson.toJson(jsonObject).toString();

  Log.i("json", jsonStr);
  // {"user_name":null,"user_id":"user_123456"}
}