How can I sum the data in the nested Room database?

Question

This will probably be the simplest question on SO so first of all I apologize but I couldn't establish the answer so I had to ask it here.

Suppose you have a flowing list like this. I want to sum the dayId0 values, then dayId1 and print them on the upper layer in Room. For example, dayId0 will continue as food_kcal_sum=350 and dayId1 food_kcal_sum=45. How should I create a loop for this?

[
   {
       "dayId": 0,
       "food_kcal": 270.0,
       "id": 1
   },
   {
       "dayId": 0,
       "food_kcal": 60.0,
       "id": 2
   },
   {
       "dayId": 0,
       "food_kcal": 20.0,
       "id": 3
   },
   {
       "dayId": 1,
       "food_kcal": 15.0,
       "id": 4
   },
   {
       "dayId": 1,
       "food_kcal": 30.0,
       "id": 5
   }
]

Answer 1

I made an attempt using Gson.

public class DriverClass {

Map<Integer, Integer> mapValues = new HashMap<>();

public static void main(String[] args) {

    Map<Integer, Integer> map = new DriverClass().sumValues();
    map.entrySet().forEach(i -> System.out.println(i.getKey() + " " + i.getValue()));

}

private Map<Integer, Integer> sumValues() {
    Gson gson = new Gson();
    JsonArray jsonArray = gson.fromJson(myJson(), JsonArray.class);
    for (int i = 0; i < jsonArray.size(); i++) {
        JsonObject jsonObject = jsonArray.get(i).getAsJsonObject();
        JsonElement jsonElementDayId = jsonObject.get("dayId");
        JsonElement jsonElementFoodCal = jsonObject.get("food_kcal");
        calculateValues(jsonElementDayId.getAsInt(), jsonElementFoodCal.getAsInt());

    }
    return mapValues;
}

private void calculateValues(int jsonElementDayId, int jsonElementFoodCal) {
    if (mapValues.containsKey(jsonElementDayId)) {
        mapValues.put(jsonElementDayId, mapValues.get(jsonElementDayId) + jsonElementFoodCal);
    } else {
        mapValues.put(jsonElementDayId, jsonElementFoodCal);
    }
}

public static String myJson() {
    return """
            [
                {
                    "dayId": 0,
                        "food_kcal": 270.0,
                        "id": 1
                },
                {
                    "dayId": 0,
                        "food_kcal": 60.0,
                        "id": 2
                },
                {
                    "dayId": 0,
                        "food_kcal": 20.0,
                        "id": 3
                },
                {
                    "dayId": 1,
                        "food_kcal": 15.0,
                        "id": 4
                },
                {
                    "dayId": 1,
                        "food_kcal": 30.0,
                        "id": 5
                }
            ]
        """;
}

}

outputs:

0 350
1 45