How to send Array of Objects in retrofit Android?

You don't need to convert your object to a JSONArray, Retrofit will do it automatically for you.

Simply change your API method declaration to:

@Headers({
        "Content-type: application/json"
})
Response getNearByParkingSpaces(@Header("Authorization") String accessToken,
                                    @Body List<String> arrayOfSids);

I encounter same issue solve this by adding this dependencies:

implementation 'com.squareup.retrofit2:converter-scalars:$version'

There are multiple existing Retrofit converters for various data formats. You can serialize and deserialize Java objects to JSON or XML or any other data format and vice versa. Within the available converters, you’ll also find a Retrofit Scalars Converter that does the job of parsing any Java primitive to be put within the request body. Conversion applies to both directions: requests and responses. https://futurestud.io/tutorials/retrofit-2-how-to-send-plain-text-request-body

then you can use your generateParkingspaceBody as value to post.

generateParkingspaceBody.toString()  as your request body 

If you want to upload array of object using Retrofit then follow the steps. It will work 100%. In my case I have 2 params one is userid and second is location_data. In second params I have to pass array of objects.

@FormUrlEncoded
@POST("api/send_array_data")
Call<StartResponseModal> postData(@Field("user_id") String user_id,
                                             @Field("location_data") String 
jsonObject);

then in your MainActivity.class.

 ArrayList<JSONObject> obj_arr;
 try {
                JSONArray jsonArray = new JSONArray();
                obj_arr = new ArrayList<>();
                 // LocationData is model class. 
                for (LocationData cart : arrayList) {
                    JSONObject jsonObject = new JSONObject();
                    jsonObject.put("latitude", cart.getLatitude());
                    jsonObject.put("longitude", cart.getLongitude());
                    jsonObject.put("address", cart.getAddress());
                    jsonObject.put("battery", cart.getBattery());
                    jsonObject.put("is_gps_on", cart.getIs_gps_on());
                    jsonObject.put("is_internet_on", cart.getIs_internet_on());
                    jsonObject.put("type", cart.getType());
                    jsonObject.put("date_time", cart.getDate_time());
                    jsonArray.put(jsonObject);
                    obj_arr.add(jsonObject);
                }
                Log.e("JSONArray", String.valueOf(jsonArray));
            } catch (JSONException jse) {
                jse.printStackTrace();
            }




 String user_id = SharedPreferenceUtils.getString(getActivity(), 
 Const.USER_ID);
    RetrofitAPI retrofitAPI = 
 APIClient.getRetrofitInstance().create(RetrofitAPI.class);
    Call<StartResponseModal> call = 
 retrofitAPI.postData(user_id,obj_arr.toString());
    call.enqueue(new Callback<StartResponseModal>() {
        @Override
        public void onResponse(Call<StartResponseModal> call, 
  Response<StartResponseModal> response) {
            // Handle success
            if (response.isSuccessful() && 
  response.body().getErrorCode().equals("0")) {
                // Process the response here
                Log.e("Hello Room data send","Success");
                deleteItemsFromDatabase();


            } else {
                // Handle API error
                Log.e("Hello Room data send","failed else");

            }
        }

        @Override
        public void onFailure(Call<StartResponseModal> call, Throwable t) {
            // Handle failure
            Log.e("Hello Room send","failure");

        }
    });