How to post array in retrofit android

As of today, running the Retrofit implementation 'com.squareup.retrofit2:retrofit:2.1.0'

This works perfectly...

@FormUrlEncoded
@POST("index.php?action=item")
Call<Reply> updateManyItem(@Header("Authorization") String auth_token, @Field("items[]") List<Integer> items, @Field("method") String method);

You can disregard the @Header and @Field("method") .... the main piece is @Field("items[]") List<Integer> items

This is what allows you to send the items. On the API side I am simply looking for an array of integers and this works perfectly.

see this example where i need to pass registration fields data as json request

@POST("magento2apidemo/rest/V1/customers")
Call<RegisterEntity> customerRegistration(@Body JsonObject registrationData);

here i have created registrationData is

private static JsonObject generateRegistrationRequest() {
        JSONObject jsonObject = new JSONObject();
        try {
            JSONObject subJsonObject = new JSONObject();
            subJsonObject.put("email", "[email protected]");
            subJsonObject.put("firstname", "abc");
            subJsonObject.put("lastname", "xyz");

            jsonObject.put("customer", subJsonObject);
            jsonObject.put("password", "password");

        } catch (JSONException e) {
            e.printStackTrace();
        }
        JsonParser jsonParser = new JsonParser();
        JsonObject gsonObject = (JsonObject) jsonParser.parse(jsonObject.toString());
        return gsonObject;
    }

@FormUrlEncoded
@POST("service_name") 
   void functionName(
        @FieldMap Map<String, String> learning_objective_uuids, @FieldMap Map<String, String> user_uuids, @Field("note") String note,
        Callback<CallBackClass> callback
    );

Better solution : Use arraylist.. Reference link : johnsonsu

@FormUrlEncoded
    @POST("service_name") 
       void functionName(
            @Field("learning_objective_uuids[]") ArrayList<String> learning_objective_uuids, @Field("user_uuids[]") ArrayList<String> user_uuids, @Field("note") String note,
            Callback<CallBackClass> callback
        );