add interface ApiInterface

 package network;

import retrofit2.Call;

import retrofit2.http.Body;

import retrofit2.http.Headers;

import retrofit2.http.POST;

import retrofit2.http.Query;

public interface ApiInterface

{

    String GET_CAT="readc.php";

   @Headers({"Content-Type: application/json","Accept: application/json"})

    @POST(GET_CAT)

    Call<contact> getdata();

}