MainActivity with Retrofit example

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
import retrofit2.http.Body;
import retrofit2.http.POST;

public class MainActivity extends AppCompatActivity {
    int token = 5926166;
    class User { // request
        public String action;
        public String nickname;

        public User(String action, String nickname) {
            this.action = action;
            this.nickname = nickname;
        }
    }
    class Registration { // response
        public String status;
        public int token;

        public Registration(String status, int token) {
            this.status = status;
            this.token = token;
        }
    }

    class FetchRequest { // request
        public String action = "fetch_cards";
        public int token;

        public FetchRequest(int token) {
            this.token = token;
        }
    }
    class FetchResponse { // response
        public String status;
        public Card[] cards;
    }
    class Card {
        int count, color, shape, fill;

        @Override
        public String toString() {
            return "{ color=" + color +", count=" + count + ", shape=" + shape + ", fill=" + fill + '}';
        }
    }

    interface JsonServer {
        @POST("/")
        Call<Registration> register(@Body User user);
        @POST("/")
        Call<FetchResponse> fetch_cards(@Body FetchRequest fetchRequest);
    }

    public void onClick(View v) {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://194.176.114.21:8050")
                .addConverterFactory(GsonConverterFactory.create())
                .build();
        JsonServer jsonServer = retrofit.create(JsonServer.class);

        String nickname = "Vasya123";
        Call<FetchResponse> call = jsonServer.fetch_cards(new FetchRequest(token));

        Callback<FetchResponse> callback = new Callback<FetchResponse>() {
            @Override
            public void onResponse(Call<FetchResponse> call, retrofit2.Response<FetchResponse> response) {
                FetchResponse fetchResponse = response.body();
                ArrayAdapter<Card> adapter = new ArrayAdapter<Card>(getApplicationContext(), R.layout.layout, fetchResponse.cards);
                ListView lv = (ListView) findViewById(R.id.cards);
                lv.setAdapter(adapter);

            }

            @Override
            public void onFailure(Call<FetchResponse> call, Throwable t) {

            }
        };
        call.enqueue(callback);
    }

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

}