Retrofit singleton

activity.java

LoginService loginService = RetrofitApi.getInstance().getLoginService();
SendUser sendUser = new SendUser(loginEditText.getText().toString(), passwordEditText.getText().toString());
Call<User> userCall = loginService.getUser(sendUser);
userCall.enqueue(new Callback<User>() {
    @Override
    public void onResponse(Call<User> call, Response<User> response) {
        if (response.isSuccessful()) {

        } else {
            Log.d(TAG, "Response error " + response.raw().toString());
        }
    }

    @Override
    public void onFailure(Call<User> call, Throwable t) {
        Log.d(TAG, "Error " + t.getMessage());
    }
});

RetrofitApi.java

import java.util.concurrent.TimeUnit;

import okhttp3.OkHttpClient;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
import ua.net.lsoft.casinoprovision.server.interfaces.DealersService;
import ua.net.lsoft.casinoprovision.server.interfaces.LoginService;
import ua.net.lsoft.casinoprovision.server.interfaces.PlayersService;

public class RetrofitApi {

    private static RetrofitApi instance;
    public static final String BASE_URL = "";

    private LoginService loginService;

    public static RetrofitApi getInstance() {
        if (instance == null) {
            instance = new RetrofitApi();
        }

        return instance;
    }

    private RetrofitApi() {
        buildRetrofit(BASE_URL);
    }

    private void buildRetrofit(String url) {

        OkHttpClient client = new OkHttpClient.Builder()
                .connectTimeout(10, TimeUnit.SECONDS)
                .readTimeout(30, TimeUnit.SECONDS).build();

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(url)
                .addConverterFactory(GsonConverterFactory.create())
                .client(client)
                .build();

        loginService = retrofit.create(LoginService.class);
    }

    public LoginService getLoginService() {
        return loginService;
    }
}