Callback

public void isAuthorised(Callback callback) {
        if (mSettings.contains(APP_PREFERENCES_LOGIN) && mSettings.contains(APP_PREFERENCES_PASSWORD)) {

            final String login = mSettings.getString(APP_PREFERENCES_LOGIN, "");
            final String passwd = mSettings.getString(APP_PREFERENCES_PASSWORD, "");

            NetworkService.getInstance().getApi().getUserWithCreds(login, passwd).enqueue(new Callback<User>() {
                @Override
                public void onResponse(Call<User> call, Response<User> response) {
                    User u = response.body();
                    callback.onResult(u.isHasPermit());
                }

                @Override
                public void onFailure(Call<User> call, Throwable t) {
                    Log.e(LOG_TAG, t.getMessage(), t);
                    callback.onResult(false);
                }
            });
        } else {
            callback.onResult(false);
        }
        
        public interface Callback {
          void onResult(boolean authSuccessful);
        }