public static class getSessionId extends AsyncTask<Void, Void, String>
    {
        private String user;
        private String password;
        private LoginAttemptedCallback loginAttemptedCallback;

        public getSessionId(String user, String password, LoginAttemptedCallback loginAttemptedCallback)
        {
            this.loginAttemptedCallback = loginAttemptedCallback;
            this.user = user;
            this.password = password;
        }

        @Override
        protected String doInBackground(Void... params)
        {
            return getSessionId(user, password);
        }

        @Override
        protected void onPostExecute(String result)
        {
            Log.d("TAG", "Session id: " + result);

            try
            {
                JSONObject results = new JSONObject(result);
                String sessionId = results.getString("session_id");
                if (sessionId != null)
                {
                    loginAttemptedCallback.onLoginSuccess(user, password, sessionId);
                }
            } catch (JSONException e)
            {
                loginAttemptedCallback.onLoginFailed();
            }
        }
    }