In the last line of this block, you should use resultStr not responseString.
You are also not logging most of the exceptions in the doInBackground
method.
Get into the habit of doing Log.e(TAG,"description of error....",exception)
instead of using e.printstackTrace()
or e.toString()
protected void onPostExecute(String resultStr) {
try {
JSONObject json = new JSONObject(responseString);