NetworkOnMainThreadException occured even having AsyncTask - android-asynctask

Question

I am new to Android app development. I have read several posts which suggest that I have to use AsyncTask to get into a PHP file and edit a database.
Here is my class with AsyncTask.
private class urlEditTask extends AsyncTask<Void, Void, Void>{
final String propName;
final String roomLoc;
final String roomType;
public urlEditTask(final String propName, final String roomLoc, final String roomType){
super();
this.propName = propName;
this.roomLoc = roomLoc;
this.roomType = roomType;
}
#Override
protected Void doInBackground(Void... params) {
editDB(propName, roomLoc, roomType);
return null;
}
protected void onPostExecute(Void... params) {
Toast.makeText(getApplicationContext(), "Loading....", Toast.LENGTH_LONG).show();
}
private void editDB(final String propName, final String roomLoc, final String roomType){
String name = etName.getText().toString();
String contact = etContact.getText().toString();
String email = etEmail.getText().toString();
String rentStart = etRentStart.getText().toString();
String rentEnd = etRentEnd.getText().toString();
List<NameValuePair> nameValuePair = new ArrayList<NameValuePair>(1);
nameValuePair.add(new BasicNameValuePair("name", name));
nameValuePair.add(new BasicNameValuePair("propName", propName));
nameValuePair.add(new BasicNameValuePair("roomLoc", roomLoc));
nameValuePair.add(new BasicNameValuePair("roomType", roomType));
nameValuePair.add(new BasicNameValuePair("contact", contact));
nameValuePair.add(new BasicNameValuePair("email", email));
nameValuePair.add(new BasicNameValuePair("rentStart", rentStart));
nameValuePair.add(new BasicNameValuePair("rentEnd", rentEnd));
try {
DefaultHttpClient httpClient = new DefaultHttpClient();
HttpPost httpPost = new HttpPost("http://hoome.hk/EditDB.php");
httpPost.setEntity(new UrlEncodedFormEntity(nameValuePair));
HttpResponse response = httpClient.execute(httpPost);
HttpEntity entity = response.getEntity();
InputStream is = entity.getContent();
String msg = "Data entered successfully";
Toast.makeText(getApplicationContext(), msg, Toast.LENGTH_LONG).show();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (ClientProtocolException e) {
Log.e("ClientProtocol", "Log_tag");
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}
I want the task to be executed after I clicked a button.
Here is my code for execution.
Button confirmEdit = (Button) findViewByID(R.id.confirmEdit);
confirmEdit.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
new urlEditTask(itemValue1, itemValue2, itemValue3).execute();
}
});
Probably, I don't quite understand how AsyncTask works.
May I know why I get NetworkOnMainThreadException with the above codes?
Here is the log:
07-20 04:14:01.480 2588-2603/hk.hoome.www.contracthandler E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1
Process: hk.hoome.www.contracthandler, PID: 2588
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:300)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
at java.util.concurrent.FutureTask.run(FutureTask.java:242)
at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
at android.os.Handler.<init>(Handler.java:200)
at android.os.Handler.<init>(Handler.java:114)
at android.widget.Toast$TN.<init>(Toast.java:336)
at android.widget.Toast.<init>(Toast.java:100)
at android.widget.Toast.makeText(Toast.java:250)
at hk.hoome.www.contracthandler.MainActivity$urlEditTask.doInBackground(MainActivity.java:443)
at hk.hoome.www.contracthandler.MainActivity$urlEditTask.doInBackground(MainActivity.java:399)
at android.os.AsyncTask$2.call(AsyncTask.java:288)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)    
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)