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How to return a value from a callback function

I want to return a String from a callback
My class which fetch date from server in a thread and i want this value in main thread as a String. I am beginner in Java.
public class InternetDate {
private final Activity activity;
private String finalDate = "";
public InternetDate(Activity activity) {
this.activity = activity;
}
public void setDateAndTimeFormat(String dateAndTimeFormat) {
mDateAndTimeFormat = dateAndTimeFormat;
}
public void getCurrentDate(OnGetDate onGetDate) {
new BackgroundTask(activity) {
#Override
public void doInBackground() {
try {
finalDate = getCurrentDateFromInternet();
} catch (Exception e) {
Log.e(TAG, e.getMessage());
}
}
#Override
public void onPostExecute() {
try {
JSONObject jb = new JSONObject(finalDate);
String name = jb.getString("UnixTimeStamp");
onGetDate.onSuccess(name);
if (progressDialog.isShowing()) {
progressDialog.dismiss();
}
} catch (JSONException e) {
}
}
}.execute();
}
private String getCurrentDateFromInternet() throws Exception {
String date_api = example.com/api;
URL url = new URL(date_api);
BufferedReader in = null;
try {
in = new BufferedReader(new InputStreamReader(url.openStream()));
return in.readLine();
} finally {
if (in != null) {
try {
in.close();
} catch (IOException e) {
Log.e(TAG, e.getMessage());
}
}
}
}
public interface OnGetDate {
void onSuccess(String date);
}
I want this as a String. Please help me to archive this String in MainThread
private String getDate(){
String currentDate = "";
InternetDate internetDate = new InternetDate(this);
internetDate.getCurrentDate(new InternetDate.OnGetDate() {
#Override
public void onSuccess(String date) {
currentDate = date; // Null return
}
});
return currentDate;
}

You might wait for the response using a semaphore, but this kind of code is blocking by nature and leads to apps with a poor user experience, because the ui thread is blocked during the whole process
//import java.util.concurrent.*;
//[...]
private String getDate() throws TimeoutException {
final String[] result = new String[]{null};
final Semaphore sem = new Semaphore(0);
new InternetDate(this).getCurrentDate(new InternetDate.OnGetDate() {
#Override
public void onSuccess(String date) {
result[0] = date; // Null return
sem.release();
}
});
try {
if (sem.tryAcquire(10, TimeUnit.SECONDS)) {
return result[0];
} else {
throw new TimeoutException("no response after 10 seconds");
}
} catch(InterruptedException e) {
return null;
}
}

You can use Executors :
private String getDate() throws ExecutionException, InterruptedException {
// This line is non-blocking:
Future<String> future = Executors.newCachedThreadPool()
.submit(() -> new InternetDate().getCurrentDateFromInternet());
// The invocation of 'get' is blocking:
return future.get();
}
I assume getCurrentDateFromInternet returns the date in the format you want.

Your problem can be solved using a MutableLiveData object without blocking the UI thread.
Let's start assuming you have a main class named MainThread where your getDate method lives.
In this class first create the following MutableLiveData object:
private MutableLiveData<String> date = new MutableLiveData<>();
The object above will be updated with the date value as soon as it's available to your program.
Next create/update the method that'll make a call to the getDate method (which we'll keep for simplicity's sake):
private void exampleDateMethod() {
// first create an observer; in the observer you put the code that does something with the Date
date.observe(this, new Observer<String>() {
#Override
public void onChanged( String date ) {
// this is where you do something with the date, an example:
findViewById( R.id.date_view ).setText( date );
}
});
// pass the MutableLiveData to the getDate method so that its value can be updated:
getDate( date );
}
And change the getDate method to pass the Date to the MutableLiveData object:
private void getDate( MutableLiveData<String> liveDate ){
internetDate.getCurrentDate(new InternetDate.OnGetDate() {
#Override
public void onSuccess(String date) {
// set the value of the MutableLiveData object, this will notify the observer and execute the code in its onChanged method
liveData.setValue( date );
}
});
}

Retrofit Task (Execute) Called from another function doesn't wait until Task finish

From my Login Activity (First Activity Opened) I always do a check if the token is still active on my server which is done through Async Task that does API call to server.
here's the code from LoginActivity :
private void checkIfAuthenticated(){
SharedPreferences reader_auth = getSharedPreferences(getString(R.string.auth_preferences), MODE_PRIVATE);
String auth_key = reader_auth.getString(getString(R.string.auth_access_key),null);
String mobile_token = reader_auth.getString(getString(R.string.auth_mobile_token),null);
if (auth_key != null) {
//THIS PART RUNS THE TOKEN CHECK TO SERVER
authGlobal = new AuthenticationGlobal(this);
// I WANT THIS FUNCTION TO FINISH FIRST BEFORE IT GOES TO THE NEXT PART OF THE CODE
authGlobal.runAuthenticationCheck(auth_key,mobile_token);
String Auth_Key = reader_auth.getString(getString(R.string.auth_access_key),null);
Log.d("Auth Key Check 0",Auth_Key);
if (Auth_Key != null) {
Log.d("Auth Key Check 1",Auth_Key);
MoveToDashboardActivity();
}
}
}
The runAuthenticationCheck(String,String) Code is located on another class (Because it was meant to be a global function which can be called from any function on any activity)
runAuthenticationCheck is located in AuthenticationGlobal Class, here's the code :
public void runAuthenticationCheck (String mobile_token, String Access_token) {
checkAuthTask = new checkAuthenticationTask(mobile_token, Access_token);
checkAuthTask.execute((Void) null);
}
public class checkAuthenticationTask extends AsyncTask<Void, Void, Boolean> {
private GetDataService service;
private String mobile_token;
private String access_token;
checkAuthenticationTask( String Access_token,String Mobile_token) {
/*Create handle for the RetrofitInstance interface*/
mobile_token = Mobile_token;
access_token = Access_token;
service = RetrofitClientInstance.getRetrofitInstance().create(GetDataService.class);
}
#Override
protected Boolean doInBackground(Void... params) {
// TODO: attempt authentication against a network service.
try {
Call<CheckAuthenticationResponse> call = service.checkAuthentication(access_token,mobile_token);
Response<CheckAuthenticationResponse> CheckAuthenticationResponse = call.execute();
if (CheckAuthenticationResponse.code() == 200){
} else{
//clear shared preferences
clearAuthentication();
Log.e("AuthKey Global","Expired0");
}
} catch (IOException ea) {
clearAuthentication();
Log.e("AuthKey Global","Expired1");
Log.e("AuthenticationResponseError Global","Network Went Wrong");
ea.printStackTrace();
}
return true;
}
#Override
protected void onPostExecute(final Boolean success) {
//mAuthTask = null;
//showProgress(false);
if (success) {
Log.e("AuthKey Global","Done");
} else {
// mPasswordView.setError(getString(R.string.error_incorrect_password));
clearAuthentication();
Log.e("AuthKey Global","Expired2");
}
}
#Override
protected void onCancelled() {
//mAuthTask = null;
//showProgress(false);
}
There are 2 Class / Activity : "LoginActivity" and "AuthenticationGlobal".
There are 3 Function :
checkIfAuthenticated => located in LoginActivity, Which in turn actually call another function from another class (Function number 2 : "runAuthenticationCheck")
runAuthenticationCheck => located in AuthenticationGlobal. which in calls a AsyncTask via .execute(...) command.
checkAuthenticationTask => located in AuthenticationGlobal. Which actually does the API Call to server.
From "LoginActivity" I run a function "checkIfAuthenticated" => which calls function "runAuthenticationCheck" located at "AuthenticationGlobal" => which runs a Task "checkAuthenticationTask" which does API Call to server and does stuff.
The problem is, when I called the first Function, the code doesn't wait until the function "checkIfAuthenticated" / "checkAuthenticationTask" is done. Is there a way for me to make the app wait until the task / function finish first??
Thank you
UPDATE :
I ONLY NEED TO ADD .get() at the end of .execute() and wrap it inside try catch.
public void runAuthenticationCheck (String mobile_token, String Access_token) {
checkAuthTask = new checkAuthenticationTask(mobile_token, Access_token);
try {
checkAuthTask.execute((Void) null).get();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

Well. I just need to add a .get() on the execute() and wrap it inside a try catch.
A dumb mistake.
here's the updated code :
public void runAuthenticationCheck (String mobile_token, String Access_token) {
checkAuthTask = new checkAuthenticationTask(mobile_token, Access_token);
try {
checkAuthTask.execute((Void) null).get();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

AsyncTask: Pass an exception value from doInBackground to onPostExecute

i am trying to pass an error message between my doInBackground into my onPostExecute. Would there be any way to do this using data structures? In my situation i'm am trying to avoid making a class and returning an object
#Override
protected User doInBackground(List<Pair<String, String>>... params) {
randomAPI api = randomAPI
.getInstance(context);
try {
**thing that will generate error**
}
} catch (RandomException e) {
Log.e(TAG, "Error", e);
}
return somethingThatIsUsableInOnPostExecute;
}
#Override
protected void onPostExecute(User result) {
}

private String exception = "";
#Override
protected User doInBackground(List<Pair<String, String>>... params) {
randomAPI api = randomAPI
.getInstance(context);
try {
**thing that will generate error**
}
} catch (RandomException e) {
Log.e(TAG, "Error", e);
exception = e.getMessage();
}
return somethingThatIsUsableInOnPostExecute;
}
#Override
protected void onPostExecute(User result) {
if(!exception.isEmpty()){
//exception path
}
}

you should try to change return type of doInBackground from
User object to Object and cast it in onPostExecute.

Jsoup.connect().get() takes only part of html file on Android

So, I try to parse Wikipedia, and my code works well at computer.
All, what I changed - .connect().get is in AsyncTask, but I get only part of html file (no "body", only half of second "script" in "title") and I can't understand why.
This is my code example for Android.
protected String doInBackground(String... params) {
try {
Document doc = Jsoup.connect(params[0]).get();
return doc.toString();
} catch (IOException e) {
//...
e.printStackTrace();
}
return null;
}
And this is simple.
String url = "https://en.wikipedia.org/wiki/Protectorate";
Document doc = null;
try {
doc = Jsoup.connect(url).get();
} catch (IOException e) {
//...
e.printStackTrace();
}
I checked, params[0] is https://en.wikipedia.org/wiki/Protectorate, here's no mistake.
If you need some extra information, I will give it, of course.

Logcat fools us here, since it shortens the message (I assume you checked your string with logcat? See related question)
If you split your result string into chunks, you will see that the whole page was loaded. Try adding something like this logAll function to your AsyncTask class to see the full output:
private class DownloadTask extends AsyncTask<String, Integer, String> {
Document doc = null;
protected String doInBackground(String... params) {
try {
doc = Jsoup.connect(params[0]).get();
return doc.toString();
} catch (Exception e) {
e.printStackTrace();
}
return doc.toString();
}
#Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
logAll("async",doc.toString());
}
void logAll(String TAG, String longString) {
int splitSize = 300;
if (longString.length() > splitSize) {
int index = 0;
while (index < longString.length()-splitSize) {
Log.e(TAG, longString.substring(index, index + splitSize));
index += splitSize;
}
Log.e(TAG, longString.substring(index, longString.length()));
} else {
Log.e(TAG, longString.toString());
}
}
}

Android getting a result from an asynctask from within and asynctask

I have two AsyncTasks running and the async task that is waiting for the result is just not getting the correct result.
I have a network class that runs like so:
public ArrayList<User> searchForFriends(String activeHash, TelephoneNumber telephone)
{
Object[] obj = {activeHash, telephone};
try
{
return new SearchForFriendsTelephone().execute(obj).get(Constants.TIMEOUT_TIME, Constants.TIMEOUT_UNIT);
}
catch (InterruptedException e)
{
return null;
}
catch (ExecutionException e)
{
return null;
}
catch (TimeoutException e)
{
return null;
}
}
private class SearchForFriendsTelephone extends AsyncTask<Object, Void, ArrayList<User>>
{
#Override
protected ArrayList<User> doInBackground(Object... searchTelephone)
{
if (config.getNetworkVersion() == config.NETWORK_PROTOCOL_VERSION_1)
{
TelephoneNumber tel = (TelephoneNumber) searchTelephone[1];
List<NameValuePair> params = new ArrayList<NameValuePair>(3);
params.add(new BasicNameValuePair(NetworkConfig.POST_ACTIVE_HASH, (String) searchTelephone[0]));
params.add(new BasicNameValuePair(NetworkConfig.POST_MOBILE_NUMBER_COUNTRY_CODE, tel.getCountryCode()));
params.add(new BasicNameValuePair(NetworkConfig.POST_MOBILE_NUMBER_RAW, tel.getNumberRaw()));
ServerCommunication csc = new ServerCommunication();
JSONObject jsonFoundFriends = csc.postToServer(config.getBaseUrl() + URL_FRIEND_SEARCH_MOBILE, params);
if (jsonFoundFriends == null || csc.networkError())
{
FriendNetworkCommunication.this.networkError = csc.getNetworkError();
return null;
}
return _processSearchFriends(jsonFoundFriends);
}
FriendNetworkCommunication.this.networkError = new NetworkError(NetworkLanguage.UNABLE_TO_PROCESS);
return null;
}
Anyway this works fine with no issues and pulls back the user/s. I know this as I tried the following code in the main ui thread and it populates a view just fine. When I call this code from another AsyncTask. I get a timeout error.
Code to all the searchForFriends code:
private class CompareNumbers extends AsyncTask<ArrayList<NameAndNumber>, Integer, Void>
{
#Override
protected Void doInBackground(ArrayList<NameAndNumber>... params)
{
for (NameAndNumber nameNumber : params[0])
{
try
{
FriendNetworkCommunication fnc = new FriendNetworkCommunication();
ArrayList<User> users = fnc.searchForFriends(CurrentUser.getInstance().getUserActiveHash(), new TelephoneNumber(String.valueOf(nameNumber.getNumber().getNationalNumber()), String.valueOf(nameNumber.getNumber().getCountryCode())));
if (users != null && users.size() == 1)
{
User u = users.get(0);
String[] s = nameNumber.getName().split(" ");
u.setFirstName(s[0]);
u.setLastName(s[1]);
((ArrayAdapter<User>) ((ListView) getView().findViewById(R.id.friend_add_fragment_search_cont_list)).getAdapter()).add(u);
}
}
catch (Exception e)
{
e.printStackTrace();
}
}
((ArrayAdapter<User>) ((ListView)getView().findViewById(R.id.friend_add_fragment_search_cont_list)).getAdapter()).notifyDataSetChanged();
return null;
}
}
Can I not run an asynctask that waits on another?
NOTE: This is all running in a fragment if this makes any difference?
NOTE2: The first Asynctask runs a network call and has to be run asynchronously and so I wanted it to be like this so if I wanted I could run it anywhere synchronously

try giving the .execute() of the second async task in the onpostexecute() of the first async task.

I have found the answer to my question and this is not possible.
A full answer can be found here:
Creating another AsyncTask inside of doInBackground