How can I get the value from the inside of a thread?

[Marin Capranov]

Hello!

I have created a new thread inside the onPostExecute method from the Async class. The thing is I want to return the value formed inside the thread. However, outside of the thread in the onPostExecute method, the value is not recognised. I want to call a method that returns the polylines, but without the value from the thread, I can't make it work. Also when I try to call the method inside the thread it gives an error : com.google.maps.api.android.lib6.common.apiexception.c: Not on the main thread. So I guess if I get the value from the thread and use it outside it won't give the error anymore and I will be able to finish my work.

@Override
    protected void onPostExecute(String s) {

        Parser parser = new Parser();
        MapsActivity mapsActivity = new MapsActivity();

        List<HashMap<String, String>> trueUrl;

        trueUrl = parser.parseTest(s);

        String origin = trueUrl.get(0).toString()
                .replace("{", "")
                .replace("}","")
                .replace("=", ":");

        String destination = trueUrl.get(0).toString()
                .replace("{", "")
                .replace("}","")
                .replace("=", ":");

        distUrl = mapsActivity.getDistanceUrl(origin,destination,mapsActivity.getPlacesId(trueUrl));

        Log.i("outerUrl", "onClick: " + distUrl + " " + trueUrl.get(0).toString());

        Thread thread = new Thread(new Runnable(){
            String[] test  = null;
            DownloadUrl downloadUrl = new DownloadUrl();
            @Override
            public void run() {
                try {
                    test = parser.parseDirections(downloadUrl.readUrl(distUrl));
                    displayDirection(test);  // Method that gives an error
                } catch (IOException e) {
                    e.printStackTrace();
                }
                Log.i("test", "run: " + Arrays.toString(test));

            }
        });
        thread.start();


        showPolyLines();
    }

    public void displayDirection(String[] directionsList)
    {

        int count = directionsList.length;
        for(int i = 0;i<count;i++)
        {
            PolylineOptions options = new PolylineOptions();
            options.color(Color.RED);
            options.width(10);
            options.addAll(PolyUtil.decode(directionsList[i]));

            mMap.addPolyline(options);
        }
    }

[Stephan van Hulst]

Please post the full exception stack trace.

Download the data in another AsyncTask. Run the method that throws the exception from another onPostExecute().

NEVER create your own threads. Use either ExecutorService or AsyncTask.

[Marin Capranov]

02-17 15:52:09.351 15930-16917/com.example.authorisationfirebase E/AndroidRuntime: FATAL EXCEPTION: Thread-5092
   Process: com.example.authorisationfirebase, PID: 15930
   com.google.maps.api.android.lib6.common.apiexception.c: Not on the main thread
       at com.google.maps.api.android.lib6.common.m.b(:com.google.android.gms.dynamite_mapsdynamite@200414046@20.04.14 (040306-0):5)
       at com.google.maps.api.android.lib6.common.r.a(:com.google.android.gms.dynamite_mapsdynamite@200414046@20.04.14 (040306-0):2)
       at com.google.maps.api.android.lib6.impl.bi.a(:com.google.android.gms.dynamite_mapsdynamite@200414046@20.04.14 (040306-0):91)
       at com.google.android.gms.maps.internal.i.a(:com.google.android.gms.dynamite_mapsdynamite@200414046@20.04.14 (040306-0):150)
       at ch.onTransact(:com.google.android.gms.dynamite_mapsdynamite@200414046@20.04.14 (040306-0):4)
       at android.os.Binder.transact(Binder.java:387)
       at com.google.android.gms.internal.maps.zza.zza(Unknown Source)
       at com.google.android.gms.maps.internal.zzg.addMarker(Unknown Source)
       at com.google.android.gms.maps.GoogleMap.addMarker(Unknown Source)
       at com.example.authorisationfirebase.GetDirectionsData.showPolyLines(GetDirectionsData.java:114)
       at com.example.authorisationfirebase.GetDirectionsData$1.run(GetDirectionsData.java:77)
       at java.lang.Thread.run(Thread.java:818)

[Marin Capranov]

And if I create another async task that won't be a problem?

[Stephan van Hulst]

The exception is thrown by GetDirectionsData.showPolyLines(), which is not called by the code you've shown us.

Creating a second AsyncTask from the onPostExecute() method of a first AsyncTask should not be a problem.

[Marin Capranov]

Yeah it's a different method but it is giving the same error.

SO you suggest creating an Asynctask what? Class or method inside the onPostExecution of the existing Asynctask. I was thinking I have to create another class Asynctask.

[Stephan van Hulst]

new AsyncTask<Sting, Void, String>() {
  
  @Override
  protected final String doInBackground(String... inputOfFirstStep) {
    return performLongRunningOperationThatDoesntInteractWithTheUserInterface(inputOfFirstStep[0]);
  }

  @Override
  protected final void onPostExecute(String resultOfFirstStep) {
    updateUserInterface(resultOfFirstStep);

    new AsyncTask<String, Void, String>() {

      @Override
      protected final String doInBackground(String... inputOfSecondStep) {
        return performAnotherLongRunningOperationThatDoesntInteractWithTheUserInterface(inputOfSecondStep[0]);
      }

      @Override
      protected final void onPostExecute(String resultOfSecondStep) {
        updateUserInterfaceAgain(resultOfSecondStep);
      }
    }.execute(resultOfFirstStep);
  }
}.execute("Input of first step");

Of course, if you're not updating the user interface in the onPostExecute() method of the first step, that begs the question why you're not performing the second long running operation in the doInBackground() method of the first step.

[Marin Capranov]

I do something wrong?

 @Override
    protected String doInBackground(Object... objects) {

        mMap = (GoogleMap) objects[0];
        url = (String) objects[1];

        DownloadUrl downloadUrl = new DownloadUrl();
        try {
            googleDirectionsData = downloadUrl.readUrl(url);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return googleDirectionsData;
    }

    @Override
    protected void onPostExecute(String s) {

        Parser parser = new Parser();
        MapsActivity mapsActivity = new MapsActivity();

        List<HashMap<String, String>> trueUrl;

        trueUrl = parser.parseTest(s);

        String origin = trueUrl.get(0).toString()
                .replace("{", "")
                .replace("}","")
                .replace("=", ":");

        String destination = trueUrl.get(0).toString()
                .replace("{", "")
                .replace("}","")
                .replace("=", ":");

        distUrl = mapsActivity.getDistanceUrl(origin,destination,mapsActivity.getPlacesId(trueUrl));

        Log.i("outerUrl", "onClick: " + distUrl + " " + trueUrl.get(0).toString());


        new  AsyncTask<Object, Object, String>() {
            @Override
            protected String doInBackground(Object... objects) {
                distUrl = (String) objects[0];
                DownloadUrl downloadUrl = new DownloadUrl();
                try {
                    testUrl = downloadUrl.readUrl(distUrl);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                Log.i("test", "doInBackground: " + testUrl);
                return testUrl;
            }

            @Override
            protected void onPostExecute(String s) {
                String[] list = parser.parseDirections(s);
                Log.i("list", "onPostExecute: " + Arrays.toString(list));
               showPolyLines();
            }
        }.execute();

    }

E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #4
   Process: com.example.authorisationfirebase, PID: 9912
   java.lang.RuntimeException: An error occurred while executing doInBackground()
       at android.os.AsyncTask$3.done(AsyncTask.java:309)
       at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
       at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
       at java.util.concurrent.FutureTask.run(FutureTask.java:242)
       at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
       at java.lang.Thread.run(Thread.java:818)
    Caused by: java.lang.ArrayIndexOutOfBoundsException: length=0; index=0
       at com.example.authorisationfirebase.GetDirectionsData$1.doInBackground(GetDirectionsData.java:74)
       at com.example.authorisationfirebase.GetDirectionsData$1.doInBackground(GetDirectionsData.java:71)
       at android.os.AsyncTask$2.call(AsyncTask.java:295)
       at java.util.concurrent.FutureTask.run(FutureTask.java:237)
       at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) 
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
       at java.lang.Thread.run(Thread.java:818) 

[Stephan van Hulst]

You're not providing enough arguments to your AsyncTask's execute() method. This is what causes the exception.

Why are you doing all that stuff in your onPostExecute() method? You can do everything in your doInBackground() method, except the call to showPolyLines(). Do that in your onPostExecute() method. Then you don't need a second AsyncTask.

Remember: doInBackground() is for everything that doesn't need to interact with your user interface. Use onPostExecute() only for interacting with the user interface.

[Marin Capranov]

Thank you! I have fixed some issues, but I still struggle at understanding these threads and async.

I have created a new parser inside the onPostExecute but it returns an error: java.lang.NullPointerException: Attempt to invoke virtual method 'int org.json.JSONArray.length()' on a null object reference


Parser.class

public String[] parseDirections(String jsonData)
    {
        JSONArray jsonArray = null;
        JSONObject jsonObject;

        try {
            jsonObject = new JSONObject(jsonData);
            jsonArray = jsonObject.getJSONArray("routes").getJSONObject(0).getJSONArray("legs").getJSONObject(0).getJSONArray("steps");
        } catch (JSONException e) {
            Log.d("parsedir", "getPaths: " + e);
        }
        return getPaths(jsonArray);       // it says its here the problem.
    }

    public String[] getPaths(JSONArray googleStepsJson )
    {
        int count = googleStepsJson.length();      // it says its here the problem.
        String[] polylines = new String[count];

        for(int i = 0;i<count;i++)
        {
            try {
                polylines[i] = getPath(googleStepsJson.getJSONObject(i));
            } catch (JSONException e) {
                Log.d("getpaths", "getPaths: " + e);
            }
        }

        return polylines;
    }


    public String getPath(JSONObject googlePathJson)
    {
        String polyline = "";
        try {
            polyline = googlePathJson.getJSONObject("polyline").getString("points");
        } catch (JSONException e) {
            Log.d("getpath", "getPaths: " + e);
        }
        return polyline;
    }

MyAsync.class

public class GetDirectionsData extends AsyncTask<Object, String, String> {

    GoogleMap mMap;
    String url;
    String googleDirectionsData;
   // String googleDirectionsData1;
    String distUrl;


    @Override
    protected String doInBackground(Object... objects) {

        mMap = (GoogleMap) objects[0];
        url = (String) objects[1];
        //distUrl = (String) objects[2];

        DownloadUrl downloadUrl = new DownloadUrl();
        try {
            googleDirectionsData = downloadUrl.readUrl(url);
            Parser parser = new Parser();

            List<HashMap<String, String>> trueUrl;

            trueUrl = parser.parseTest(googleDirectionsData);

            String origin = trueUrl.get(0).toString()
                    .replace("{", "")
                    .replace("}","")
                    .replace("=", ":");

            String destination = trueUrl.get(0).toString()
                    .replace("{", "")
                    .replace("}","")
                    .replace("=", ":");

            distUrl = getDistanceUrl(origin,destination,getPlacesId(trueUrl));

           // googleDirectionsData1 = downloadUrl.readUrl(distUrl);

            Log.i("outerUrl", "onClick: " + distUrl + " " + trueUrl.get(0).toString());
        } catch (IOException | RuntimeException e) {
            Log.d("error", "doInBackground: " + e);
            e.printStackTrace();
        }

        return distUrl;
    }

    @Override
    protected void onPostExecute(String s) {

                Parser parser1 = new Parser();
                String[] list;
                list = parser1.parseDirections(s);   // it says its here the problem.

                displayDirection(list);

        Log.i("test", "onPostExecute: " + Arrays.toString(list) );
    }

    public void displayDirection(String[] directionsList)
    {

        int count = directionsList.length;
        for(int i = 0;i<count;i++)
        {
            PolylineOptions options = new PolylineOptions();
            options.color(Color.RED);
            options.width(10);
            options.addAll(PolyUtil.decode(directionsList[i]));

            mMap.addPolyline(options);
        }
    }

I believe that the parser does not work onPostExecute and should a background task as you suggested earlier but it did not work in the doInBackground either.

What can I do?

[Marin Capranov]

If you need the JSON to have a look that's the link

https://maps.googleapis.com/maps/api/directions/json?origin=place_id:ChIJ5zXmY228cEgRDDF7Z1-YQL8&destination=place_id:ChIJ5zXmY228cEgRDDF7Z1-YQL8&alternatives=true&mode=DRIVING&waypoints=place_id:ChIJ5zXmY228cEgRDDF7Z1-YQL8%7Cplace_id:ChIJRyztkXO8cEgRaO_-EaZwzm0%7Cplace_id:ChIJ-SIh8wq8cEgRBDMi070G2z8%7Cplace_id:ChIJCUUGEQ28cEgR4QmYc8pahII%7Cplace_id:ChIJp5AmknS8cEgRQzLse47kXyw%7Cplace_id:ChIJNZobMIq7cEgRQOz35HLPzMo%7Cplace_id:ChIJc5edxoi8cEgRvPxRxxTEQ9Y%7Cplace_id:ChIJK_W0bRu8cEgRmb9ypOwhOM0%7Cplace_id:ChIJ9d9D8Yy7cEgRjuOtcq2zPJM%7Cplace_id:ChIJFUBd9By8cEgR7cd5lUxjj64%7Cplace_id:ChIJcQCpsBu9cEgRuanc7ozkkRg%7Cplace_id:ChIJVWc_v8-7cEgRZ-mHpzDMwoE%7Cplace_id:ChIJCyYL4wUR2kcRekCDGlhBnOE%7Cplace_id:ChIJvTruyZi7cEgRMkNRD-9f5QE%7Cplace_id:ChIJ4ewwisq9cEgRt2-lHo1LQqg%7Cplace_id:ChIJsarvTVa8cEgRXhKeO70GIxM%7Cplace_id:ChIJ_Xoarrq7cEgRDRjGivZCGII%7Cplace_id:ChIJW_pynAi9cEgR647DNng_82o%7Cplace_id:ChIJO2sEHUG7cEgR_afhKlVAJ_c%7Cplace_id:ChIJ2eVJkPu7cEgRU_uU6jmPzTc&key=AIzaSyCuRGuOxVFfA2rs5gT-w2Y8K_RSlgzualg

[Stephan van Hulst]

When posting exception messages, please post the full stack trace. The message by itself is usually not so helpful.

The problem has nothing to do with threads. You're just not passing data correctly.

Why is distUrl an input of your AsyncTask? Why are you overwriting it in doInBackground()? Why are you returning it again from doInBackGround()? If you're returning an URL from doInBackground(), then why are you treating it as JSON data in onPostExecute()?

[Marin Capranov]

Sorry for the confusion. I have achieved what I wanted. However, if you got any experience with direction API could you help me to decode the path that I have got from the overview_polylines object. I want to display on the map and it doesn't work.

@Override
    protected void onPostExecute(String s) {

        new AsyncTask<Object, String, List<HashMap<String, String>>>() {
            @Override
            protected List<HashMap<String, String>> doInBackground(Object... objects) {
                Parser parser1 = new Parser();
                DownloadUrl downloadUrl1 = new DownloadUrl();
                String distObj = distUrl;
                String url1 = "";
                try {
                    url1 = downloadUrl1.readUrl(distObj);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                List<HashMap<String,String>> list;
                list = parser1.parseTesting(url1);

                return  list;
            }

            @Override
            protected void onPostExecute(List<HashMap<String, String>> string) {
                 displayDirection(string);
                Log.i("test", "onPostExecute: " + string);
            }
        }.execute(s);

    }

    public void displayDirection(List<HashMap<String, String>> directionsList)
    {

        int count = directionsList.size();
        for(int i = 0;i<count;i++)
        {
            PolylineOptions options = new PolylineOptions();
            options.color(Color.RED);
            options.width(10);
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                options.addAll(PolyUtil.decode(String.valueOf(directionsList.get(i))));
            }

            mMap.addPolyline(options);
        }
    }

Parser

 public List<HashMap<String,String>> parseTesting(String jsonFile){
        JSONObject jsonObject;
        JSONArray jsonArray;

        List<HashMap<String, String>> list = new ArrayList<>();
        HashMap<String, String> hashMap = new HashMap<>();
        
        try {
            jsonObject = new JSONObject(jsonFile);
            jsonArray = jsonObject.getJSONArray ("routes");
            JSONObject route = jsonArray.getJSONObject (0);
            JSONObject poly = route.getJSONObject ("overview_polyline");
            String polyline = poly.getString ("points");
            hashMap.put("obj", polyline);
            list.add(hashMap);
        } catch (JSONException e) {
            e.printStackTrace();
        }
        return list;
    }

the message from log "test" shows:

I/test: onPostExecute: [{obj={kd_IljoJhG`BpAuKjAmHhC}BhREnC}F?AAC?EmHmJtHdJDF?D?@JNnHxJj@jHu@`OuB|DoIrCyHaEkFkBsIcCJmFdBkLlCyNrBwOcC]A@?@A@A?C?CGyC}@IGyKyDwDTd@[tPrD?RE?uJzd@`@zCq@vHbHjBnNbEtCrAjIbD|HsCfP~BrS|VtEcBdJnDnK[mHs@lFaKgF{PgGsI{Fc\]eBoGoRx@ab@iBwC{F]_CTCFKBIQQu@g^oWoGoDqKv@_H`GrF_F~SqMfEpQpHhGrPzNJAFN?HfBK`KhBmAxc@fG`Qx@dC?TvFh\~B~FjFj@bJzGNKJR|ClVwBGfG\nC~HDqOoAsVABC@C?EKDKzEaPiJgQkPwPoJaKsN{F[oc@lF{QAK}F_q@oL`@iMhFe@_GlBrErMaFbEeHbAcYdBc^qU{FiIPqEoAcK|\aIjL{JrYk@|FaFxSyC_H_WuDoVmCyLiF{G_YiEfNlDvE`InA`@`@dJfA^tDk\xdA_G`TcJ{@GvD~KtIzKdNnUje@bIfEm]roB}Qlm@|TpNdTnDnK`QzLb[fFpQnEfPg@vHlGdHzIvK@AF?xBgGtBqCpApAdFhBhAbBcJ}FgF|Ha@`A@HILkQ`KsEbAs_@yF}QReKoLwOahAsDyK{@`CxBjc@h@xKtF}LiDoYsDyKsAxAwBwDhHyYzUct@nQ}gAdJgh@kCoEmVsb@{D}G}VoUqCiCjIiFxM{NdNe[~Kil@fNsWdJiYr@kS}Vun@kBfD\zGIcEn@gCl@jFqApBUyIvAmHrAr@xKl\~Hx_@yLrd@mKtYeYh_AsGtUeDdDaFuAcCpB`IfHvPxPpRta@rJtHq@|JkX|wAaFhZkLh[gJ|j@kBbEoCqJcC{GuArEJbEx@`AHj@c@xEfCrBz@qDr@gCHkAhC{A~C}VzUct@nQ}gAdJgh@kCoEmVsb@{D}G}VoUqCiCjIiFxM{NdNe[~Kil@fNsWdJiYr@kS}Vun@jC}Zp@}AlGrCbGnFjNrJvU_CjFFpFm@pKxKzWvHPGxYtHl]tU`G}@fBrGlAx`@~DbWl@`T_Fhw@tS~j@lC~GtWd_AFn@QAiG|T{Brm@DH?JKFu@jJpCtQ~HrWnIlTpBlKqBmKwGqFmBN{\ePCFI?CQkHqDaMgGwIsEqJaAyLhEoPmHmByKwXaIw_@wOgIeLsGaGmDmEvBsZfAkOmCaBcBSsMvHwPj@mOmVoUwb@_TcM_AgI}TilBRzB~ArS|Gla@hHxm@d^|_@xJvXpQlTnEfEtJSrMwHpIlB}]

[Stephan van Hulst]

There really is no point in starting up another AsyncTask in the onPostExecute() method if that's all you're gonna do. Put the code of the inner doInBackground() at the end of the outer doInBackground(), and replace the contents of the outer onPostExecute() with the contents of the inner onPostExecute().

The distObj variable is unused.

Why are you continuing execution of doInBackground() if readUrl() throws an exception?

Why are you passing a variable named url1 into a method that expects a JSON string?

Why is a parameter of type List named 'string'? Use a descriptive name. What does the list represent?

Don't use HashMap as your type argument. Use Map instead.

Instead of parsing JSON by hand, use a JSON binding API such as Jackson or JSON-B. Or use the Google Maps Services Java Client.

As for decoding the polyline, the developer manual is here: https://developers.google.com/maps/documentation/utilities/polylinealgorithm

Instead of decoding the polyline manually, you may want to use the Google Maps Services Java Client. Running the client directly on Android is not recommended, as per the developer manual, but if you REALLY want to send requests directly to the Maps service instead of through your own proxy server, you can probably use the PolylineEncoding class to achieve what you want. The client library can be found here: https://github.com/googlemaps/google-maps-services-java