app crash when trying to call function from different class

Question

I am trying to call function getData() after they sort spinner change but i keep getting error and app crash.

This is the function inside my homepageActivity :

private JsonArrayRequest getDataFromServer(int requestCount, int SortType) {
        //Initializing ProgressBar
        final ProgressBar progressBar = (ProgressBar) findViewById(R.id.progressBar1);
        String FINAL_URL = "";
        //Displaying Progressbar
        progressBar.setVisibility(View.VISIBLE);
        setProgressBarIndeterminateVisibility(true);

        //JsonArrayRequest of volley
        if(SortType == 0){
            FINAL_URL = Config.DATA_URL + String.valueOf(requestCount);
        }else if (SortType == 1){
            FINAL_URL = Config.DATA_URL + String.valueOf(requestCount);
        }else{
            Log.d("ERROR:","sort is more than 1");
        }
        JsonArrayRequest jsonArrayRequest = new JsonArrayRequest(FINAL_URL,
                new Response.Listener<JSONArray>() {
                    @Override
                    public void onResponse(JSONArray response) {
                        //Calling method parseData to parse the json response
                        parseData(response);
                        //Hiding the progressbar
                        progressBar.setVisibility(View.GONE);
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        progressBar.setVisibility(View.GONE);
                        //If an error occurs that means end of the list has reached
                        Toast.makeText(homepageActivity.this, "No More Items Available", Toast.LENGTH_SHORT).show();
                    }
                });

        //Returning the request
        return jsonArrayRequest;
    }

    //This method will get data from the web api
    public void getData(int sort) {
        //Adding the method to the queue by calling the method getDataFromServer
        requestQueue.add(getDataFromServer(requestCount, sort));
        //Incrementing the request counter
        requestCount++;
    }

    //This method will parse json data
    private void parseData(JSONArray array) throws RuntimeException {
        for (int i = 0; i < array.length(); i++) {
            //Creating the superhero object
            SuperHero superHero = new SuperHero();
            JSONObject json = null;
            try {
                //Getting json
                json = array.getJSONObject(i);
                //Adding data to the superhero object
                superHero.setImageUrl(json.getString(Config.TAG_IMAGE_URL));
                superHero.setName(json.getString(Config.TAG_NAME));
                superHero.setPublisher(json.getString(Config.TAG_PUBLISHER));
            } catch (JSONException e) {
                e.printStackTrace();

            }
            //Adding the superhero object to the list
            listSuperHeroes.add(superHero);
        }
        //Notifying the adapter that data has been added or changed
        Log.d("ADebugTag", "Value: " + listSuperHeroes);

        adapter.notifyDataSetChanged();
    }

and this is what I have inside my cardadapter :

HPC = new homepageActivity();
                Boolean checkFirst = true;
                @Override
                public void onItemSelected(AdapterView<?> parent, View view, int position, long l) {
                    if(!checkFirst){
                        Log.d("ERROR:","hello world is clicked" + position);
                        homepageActivity.sortDataOn = false;
                        homepageActivity.listSuperHeroes.clear();
                        homepageActivity.adapter.notifyDataSetChanged();
                        homepageActivity.sortType = position;
                        HPC.getData(homepageActivity.sortType);
                    }

                    checkFirst = false;
                }

The app works okay until I change spinner and it tries to call getData function and then it crash and it gives this error below:

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.yemencar.yemencar, PID: 4714
                  java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                      at android.support.v7.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:247)
                      at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:182)
                      at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520)
                      at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:191)
                      at com.yemencar.yemencar.homepageActivity.getDataFromServer(homepageActivity.java:91)
                      at com.yemencar.yemencar.homepageActivity.getData(homepageActivity.java:131)
                      at com.yemencar.yemencar.CardAdapter$VHHeader$1.onItemSelected(CardAdapter.java:162)
                      at android.widget.AdapterView.fireOnSelected(AdapterView.java:944)
                      at android.widget.AdapterView.dispatchOnItemSelected(AdapterView.java:933)
                      at android.widget.AdapterView.-wrap1(Unknown Source:0)
                      at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:898)
                      at android.os.Handler.handleCallback(Handler.java:789)
                      at android.os.Handler.dispatchMessage(Handler.java:98)
                      at android.os.Looper.loop(Looper.java:164)
                      at android.app.ActivityThread.main(ActivityThread.java:6541)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

--EDIT--

based on Mike M comment I cant use HPC = new homepageActivity(); So to move forward what options I have to achieve what I trying to do here , to make it clear I have a recyclerView with a sort spinner in its header, so when this sort is change e.g "by date" then I want to update the recyclerView content.

Answer 1

It sounds like your goal is to somehow communicate with your Activity from inside your onItemSelected() callback.

One way to do this is to leverage the parent argument to the callback... all View instances have a getContext() method, and unless something non-standard is going on that context will be your activity. So you could do something like this:

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long l) {
    homepageActivity activity = (homepageActivity) parent.getContext();
    // now you can call activity.sortDataOn etc
}

It's also possible that you'd be better off passing a reference to your activity to your adapter whenever you create it, but without your full code it's hard to say.