NullPointerException when notifyDataSetChanged() on ArrayAdapter (ListView in ViewPager) - android-listview

Question

I have an Activity with ViewPager which manage 2 Fragments using FragmentStatePagerAdapter,
The fragments have ListViews, each of the ListView shows data and I am able to go from Fragment_A to Fragment_B by swipe or item tapping on the ListView of Fragment_A.
I am also able to update the underlying data source for Fragment_B based on the item selected in Fragment_A. But when I try to notifyDataSetChanged() on the DataAdapter of Fragment_B, I am getting NullPointerException .
I want to update the data in Fragment_B according to the item selected in Fragment_A.
Here is my code
public class MyActivity extends FragmentActivity implements AdapterView.OnItemClickListener{
ViewPager viewPager = null;
int mSelectedItemPosition;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.fragment_fail_activity);
viewPager = (ViewPager) findViewById(R.id.fail_pager_VP);
viewPager.setAdapter(new pagerAdapter(getSupportFragmentManager()));
}
#Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
mSelectedItemPosition = position;
viewPager.setCurrentItem(1);
FragmentStatePagerAdapter fspa = (FragmentStatePagerAdapter) viewPager.getAdapter();
Fragment_B item = (Fragment_B) fspa.getItem(1);
item.setData(position);
item.refresh();
}
}
class pagerAdapter extends FragmentStatePagerAdapter {
public pagerAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int position) {
Fragment fragment = null;
if (position == 0) {
fragment = new Fragment_A();
} else if (position == 1) {
fragment = new Fragment_B();
}
return fragment;
}
#Override
public int getCount() {
return 2;
}
#Override
public CharSequence getPageTitle(int position) {
String title = "";
if (position == 0) {
return "a_title";
}else if (position == 1) {
return "b_title";
}
}
}
public class Fragment_A extends android.support.v4.app.Fragment {
private ListView frag_a_listView;
DataAdapter dataAdapter;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public void onViewCreated(View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
ListView frag_a_listView = (ListView) getActivity().findViewById(R.id.frag_a_LV);
frag_a_listView.setOnItemClickListener((AdapterView.OnItemClickListener) getActivity());
}
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View layout = inflater.inflate(R.layout.fragment_fail_main, container, false);
frag_a_listView = (ListView) layout.findViewById(R.id.frag_a_LV);
dataAdapter = new DataAdapter(getActivity(),
R.layout.data_row,
R.id.data_label_TV,
getFrag_aData());
frag_a_listView.setAdapter(dataAdapter);
return layout;
}
private List<String> getFrag_aData() {
...
return someData;
}
}
public class Fragment_B extends android.support.v4.app.Fragment {
ListView frag_b_listView;
List<String> mData;
DataAdapter dataAdapter;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public void onViewCreated(View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
ListView frag_b_listView = (ListView) getActivity().findViewById(R.id.frag_b_LV);
frag_b_listView.setOnItemClickListener((AdapterView.OnItemClickListener) getActivity());
}
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View layout = inflater.inflate(R.layout.fragment_frag_b, container, false);
frag_b_listView = (ListView) layout.findViewById(R.id.frag_b_LV);
setData(0);
dataAdapter = new DataAdapter(getActivity(),
R.layout.data_row,
R.id.data_label_TV,
mData);
frag_b_listView.setAdapter(dataAdapter);
return layout;
}
public void setData(int position) {
...
mData = some List<String>;
}
public void refresh() {
dataAdapter.notifyDataSetChanged(); <------- this crashes the app
}
}
public class DataAdapter extends ArrayAdapter {
public DataAdapter(Context context, int resource, int textViewResourceId, List objects) {
super(context, resource, textViewResourceId, objects);
}
#Override
public View getView(int position, View convertView, ViewGroup parent) {
convertView = super.getView(position, convertView, parent);
return convertView;
}
}
here is the crash log
06-25 16:51:46.813 21537-21537/au.myCity.eight E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: au.myCity.eight, PID: 21537
java.lang.NullPointerException: Attempt to invoke virtual method 'void au.myCity.eight.dataAdapter.notifyDataSetChanged()' on a null object reference
at au.myCity.eight.fragments.Fragment_B.refresh(Fragment_B.java:56)
at au.myCity.eight.MyActivity.onItemClick(MyActivity.java:37)
at android.widget.AdapterView.performItemClick(AdapterView.java:305)
at android.widget.AbsListView.performItemClick(AbsListView.java:1146)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3053)
at android.widget.AbsListView$3.run(AbsListView.java:3860)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Answer 1

After few tries with Log.d and checking some variable references.
The problem was the reference to Fragment_B changed by the time I called dataAdapter.notifyDataSetChanged(), to fix this I used the solution suggested in SO here, thanks to the "Streets Of Boston".
Summery, In the PagerAdapter:
Save a reference to initialized fragments in a SparseArray, use getRegisteredFragment in
onItemClick instead of getItem in MyActivity.
Override instantiateItem and destroyItem as suggested in the link.