NullPointerException when notifyDataSetChanged() on ArrayAdapter (ListView in ViewPager)

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)

Looks like your fragment's view never created and `dataAdapter` never initialized. — Anton Kovalyov, Jun 25 '15 at 07:27
Thank you. But they are created and initialized in the fragment's onCreateView. (dataAdapter = new DataAdapter ...) I must be missing something? — Fred J., Jun 25 '15 at 07:33

score 1 · Accepted Answer · edited May 23 '17 at 11:46

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.

NullPointerException when notifyDataSetChanged() on ArrayAdapter (ListView in ViewPager)

1 Answers1