NullPointerException notifydatasetchanged（）

Question

I have a main activity with a View Pager and a Spinner. When I change the spinner, i want the fragments in the ViewPager to update to use new data.

I have tried to communicate from the Activity to the Fragment when a selection has been made in the Spinner but I keep getting a NullPointerException.

i call the updateListView() method from the Activity whenever the Spinner selection has been confirmed. The commented line of code also causes it to crash.

ListFragment lf = (ListFragment) mSectionsPagerAdapter.getItem(0);
                lf.updateListView();

Here is my code

    public class ListFragment extends Fragment {

ArrayList<Psalm> psalms;
ListViewAdapter adapter;
View v;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState){
    v = inflater.inflate(R.layout.fragment_list, parent, false);

    psalms = GlobalSingleton.get(getActivity()).getPsalms().getPsalms();

    adapter = new ListViewAdapter(getActivity(), psalms);

    ListView l = (ListView) v.findViewById(R.id.fragmentListListView);
    l.setAdapter(adapter);

    return v;

}

public void updateListView(){
    psalms = GlobalSingleton.get(getActivity()).getPsalms().getPsalms();

    System.out.println("HERE");
    ((ListViewAdapter) ((ListView) v.findViewById(R.id.fragmentListListView)).getAdapter()).notifyDataSetChanged(); 
    //adapter.notifyDataSetChanged();

}

}

main activity

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {


/**
 * The {@link android.support.v4.view.PagerAdapter} that will provide
 * fragments for each of the sections. We use a
 * {@link android.support.v4.app.FragmentPagerAdapter} derivative, which
 * will keep every loaded fragment in memory. If this becomes too memory
 * intensive, it may be best to switch to a
 * {@link android.support.v4.app.FragmentStatePagerAdapter}.
 */
SectionsPagerAdapter mSectionsPagerAdapter;

/**
 * The {@link ViewPager} that will host the section contents.
 */
ViewPager mViewPager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    try {

        String[] files = getAssets().list("");

        ArrayList<Psalm> singPsalms = new ArrayList<Psalm>();
        ArrayList<Psalm> scottishPsalter = new ArrayList<Psalm>();

        for(String s: files){
            if(s.split("_")[0].equals("SingPsalms")){
                singPsalms.add(new Psalm(s.split("_")[1],  s.split("_")[2], s.split("_")[3].replace(".txt", "")));
            } else if(s.split("_")[0].equals("ScottishPsalter")){
                scottishPsalter.add(new Psalm(s.split("_")[1] ,  s.split("_")[2], s.split("_")[3].replace(".txt", "")));
            }
        }

        Collections.sort(singPsalms, new PsalmComparator());

        GlobalSingleton.get(this).setSingPsalms(new Psalmody("Sing Psalms", singPsalms));
        GlobalSingleton.get(this).setScottishPsalter(new Psalmody("Scottish Psalter", scottishPsalter));
        GlobalSingleton.get(this).setSelectedPsalmody("Scottish Psalter");


    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }


    // Adapter
    Spinner psalter = (Spinner) findViewById(R.id.spinner1);
    SpinnerAdapter adapter = ArrayAdapter.createFromResource(this, R.array.psalter, android.R.layout.simple_spinner_dropdown_item);
    psalter.setAdapter(adapter);
    psalter.setOnItemSelectedListener( new OnItemSelectedListener() {

        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            String selected =  parent.getItemAtPosition(position).toString();

            if (!selected.equals(GlobalSingleton.get(getApplicationContext()).getSelectedPsalmody())){
                GlobalSingleton.get(getApplicationContext()).setSelectedPsalmody(selected);

                System.out.println("HERE");
                ListFragment lf = (ListFragment) mSectionsPagerAdapter.getItem(0);
                lf.updateListView();

            }

        }

        public void onNothingSelected(AdapterView<?> parent) {
        }
    });


    // Set up the action bar.
    final ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    // Create the adapter that will return a fragment for each of the three
    // primary sections of the app.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    // When swiping between different sections, select the corresponding
    // tab. We can also use ActionBar.Tab#select() to do this if we have
    // a reference to the Tab.
    mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }
    });

    // For each of the sections in the app, add a tab to the action bar.
    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
        // Create a tab with text corresponding to the page title defined by
        // the adapter. Also specify this Activity object, which implements
        // the TabListener interface, as the callback (listener) for when
        // this tab is selected.
        actionBar.addTab(
                actionBar.newTab()
                .setText(mSectionsPagerAdapter.getPageTitle(i))
                .setTabListener(this));
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    // When the given tab is selected, switch to the corresponding page in
    // the ViewPager.
    mViewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}

/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a DummySectionFragment (defined as a static inner class
        // below) with the page number as its lone argument.
        Fragment fragment = null;

        if (position == 0){
            fragment = new ListFragment();


        } else {
            fragment = new GridFragment();

        }
        return fragment;
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
        case 0:
            return "List".toUpperCase(l);
        case 1:
            return "Grid".toUpperCase(l);
        case 2:
            return "Metre Seperated".toUpperCase(l);
        }
        return null;
    }
}

}

Answer 1

Use same object of ListView on which you are calling setAdapter in onCreateView for calling notifyDataSetChanged as:

    ....
    l = (ListView) v.findViewById(R.id.fragmentListListView);
    l.setAdapter(adapter);
    return v;
}
ListView l;
public void updateListView(){ 
    //....
    l.getAdapter()).notifyDataSetChanged(); 
    //adapter.notifyDataSetChanged();
}

Answer 2

try below code. do not use view and use member listview.

public class ListFragment extends Fragment {

ArrayList<Psalm> psalms;
ListViewAdapter adapter;
ListView l;
View v;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState){
    v = inflater.inflate(R.layout.fragment_list, parent, false);

    psalms = GlobalSingleton.get(getActivity()).getPsalms().getPsalms();

    adapter = new ListViewAdapter(getActivity(), psalms);

    l = (ListView) v.findViewById(R.id.fragmentListListView);
    l.setAdapter(adapter);

    return v;

}

public void updateListView(){
    psalms = GlobalSingleton.get(getActivity()).getPsalms().getPsalms();

    System.out.println("HERE");
    if(l != null){
         l.getAdapter()).notifyDataSetChanged(); 
    }

}