Android Set Text of TextView in Fragment that is in FragmentPagerAdapter

Question

This one is driving me nuts. Basically, I want to create a ViewPager and add a few Fragments to it. Then, all I want to do, it set a value in one of the Fragment's TextViews. I can add the Fragments fine, and they attach, but when I go to findViewById() for one of the TextViews in the first Fragment it throws a NullPointerException. I, for the life of me, can't figure out why.

Here's my code so far, let me know if more is needed please.

public class SheetActivity extends FragmentActivity {

    // /////////////////////////////////////////////////////////////////////////
    // Variable Declaration
    // /////////////////////////////////////////////////////////////////////////
    private ViewPager               viewPager;
    private PagerTitleStrip         titleStrip;
    private String                  type;
    private FragmentPagerAdapter    fragmentPager;  //UPDATE

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

        viewPager = (ViewPager) findViewById(R.id.viewPager);
        titleStrip = (PagerTitleStrip) findViewById(R.id.viewPagerTitleStrip);

        // Determine which type of sheet to create
        Intent intent = getIntent();
        this.type = intent.getStringExtra("type");
        FragmentManager manager = getSupportFragmentManager();
        switch (type) {
            case "1":
                viewPager.setAdapter(new InstallAdapter(manager));
                break;
            case "2":
                viewPager.setAdapter(new InstallAdapter(manager));
                break;
        }
        fragmentPager = (FragmentPagerAdapter) viewPager.getAdapter();  //UPDATE
    }

    @Override
    public void onResume() {
        super.onResume();

        fragmentPager.getItem(0).setText("something"); //UPDATE
    }

    class MyAdapter extends FragmentPagerAdapter {

        private final String[]      TITLES      = { "Title1", "Title2" };
        private final int           PAGE_COUNT  = TITLES.length;
        private ArrayList<Fragment> FRAGMENTS   = null;

        public MyAdapter(FragmentManager fm) {
            super(fm);
            FRAGMENTS = new ArrayList<Fragment>();
            FRAGMENTS.add(new FragmentA());
            FRAGMENTS.add(new FragmentB());
        }

        @Override
        public Fragment getItem(int pos) {
            return FRAGMENTS.get(pos);
        }

        @Override
        public int getCount() {
            return PAGE_COUNT;
        }

        @Override
        public CharSequence getPageTitle(int pos) {
            return TITLES[pos];
        }
    }
}

All of Fragments I created only have the onCreateView() method overridden so I can display the proper XML layout. Other than that they are 'stock'. Why can't I interact with elements in any of the Fragments?

UPDATE:

So do something like this?

public class FragmentA extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle inState) {
        return inflater.inflate(R.layout.fragment_a, container, false);
    }

    public void setText(String text) {
        TextView t = (TextView) getView().findViewById(R.id.someTextView);  //UPDATE
        t.setText(text);
    }
}

XML LAYOUT FOR FRAGMENT A

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/someTextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:textSize="22sp" />

</LinearLayout>

Answer 1

Unless you are planning to change the value at runtime, you can pass the value into the fragment as a parameter. It is done my using a Bundle and passing it as args into a Fragment, which then retrieves it from it's args. More info here. If you implement this, your instantiation of new Fragments might look something like this:

public InstallAdapter(FragmentManager fm) {
            super(fm);
            FRAGMENTS = new ArrayList<Fragment>();
            FRAGMENTS.add(FragmentA.newInstance("<text to set to the TextView>"));
            FRAGMENTS.add(FragmentB.newInstance("<text to set to the TextView>"));
        }

If, however, you are planning to update the value at runtime (it will change as user is running the app), then you want to use an Interface to channell communication between your fragment and your activity. Info here. This is what it might look like:

//Declare your values for activity;
    ISetTextInFragment setText;
    ISetTextInFragment setText2;
...
//Add interface
public interface ISetTextInFragment{
    public abstract void showText(String testToShow);
}
...
//your new InstallAdapter
public InstallAdapter(FragmentManager fm) {
        super(fm);

        FRAGMENTS = new ArrayList<Fragment>();

        Fragment fragA = new FragmentA();
        setText= (ISetTextInFragment)fragA;
        FRAGMENTS.add(fragA);

        Fragment fragB = new FragmentB();
        setText2= (ISetTextInFragment)fragB;
        FRAGMENTS.add(fragB);
}

//then, you can do this from your activity:
...
setText.showText("text to show");
...

and it will update your text view in the fragment.

While it can be done "more easily", these methods are recomended because they reduce chances of bugs and make code a lot more readable and maintainable.

EDIT: this is what your Fragment should look like (modified your code):

public class FragmentA extends Fragment implements ISetTextInFragment {

    TextView myTextView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle inState) {
        View v = inflater.inflate(R.layout.fragment_a, container, false);
        myTextView = (TextView)v.findViewbyId(R.id.someTextView)
        return v;
    }

    @Override
    public void showText(String text) {
        myTextView.setText(text);
    }
}

If after that you are still getting a null pointer exception, your TextView is NOT located where it needs to me, namely in the R.layout.fragment_a filem, and it needs to be located there. Unless you are calling the interface method BEFORE the fragment finished loading, of course.