FragmentManager给了我NPE

Question

I'm checking fragments for the first time and I created a LinearLayout with a FrameLayout within and for my layout-sw600dp a LinearLayout with 2 FrameLayouts.

The one has an id of "main" and the other is "details".

The error I get is :

 Caused by: java.lang.NullPointerException
        at android.app.BackStackRecord.doAddOp(BackStackRecord.java:395)
        at android.app.BackStackRecord.add(BackStackRecord.java:385)
        at mes.fallstudio.tvfall.MainActivity.onCreate(MainActivity.java:28)

where line 28 is the last line here:

public class MainActivity extends AppCompatActivity {
private android.support.v4.app.Fragment mainFragment;
private android.app.Fragment detailsFragment;
private Boolean isDualPane = false;

private android.support.v4.app.FragmentManager fm;

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

    android.support.v4.app.FragmentManager fragmentManager = getSupportFragmentManager();
    mainFragment = fragmentManager.findFragmentById(R.id.main);

    android.support.v4.app.FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction.add(R.id.main, mainFragment).commit();


}
}

XML layout files :

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


<FrameLayout
    android:id="@+id/main"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
</FrameLayout>

<FrameLayout
    android:id="@+id/details"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
</FrameLayout>

this is for layout-sw600dp and the same without the 2nd FrameLayout ( details) for the xml file for the layout folder.

I'm extending AppCompatActivity if this helps. Thanks you

Answer 1

AppCompatActivity uses getSupportFragmentManager() instead of getFragmentManager() .

Reference: https://developer.android.com/reference/android/support/v7/app/AppCompatActivity.html

EDIT:

In your code you try to get mainFragment from the XML layout. However, you do not define it there. You can either instantiate mainFragment in your code:

mainFragment = new MainFragment();

Or define it in your XML:

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

<fragment android:name="mes.fallstudio.tvfall.MainFragment"
    android:id="@+id/main"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<FrameLayout
    android:id="@+id/details"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
</FrameLayout>

In the latter case you don't need to use the FragmentManager at all in your code unless you need to manipulate the fragments at run time.

In both cases you also need the actual fragment:

public class MainFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.main_fragment, container, false);
    }
}