FragmentActivity with ViewPager: Crash on orientation change

Question

I have used ViewPager in FragmentActivity. I am getting a crash when orientation changes.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list);
    context = this;
    loadInitialData();
    setActionBar();
    initializeUiComponents();
}

private void initializeUiComponents() {
    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    }

My data is coming dynamically from the database, and I have to call the Sysnctask which is getting the datat from database in onResume(). Here is onPostExecute() of the Asynctask.

protected void onPostExecute(Void result) {
        
            mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
            mViewPager.setAdapter(mSectionsPagerAdapter);
        }
        mSectionsPagerAdapter.notifyDataSetChanged();
        }
}

Adding FragmentAdapterCode:

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 = new PayeeListSectionFragment();
        Bundle args = new Bundle();
        args.putInt(PayeeListSectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }
    
    @Override
    public int getCount() {
        return 2;
    }
    
    @Override
    public CharSequence getPageTitle(int position) {
        
        return string;
    }
}

Adding Fragment Code:

public class PayeeListSectionFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this fragment.
     */
    public static final String ARG_SECTION_NUMBER = "section_number";
    
    public PayeeListSectionFragment() {
    }
    
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = null;
        \\ rootView inflated
        return rootView;
    }
}

The xml is used is:

 <android.support.v4.view.ViewPager
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".PayeeListActivity" >

    <!--
This title strip will display the currently visible page title, as well as the page
titles for adjacent pages.

    -->

    <android.support.v4.view.PagerTitleStrip
        android:id="@+id/pager_title_strip"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="top"
        android:background="#33b5e5"
        android:paddingBottom="4dp"
        android:paddingTop="4dp"
        android:textColor="#fff" />
</android.support.v4.view.ViewPager>

I am getting the following crash on orientation change:

06-07 11:01:57.834: E/AndroidRuntime(766): FATAL EXCEPTION: main  
06-07 11:01:57.834: E/AndroidRuntime(766): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.walletv2.activity/com.walletv2.activity.PayeeListActivity}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment: make sure class name exists, is public, and has an empty constructor that is public  
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3692)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.access$700(ActivityThread.java:141)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1240)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.os.Handler.dispatchMessage(Handler.java:99)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.os.Looper.loop(Looper.java:137)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.main(ActivityThread.java:5041)
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.reflect.Method.invokeNative(Native Method)
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.reflect.Method.invoke(Method.java:511)
06-07 11:01:57.834: E/AndroidRuntime(766):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
06-07 11:01:57.834: E/AndroidRuntime(766):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
06-07 11:01:57.834: E/AndroidRuntime(766):  at dalvik.system.NativeStart.main(Native Method)
06-07 11:01:57.834: E/AndroidRuntime(766): Caused by: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment: make sure class name exists, is public, and has an empty constructor that is public
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.Fragment.instantiate(Fragment.java:405)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.FragmentState.instantiate(Fragment.java:97)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1767)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:208)
06-07 11:01:57.834: E/AndroidRuntime(766):  at com.walletv2.activity.PayeeListActivity.onCreate(PayeeListActivity.java:78)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.Activity.performCreate(Activity.java:5104)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
06-07 11:01:57.834: E/AndroidRuntime(766):  ... 12 more
06-07 11:01:57.834: E/AndroidRuntime(766): Caused by: java.lang.InstantiationException: can't instantiate class com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment; no empty constructor
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.Class.newInstanceImpl(Native Method)
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.Class.newInstance(Class.java:1319)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.Fragment.instantiate(Fragment.java:394)
06-07 11:01:57.834: E/AndroidRuntime(766):  ... 19 more

Answer 1

There's a couple of hints on the following lines:

Caused by: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment: make sure class name exists, is public, and has an empty constructor that is public

And:

06-07 11:01:57.834: E/AndroidRuntime(766): Caused by: java.lang.InstantiationException: can't instantiate class com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment; no empty constructor

That suggests that you've added a parameterized constructor to your PayeeListSectionFragment inner class. Unfortunately, that won't work, as Android relies on invoking a non-parameterized constructor using reflection in order to restore a fragment's state on configuration changes (amongst other things).

If you want to supply one or more parameters to a fragment, you'll have to plug them into a Bundle and set it as argument, using setArguments(Bundle). Have a look at the DetailsFragment and CountingFragment in the documentation for an example on how to do that.

---

This problem can also arise in another situation, which may be less obvious for those not too familiar with Java. If your fragment is a nested class of say some Activity, make sure you declare the inner class static. For example, in the context of above question, it should be a public static class PayeeListSectionFragment (with an emphasis on the static modifier). That way the inner class won't keep a reference to the outer class and can have its own life cycle, without any dependencies on the outer class. Without the static modifier, the nested class can't be instantiated without instantiating the outer class, which means Android will run into issues when attempting to reinstantiate your fragment class.

An alternative solution would be to move the inner class to its own .java file. That way any outer class dependencies are automatically removed.