Dynamically add fragment into fragment

As the documentation states "A fragment must always be embedded in an activity". So when you add a "sub-fragment" it will always belong to the activity even if you add it within your fragment class. For example if you later decide to remove the containing fragment the sub fragments won't be automatically removed. When I had to do the same I had to store in a vector the sub fragments and manually remove them in the onDestroy methods of my container fragment.

I think that fragments are not thought to be used like this

I assume the problem that you are running into is that there is not an inflated view to add the fragment to because the original fragment, FRAG4_TAG, has not been inflated before you are trying to add it.

You can pass enough information to FRAG4_TAG in the Arguments to let it know that it should create and add a fragment (or what all fragments you need it to have) to itself during it's onCreateView, after the view has been inflated...

The layout for the activity...

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
        <TextView
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="MyActivity"/>

    <LinearLayout
        android:orientation="vertical"
        android:id="@+id/main_frag_container"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"/>
</LinearLayout>

The Activity...

public class MyActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        FragmentManager fm = getFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();

        Fragment fragOne = new MyFragment();
        Bundle arguments = new Bundle();
        arguments.putBoolean("shouldYouCreateAChildFragment", true);
        fragOne.setArguments(arguments);
        ft.add(R.id.main_frag_container, fragOne);
        ft.commit();

    }
}

The layout for the fragment...

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:padding="20dp">
    <TextView
        android:id="@+id/text"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Some fragment"/>

    <LinearLayout
        android:orientation="vertical"
        android:id="@+id/frag_container"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"/>
</LinearLayout>

The fragment...

public class MyFragment extends Fragment {
    @Override
    public View onCreateView(final LayoutInflater inflater, final ViewGroup container, final Bundle savedInstanceState) {
        ViewGroup layout = (ViewGroup) inflater.inflate(R.layout.frag_layout, container, false);

        boolean shouldCreateChild = getArguments().getBoolean("shouldYouCreateAChildFragment");

        if (shouldCreateChild) {
            FragmentManager fm = getFragmentManager();
            FragmentTransaction ft = fm.beginTransaction();

            fm.beginTransaction();
            Fragment fragTwo = new MyFragment();
            Bundle arguments = new Bundle();
            arguments.putBoolean("shouldYouCreateAChildFragment", false);
            fragTwo.setArguments(arguments);
            ft.add(R.id.frag_container, fragTwo);
            ft.commit();

        }

        return layout;
    }
}

This example covers the case where you need to dynamically add fragments to a fragment that HAS NOT already been inflated and added to the hierarchy. Adding a fragment to a fragment that HAS already been inflated and added to the hierarchy is as simple as just specifying the target fragments container that you want to add to by ID like you would normally.