Add fragment with 0 containerViewId
<p>Where does android add fragment when I call <code>FragmentTransaction add (Fragment fragment, String tag)</code> </p>

<p>I have written this code, but I can't see fragment's layout. It displays an empty screen.</p>

<pre><code> @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Fragment fragment = new TestFragment();
        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.add(fragment, "test");
        ft.commit();
    } // onCreate

 public class TestFragment extends Fragment {

        @Nullable
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup        container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.frag_layout, container, false);
        }
    } // TestFragment
</code></pre>