[android] How to get the fragment instance from the FragmentActivity?

I set the content view in the FragmentActivity, and the activity will create the fragment instance for me according to the class name specified in the layout file. But how can I get that fragment instance?

public class MyActivity extends FragmentActivity {

    @Override
    protected void onCreate(Bundle extra) {
        super.onCreate(extra);
        setContentView(R.layout.page_fragment);
    }
}

layout/page_fragment.xml:

<?xml version="1.0" encoding="utf-8"?>
<fragment xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/pageview"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:name="org.xi.android.PageFragment" />

To get the fragment instance in a class that extends FragmentActivity:

MyclassFragment instanceFragment=
    (MyclassFragment)getSupportFragmentManager().findFragmentById(R.id.idFragment);

To get the fragment instance in a class that extends Fragment:

MyclassFragment instanceFragment =  
    (MyclassFragment)getFragmentManager().findFragmentById(R.id.idFragment);