Fragment 怎么用?

错误信息:Caused by: java.lang.ClassCastException: com.pis.fragment.FragmentOne cannot be cast to android.app.Fragment代码:public class Main extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_fragment);
}
}
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >    <fragment
        android:id="@+id/list"
        android:name="com.pis.fragment.FragmentOne"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1" />    <fragment
        android:id="@+id/viewer"
        android:name="com.pis.fragment.FragmentTwo"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1" /></LinearLayout>
public class FragmentOne extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
return inflater.inflate(R.layout.fragment_one, container, false);
}

}
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"  >

    <TextView 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="20sp"
        android:text="FragmentOne"/>
</LinearLayout>
应用起不来