Align parent in RelativeLayout

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  >
  <AnalogClock android:id="@+id/analog"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_alignParentTop="true"
    />
  <DigitalClock android:id="@+id/digital"
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_below="@id/analog"
    />
</RelativeLayout>

import android.app.Activity;
import android.os.Bundle;
//from w  w w .  j a v  a 2s.  c  o m
public class MainActivity extends Activity {
  @Override
  public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.main);
  }
}