Android :: Does / Res / Layout / Main.xml Describe View Or ViewGroup?

<?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="@string/hello"
/>
</LinearLayout>

Since the root element is a LinearLayout, which extends ViewGroup, why does main.xml get turned into a View instead of a ViewGroup? For example, in my main Activity class, I try to get the number of subviews that the LinearLayout contains like this:

public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

ViewGroup vg = (ViewGroup) findViewById(R.layout.main);
Log.v("myTag", "num children: " + vg.getChildCount());

but it crashes when i call vg.getChildCount(). What is the right way of doing it?