// main.xml выведет "Hello"
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
     android:orientation="vertical">
    <TextView
         android:layout_width="fill_parent"
         android:layout_height="wrap_content"
         android:text="Hello!"
    </TextView>
</LinearLayout>