<?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"
    android:gravity="center" >

    <Button android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="center"/>

</LinearLayout>