<LinearLayout
   android:layout_width="match_parent"
   android:layout_height="200dp"
   android:orientation="vertical">
  <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:text="我要居中对齐呀~"/>
</LinearLayout>
 

<LinearLayout
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="horizontal" > 
  <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:text="我要垂直对齐呀~" />
</LinearLayout>