Gravity attribute : Android Notes

The gravity attribute:
Use the gravity attribute to position objects within a larger container. This will only work if the container is larger than the object it contains.

For example: We set the gravity attribute within the LinearLayout container to center. This pushes all its child views into the vertical and horizontal center of the screen.

This and more at:

http://www.101apps.co.za/index.php/articles/android-s-gravity-layout-gravity-padding-and-layout-margin-attributes.html
and at:
http://alvinalexander.com/android/how-to-center-android-linearlayout-components-gravity