Now that you have written some Android code you know that Activity, View, and the layout and widget subclasses of View are among the most important classes in Android. Typically, an Android user interface is built from widget views organized in layouts: a ListView in a LinearLayout, for instance. A single hierarchy of view objects gets loaded from a resource (or created by code) when an Activity is started. It is initialized and displayed on the device screen.

For small screens, this is fine: users move from screen to screen to access different parts of a program’s UI, and the Activity class (Android’s concept of a task) supports a back stack that enables quick and intuitive traversal through the strictly tree-structured interface. This changes completely, however, when the UI is spread over the surface of a larger tablet screen. Some parts of the screen remain constant over longer durations than others. Some parts of the screen determine the contents of other parts. A card-stack metaphor just doesn’t cut it.