User interface (UI) design, user experience (UX), human computer interaction (HCI), and usability are huge topics that aren't covered in great depth in this book. Nonetheless, it's important that you get them right when creating your user interfaces.
Android introduces some new terminology for familiar programming metaphors that will be explored in detail in the following sections:
> Views Views are the base class for all visual interface elements (commonly known as controls or widgets). All UI controls, including the layout classes, are derived from view.
> View Groups View Groups are extensions of the View class that can contain multiple child Views. Extend the ViewGroup class to create compound controls made up of interconnected child Views. The ViewGroup class is also extended to provide the layout managers that help you lay out controls within your Activities.
> Activities Activities, described in detail in the previous chapter, represent the window, or screen, being displayed. Activities are the Android equivalent of Forms. To display a user interface you assign a View (usually a layout) to an Activity.
Android provides several common UI controls, widgets, and layout managers.
For most graphical applications it's likely that you'll need to extend and modify these standard Views — or create composite or entirely new Views — to provide your own user experience.
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