User interface (UI) design, user experience (UX), human computer interaction (HCI), and usability are huge topics that can’t be covered in the depth they deserve within the confines of this book. Nonetheless, the importance of creating a UI that your users will understand and enjoy using can’t be overstated
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
Fragments — Fragments, introduced in Android 3.0 (API level 11), are used to encapsulate portions of your UI. This encapsulation makes Fragments particularly useful when optimizing your UI layouts for different screen sizes and creating reusable UI elements. Each Fragment includes its own UI layout and receives the related input events but is tightly bound to the Activity into which each must be embedded. Fragments are similar to UI View Controllers in iPhone development
Activities — Activities, described in detail in the previous chapter, represent the window, or screen, being displayed. Activities are the Android equivalent of Forms in traditional Windows desktop development. To display a UI, you assign a View (usually a layout or Fragment) to an Activity.