The graphical user interface for an Android app is built using a hierarchy of View and ViewGroup objects. View objects are usually UI widgets such as buttons or text fields and ViewGroupobjects are invisible view containers that define how the child views are laid out, such as in a grid or a vertical list.
Android provides an XML vocabulary that corresponds to the subclasses of View and ViewGroup so you can define your UI in XML using a hierarchy of UI elements.
Figure 1. Illustration of how ViewGroup objects form branches in the layout and contain other View objects.
In this lesson, you'll create a layout in XML that includes a text field and a button. In the following lesson, you'll respond when the button is pressed by sending the content of the text field to another activity.