Introduction

In Android, the User Interface is defined in a Layout. A layout can be declared in XML or created dynamically in code. (It's recommended to declare the layout in XML rather than in code to keep the presentation layer separate from the implementation layer.) A layout can define an inpidual ListItem, a fragment, or even the entire Activity. Layout files are stored in the /res/layout folder and referenced in code with the following identifier: R.layout.<filename_without_extension>.

Android provides a useful variety of Layout classes that contain and organize inpidual elements of an activity (such as buttons, checkboxes, and other Views). The ViewGroup object is a container object that serves as the base class for Android's family of Layout classes. The Views placed in a layout form a hierarchy, with the topmost layout being the parent.

Android provides several built-in layout types designed for specific purposes, such as the RelativeLayout, which allows Views to be positioned with respect to other elements. The LinearLayout can stack Views or align them horizontally, depending on the orientation specified. The TableLayout can be used for laying out a grid of Views. Within various layouts, we can also justify Views with Gravity and provide proportional size with Weight control. Layouts and ViewGroups can be nested within each other to create complex configurations. Over a dozen different Layout objects are provided for managing widgets, lists, tables, galleries, and other display formats, plus you can always derive from the base classes to create your own custom layouts.