A resource is a piece of your application that is not code – things like image files, audio files, and XML files.
To access a resource in code, you use its resource ID
resource ID is int
The at symbol (@) signifies that you’re referring to a resource rather than providing a literal value. The plus (+) indicates that you are creating a new resource reference; without it, you’re referring to an existing resource reference. Then comes id, defining what type of resource it is. Finally, you have the name of the resource
directory res is for resource
By using resource “qualifiers,” you can let Android take care of this for you.
A qualifier is a portion of a directory’s name that marks its contents as being used for a specific situation
resource folder name
animator: Property animations defined in XML.
anim: View animations defined in XML.
color: State lists of colors defined in XML. State lists are covered later in this chapter.
drawable: Drawable assets that can be defined in XML or image files (PNG, GIF, or JPG).
layout: Layouts defined in XML.
menu: Menus such as the action bar menu defined in XML.
raw: Any raw files such as audio files and custom bytecode.
values: Various simple values defined in XML, such as strings, floats, and integer colors.
xml: Any XML files you wish to read at runtime.