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An XML Namespace has the properties: Namespace URI: Namespace name expressed as a URI to which the prefix is bound. prefix: syntactically, this is the part of the attribute name following the XMLConstants. XMLNS_ATTRIBUTE ("xmlns") in the Namespace declaration.
An XML namespace is a collection of names that can be used as element or attribute names in an XML document. The namespace qualifies element names uniquely on the Web in order to avoid conflicts between elements with the same name.
XML tags define the data and used to store and organize data. It's easily scalable and simple to develop. In Android, the XML is used to implement UI-related data, and it's a lightweight markup language that doesn't make layout heavy. XML only contains tags, while implementing they need to be just invoked.
android is usually used for attribute coming from Android SDK itself. app is often used if you are using the support library. You may also see other namespaces if you are using custom views (of your own or form a library). Thanks.
The app namespace is not specific to a library, but it is used for all attributes defined in your app, whether by your code or by libraries you import, effectively making a single global namespace for custom attributes - i.e., attributes not defined by the android system.
In this case, the appcompat-v7 library uses custom attributes mirroring the android: namespace ones to support prior versions of android (for example: android:showAsAction was only added in API11, but app:showAsAction (being provided as part of your application) works on all API levels your app does) - obviously using the android:showAsAction wouldn't work on API levels where that attribute is not defined.
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