Question

Why is this line needed in xml layout file?

xmlns:android="http://schemas.android.com/apk/res/android" 

Answer 1

In XML, xmlns declares a Namespace. In fact, when you do:

<LinearLayout android:id>
</LinearLayout>

Instead of calling android:id, the xml will use http://schemas.android.com/apk/res/android:id to be unique. Generally this page doesn’t exist (it’s a URI, not a URL), but sometimes it is a URL that explains the used namespace.

The namespace has pretty much the same uses as the package name in a Java application.

Here is an explanation.

Uniform Resource Identifier (URI)

A Uniform Resource Identifier (URI) is a string of characters which
identifies an Internet Resource.

The most common URI is the Uniform Resource Locator (URL) which
identifies an Internet domain address. Another, not so common type of
URI is the Universal Resource Name (URN).

In our examples we will only use URLs.