Defining the Namespaces

ref: https://www.drupal.org/node/1353118

Modules #

Modules creating classes should place their code inside a custom namespace.
The convention for those namespaces is

namespace Drupal\example_module
To support autoloading in Drupal 8, these classes should be placed inside the correct folder:
modules/example_module/src

The pattern is:

/src/

Drupal 8 Examples
Class Drupal\example_module\Foo in namespace Drupal\example_module should be in a file named example_module/src/Foo.php
Class Drupal\example_module\Foo\Bar in namespace Drupal\example_module\Foo should be in a file named example_module/src/Foo/Bar.php