hello_world.module file

<?php

/**
 * @file
 * Implements hello_world module.
 */

use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Entity\Display\EntityViewDisplayInterface;

/**
 * Implements hook_node_view_alter().
 */
function hello_world_node_view_alter(&$build, $node, $display, $view_mode, $langcode) {
  if ($node->getType() == 'article') {
    $build['hello_world_new_content'] = array(
      '#markup' => t('Hello World!'),
    );
  }
}

use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Entity\Display\EntityViewDisplayInterface;

Classes imported

function hello_world_node_view_alter(&$build, $node, $display, $view_mode, $langcode) {
 // My code.
}

Drupal\Core\Entity\EntityInterface

Drupal\Core\Entity\Display\EntityViewDisplayInterface

Want to know more?

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