diff --git a/core/modules/comment/lib/Drupal/comment/CommentManager.php b/core/modules/comment/lib/Drupal/comment/CommentManager.php
index 4b3ab0b..5454a1b 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentManager.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentManager.php
@@ -14,7 +14,7 @@
 /**
  * Comment manager contains common functions to manage comment fields.
  */
-class CommentManager {
+class CommentManager implements CommentManagerInterface {
 
   /**
    * The field info service.
@@ -44,13 +44,7 @@ public function __construct(FieldInfo $field_info, EntityManager $entity_manager
   }
 
   /**
-   * Utility function to return URI of the comment's parent entity.
-   *
-   * @param \Drupal\comment\CommentInterface $comment
-   *   The comment entity.
-   *
-   * @return array
-   *   An array returned by \Drupal\Core\Entity\EntityInterface::uri().
+   * {@inheritdoc}
    */
   public function getParentEntityUri(CommentInterface $comment) {
     return $this->entityManager
@@ -60,19 +54,7 @@ public function getParentEntityUri(CommentInterface $comment) {
   }
 
   /**
-   * Utility function to return an array of comment fields.
-   *
-   * @param string $entity_type
-   *   The entity type to return fields which are attached on.
-   *
-   * @return array
-   *   An array of comment field map definitions, keyed by field name. Each
-   *   value is an array with two entries:
-   *   - type: The field type.
-   *   - bundles: The bundles in which the field appears, as an array with entity
-   *     types as keys and the array of bundle names as values.
-   *
-   * @see field_info_field_map()
+   * {@inheritdoc}
    */
   public function getFields($entity_type = NULL) {
     $map = $this->getAllFields();
@@ -83,7 +65,7 @@ public function getFields($entity_type = NULL) {
   }
 
   /**
-   * Utility function to return all comment fields.
+   * {@inheritdoc}
    */
   public function getAllFields() {
     $map = $this->fieldInfo->getFieldMap();
@@ -100,20 +82,7 @@ public function getAllFields() {
   }
 
   /**
-   * Utility method to add the default comment field to an entity.
-   *
-   * Attaches a comment field named 'comment' to the given entity type and
-   * bundle. Largely replicates the default behaviour in Drupal 7 and earlier.
-   *
-   * @param string $entity_type
-   *   The entity type to attach the default comment field to.
-   * @param string $bundle
-   *   The bundle to attach the default comment field instance to.
-   * @param string $field_name
-   *   (optional) Field name to use for the comment field. Defaults to 'comment'.
-   * @param int $default_value
-   *   (optional) Default value, one of COMMENT_HIDDEN, COMMENT_OPEN,
-   *   COMMENT_CLOSED. Defaults to COMMENT_OPEN.
+   * {@inheritdoc}
    */
   public function addDefaultField($entity_type, $bundle, $field_name = 'comment', $default_value = COMMENT_OPEN) {
     // Make sure the field doesn't already exist.
@@ -170,12 +139,7 @@ public function addDefaultField($entity_type, $bundle, $field_name = 'comment',
   }
 
   /**
-   * Creates a comment_body field instance.
-   *
-   * @param string $entity_type
-   *   The type of the entity to which the comment field attached.
-   * @param string $field_name
-   *   Name of the comment field to add comment_body field.
+   * {@inheritdoc}
    */
   public function addBodyField($entity_type, $field_name) {
     // Create the field if needed.
@@ -224,13 +188,7 @@ public function addBodyField($entity_type, $field_name) {
   }
 
   /**
-   * Builds human readable page title for field_ui management screens.
-   *
-   * @param string $field_name
-   *   The comment field for which the overview is to be displayed.
-   *
-   * @return string
-   *   The human readable field name.
+   * {@inheritdoc}
    */
   public function getFieldUIPageTitle($field_name) {
     list($entity_type, $field) = explode('__', $field_name, 2);
diff --git a/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php b/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
new file mode 100644
index 0000000..b4c16e3
--- /dev/null
+++ b/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
@@ -0,0 +1,87 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\comment\CommentManagerInterface.
+ */
+
+namespace Drupal\comment;
+
+/**
+ * Comment manager contains common functions to manage comment fields.
+ */
+interface CommentManagerInterface {
+
+  /**
+   * Utility function to return URI of the comment's parent entity.
+   *
+   * @param \Drupal\comment\CommentInterface $comment
+   *   The comment entity.
+   *
+   * @return array
+   *   An array returned by \Drupal\Core\Entity\EntityInterface::uri().
+   */
+  public function getParentEntityUri(CommentInterface $comment);
+
+  /**
+   * Utility function to return an array of comment fields.
+   *
+   * @param string $entity_type
+   *   The entity type to return fields which are attached on.
+   *
+   * @return array
+   *   An array of comment field map definitions, keyed by field name. Each
+   *   value is an array with two entries:
+   *   - type: The field type.
+   *   - bundles: The bundles in which the field appears, as an array with entity
+   *     types as keys and the array of bundle names as values.
+   *
+   * @see field_info_field_map()
+   */
+  public function getFields($entity_type = NULL);
+
+  /**
+   * Utility function to return all comment fields.
+   */
+  public function getAllFields();
+
+  /**
+   * Utility method to add the default comment field to an entity.
+   *
+   * Attaches a comment field named 'comment' to the given entity type and
+   * bundle. Largely replicates the default behaviour in Drupal 7 and earlier.
+   *
+   * @param string $entity_type
+   *   The entity type to attach the default comment field to.
+   * @param string $bundle
+   *   The bundle to attach the default comment field instance to.
+   * @param string $field_name
+   *   (optional) Field name to use for the comment field. Defaults to 'comment'.
+   * @param int $default_value
+   *   (optional) Default value, one of COMMENT_HIDDEN, COMMENT_OPEN,
+   *   COMMENT_CLOSED. Defaults to COMMENT_OPEN.
+   */
+  public function addDefaultField($entity_type, $bundle, $field_name = 'comment', $default_value = COMMENT_OPEN);
+
+  /**
+   * Creates a comment_body field instance.
+   *
+   * @param string $entity_type
+   *   The type of the entity to which the comment field attached.
+   * @param string $field_name
+   *   Name of the comment field to add comment_body field.
+   */
+  public function addBodyField($entity_type, $field_name);
+
+  /**
+   * Builds human readable page title for field_ui management screens.
+   *
+   * @param string $field_name
+   *   The comment field for which the overview is to be displayed.
+   *
+   * @return string
+   *   The human readable field name.
+   */
+  public function getFieldUIPageTitle($field_name);
+
+}
diff --git a/core/modules/comment/lib/Drupal/comment/Controller/AdminController.php b/core/modules/comment/lib/Drupal/comment/Controller/AdminController.php
index 4f49850..38260ec 100644
--- a/core/modules/comment/lib/Drupal/comment/Controller/AdminController.php
+++ b/core/modules/comment/lib/Drupal/comment/Controller/AdminController.php
@@ -10,7 +10,7 @@
 use Drupal\Component\Utility\String;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
-use Drupal\comment\CommentManager;
+use Drupal\comment\CommentManagerInterface;
 use Drupal\field\FieldInfo;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
@@ -29,7 +29,7 @@ class AdminController extends ControllerBase implements ContainerInjectionInterf
   /**
    * The comment manager service.
    *
-   * @var \Drupal\comment\CommentManager
+   * @var \Drupal\comment\CommentManagerInterface
    */
   protected $commentManager;
 
@@ -48,10 +48,10 @@ public static function create(ContainerInterface $container) {
    *
    * @param \Drupal\field\FieldInfo $field_info
    *   The field info service.
-   * @param \Drupal\comment\CommentManager $comment_manager
+   * @param \Drupal\comment\CommentManagerInterface $comment_manager
    *   The comment manager service.
    */
-  public function __construct(FieldInfo $field_info, CommentManager $comment_manager) {
+  public function __construct(FieldInfo $field_info, CommentManagerInterface $comment_manager) {
     $this->fieldInfo = $field_info;
     $this->commentManager = $comment_manager;
   }
diff --git a/core/modules/comment/lib/Drupal/comment/Controller/CommentController.php b/core/modules/comment/lib/Drupal/comment/Controller/CommentController.php
index 7775219..1c2ffa9 100644
--- a/core/modules/comment/lib/Drupal/comment/Controller/CommentController.php
+++ b/core/modules/comment/lib/Drupal/comment/Controller/CommentController.php
@@ -8,7 +8,7 @@
 namespace Drupal\comment\Controller;
 
 use Drupal\comment\CommentInterface;
-use Drupal\comment\CommentManager;
+use Drupal\comment\CommentManagerInterface;
 use Drupal\field\FieldInfo;
 use Drupal\Core\Access\CsrfTokenGenerator;
 use Drupal\Core\Controller\ControllerBase;
@@ -61,7 +61,7 @@ class CommentController extends ControllerBase implements ContainerInjectionInte
   /**
    * The comment manager service.
    *
-   * @var \Drupal\comment\CommentManager
+   * @var \Drupal\comment\CommentManagerInterface
    */
   protected $commentManager;
 
@@ -76,10 +76,10 @@ class CommentController extends ControllerBase implements ContainerInjectionInte
    *   The current user service.
    * @param \Drupal\field\FieldInfo $field_info
    *   Field Info service.
-   * @param \Drupal\comment\CommentManager $comment_manager
+   * @param \Drupal\comment\CommentManagerInterface $comment_manager
    *   The comment manager service.
    */
-  public function __construct(HttpKernelInterface $httpKernel, CsrfTokenGenerator $csrf_token, AccountInterface $current_user, FieldInfo $field_info, CommentManager $comment_manager) {
+  public function __construct(HttpKernelInterface $httpKernel, CsrfTokenGenerator $csrf_token, AccountInterface $current_user, FieldInfo $field_info, CommentManagerInterface $comment_manager) {
     $this->httpKernel = $httpKernel;
     $this->csrfToken = $csrf_token;
     $this->currentUser = $current_user;
diff --git a/core/modules/comment/lib/Drupal/comment/Form/DeleteForm.php b/core/modules/comment/lib/Drupal/comment/Form/DeleteForm.php
index 2c13db5..07f3146 100644
--- a/core/modules/comment/lib/Drupal/comment/Form/DeleteForm.php
+++ b/core/modules/comment/lib/Drupal/comment/Form/DeleteForm.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\comment\Form;
 
-use Drupal\comment\CommentManager;
+use Drupal\comment\CommentManagerInterface;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\ContentEntityConfirmFormBase;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -20,17 +20,17 @@ class DeleteForm extends ContentEntityConfirmFormBase {
   /**
    * The comment manager.
    *
-   * @var \Drupal\comment\CommentManager
+   * @var \Drupal\comment\CommentManagerInterface
    */
   protected $commentManager;
 
   /**
    * Constructs a DeleteForm object.
    *
-   * @param \Drupal\comment\CommentManager $comment_manager
+   * @param \Drupal\comment\CommentManagerInterface $comment_manager
    *   The comment manager service.
    */
-  public function __construct(CommentManager $comment_manager) {
+  public function __construct(CommentManagerInterface $comment_manager) {
     $this->commentManager = $comment_manager;
   }