diff --git a/core/modules/user/lib/Drupal/user/UserData.php b/core/modules/user/lib/Drupal/user/UserData.php
index 13855d2..e607d87 100644
--- a/core/modules/user/lib/Drupal/user/UserData.php
+++ b/core/modules/user/lib/Drupal/user/UserData.php
@@ -12,7 +12,7 @@
 /**
  * Defines the user data service.
  */
-class UserData {
+class UserData implements UserDataInterface {
 
   /**
    * The database connection to use.
@@ -32,26 +32,7 @@ public function __construct(Connection $connection) {
   }
 
   /**
-   * Returns data stored for a user account.
-   *
-   * @param string $module
-   *   The name of the module the data is associated with.
-   * @param integer $uid
-   *   (optional) The user account ID the data is associated with.
-   * @param string $name
-   *   (optional) The name of the data key.
-   *
-   * @return mixed|array
-   *   The requested user account data, depending on the arguments passed:
-   *   - For $module, $name, and $uid, the stored data is returned, or NULL if
-   *     no data was found.
-   *   - For $module and $uid, an associative array is returned that contains
-   *     the stored data name/value pairs.
-   *   - For $module and $name, an associative array is returned whose keys are
-   *     user IDs and whose values contain the stored data.
-   *   - For $module only, an associative array is returned that contains all
-   *     existing data for $module in all user accounts, keyed first by user ID
-   *     and $name second.
+   * Implements \Drupal\user\UserDataInterface::get().
    */
   public function get($module, $uid = NULL, $name = NULL) {
     $query = $this->connection->select('users_data', 'ud')
@@ -99,18 +80,7 @@ public function get($module, $uid = NULL, $name = NULL) {
   }
 
   /**
-   * Stores data for a user account.
-   *
-   * @param string $module
-   *   The name of the module the data is associated with.
-   * @param integer $uid
-   *   The user account ID the data is associated with.
-   * @param string $name
-   *   The name of the data key.
-   * @param mixed $value
-   *   The value to store. Non-scalar values are serialized automatically.
-   *
-   * @return void
+   * Implements \Drupal\user\UserDataInterface::set().
    */
   public function set($module, $uid, $name, $value) {
     $serialized = 0;
@@ -132,20 +102,7 @@ public function set($module, $uid, $name, $value) {
   }
 
   /**
-   * Deletes data stored for a user account.
-   *
-   * @param string|array $module
-   *   (optional) The name of the module the data is associated with. Can also
-   *   be an array to delete the data of multiple modules.
-   * @param integer|array $uid
-   *   (optional) The user account ID the data is associated with. If omitted,
-   *   all data for $module is deleted. Can also be an array of IDs to delete
-   *   the data of multiple user accounts.
-   * @param string $name
-   *   (optional) The name of the data key. If omitted, all data associated with
-   *   $module and $uid is deleted.
-   *
-   * @return void
+   * Implements \Drupal\user\UserDataInterface::delete().
    */
   public function delete($module = NULL, $uid = NULL, $name = NULL) {
     $query = $this->connection->delete('users_data');
diff --git a/core/modules/user/lib/Drupal/user/UserDataInterface.php b/core/modules/user/lib/Drupal/user/UserDataInterface.php
new file mode 100644
index 0000000..a12eba5
--- /dev/null
+++ b/core/modules/user/lib/Drupal/user/UserDataInterface.php
@@ -0,0 +1,75 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\user\UserDataInterface.
+ */
+
+namespace Drupal\user;
+
+use Drupal\Core\Database\Connection;
+
+/**
+ * Defines the user data service interface.
+ */
+class UserDataInterface {
+
+  /**
+   * Returns data stored for a user account.
+   *
+   * @param string $module
+   *   The name of the module the data is associated with.
+   * @param integer $uid
+   *   (optional) The user account ID the data is associated with.
+   * @param string $name
+   *   (optional) The name of the data key.
+   *
+   * @return mixed|array
+   *   The requested user account data, depending on the arguments passed:
+   *   - For $module, $name, and $uid, the stored value is returned, or NULL if
+   *     no value was found.
+   *   - For $module and $uid, an associative array is returned that contains
+   *     the stored data name/value pairs.
+   *   - For $module and $name, an associative array is returned whose keys are
+   *     user IDs and whose values contain the stored values.
+   *   - For $module only, an associative array is returned that contains all
+   *     existing data for $module in all user accounts, keyed first by user ID
+   *     and $name second.
+   */
+  public function get($module, $uid = NULL, $name = NULL);
+
+  /**
+   * Stores data for a user account.
+   *
+   * @param string $module
+   *   The name of the module the data is associated with.
+   * @param integer $uid
+   *   The user account ID the data is associated with.
+   * @param string $name
+   *   The name of the data key.
+   * @param mixed $value
+   *   The value to store. Non-scalar values are serialized automatically.
+   *
+   * @return void
+   */
+  public function set($module, $uid, $name, $value);
+
+  /**
+   * Deletes data stored for a user account.
+   *
+   * @param string|array $module
+   *   (optional) The name of the module the data is associated with. Can also
+   *   be an array to delete the data of multiple modules.
+   * @param integer|array $uid
+   *   (optional) The user account ID the data is associated with. If omitted,
+   *   all data for $module is deleted. Can also be an array of IDs to delete
+   *   the data of multiple user accounts.
+   * @param string $name
+   *   (optional) The name of the data key. If omitted, all data associated with
+   *   $module and $uid is deleted.
+   *
+   * @return void
+   */
+  public function delete($module = NULL, $uid = NULL, $name = NULL);
+
+}