diff --git a/core/lib/Drupal/Core/Field/FieldConfigBase.php b/core/lib/Drupal/Core/Field/FieldConfigBase.php
index fa34ca3..e25d94f 100644
--- a/core/lib/Drupal/Core/Field/FieldConfigBase.php
+++ b/core/lib/Drupal/Core/Field/FieldConfigBase.php
@@ -146,7 +146,7 @@
    *
    * @var array
    */
-  public $default_value = array();
+  protected $default_value = array();
 
   /**
    * The name of a callback function that returns default values.
diff --git a/core/lib/Drupal/Core/Field/FieldConfigInterface.php b/core/lib/Drupal/Core/Field/FieldConfigInterface.php
index ae45a21..11da0df 100644
--- a/core/lib/Drupal/Core/Field/FieldConfigInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldConfigInterface.php
@@ -77,6 +77,23 @@ public function setSettings(array $settings);
   public function setSetting($setting_name, $value);
 
   /**
+   * Sets whether the field can be empty.
+   *
+   * If a field is required, an entity needs to have at least a valid,
+   * non-empty item in that field's FieldItemList in order to pass validation.
+   *
+   * An item is considered empty if its isEmpty() method returns TRUE.
+   * Typically, that is if at least one of its required properties is empty.
+   *
+   * @param bool $required
+   *   TRUE if the field is required. FALSE otherwise.
+   *
+   * @return $this
+   *   The current object, for a fluent interface.
+   */
+  public function setRequired($required);
+
+  /**
    * Sets a default value.
    *
    * Note that if a default value callback is set, it will take precedence over
diff --git a/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php b/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
index 8e9391b..5e00f7e 100644
--- a/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
@@ -160,42 +160,6 @@ public function isDisplayConfigurable($display_context);
   public function getDisplayOptions($display_context);
 
   /**
-   * Returns whether the field can be empty.
-   *
-   * If a field is required, an entity needs to have at least a valid,
-   * non-empty item in that field's FieldItemList in order to pass validation.
-   *
-   * An item is considered empty if its isEmpty() method returns TRUE.
-   * Typically, that is if at least one of its required properties is empty.
-   *
-   * @return bool
-   *   TRUE if the field is required.
-   *
-   * @see \Drupal\Core\TypedData\Plugin\DataType\ItemList::isEmpty()
-   * @see \Drupal\Core\Field\FieldItemInterface::isEmpty()
-   * @see \Drupal\Core\TypedData\DataDefinitionInterface:isRequired()
-   * @see \Drupal\Core\TypedData\TypedDataManager::getDefaultConstraints()
-   */
-  public function isRequired();
-
-  /**
-   * Set whether the field can be empty.
-   *
-   * If a field is required, an entity needs to have at least a valid,
-   * non-empty item in that field's FieldItemList in order to pass validation.
-   *
-   * An item is considered empty if its isEmpty() method returns TRUE.
-   * Typically, that is if at least one of its required properties is empty.
-   *
-   * @param bool $required
-   *   TRUE if the field is required. FALSE otherwise.
-   *
-   * @return $this
-   *   The current object, for a fluent interface.
-   */
-  public function setRequired($required);
-
-  /**
    * Returns the default value for the field in a newly created entity.
    *
    * @param \Drupal\Core\Entity\FieldableEntityInterface $entity