diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 6a81eb0..8e59e0f 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -190,9 +190,12 @@ public function getForm($form_arg) {
   /**
    * {@inheritdoc}
    */
-  public function buildForm($form_id, &$form_state) {
+  public function buildForm($form_id, array &$form_state) {
     // Ensure some defaults; if already set they will not be overridden.
-    $form_state += $this->getFormStateDefaults();
+    $form_state = NestedArray::mergeDeep($form_state, $this->getFormStateDefaults());
+
+    // Ensure the form ID is prepared.
+    $form_id = $this->getFormId($form_id, $form_state);
 
     if (!isset($form_state['input'])) {
       $form_state['input'] = $form_state['method'] == 'get' ? $_GET : $_POST;
diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index bbe355a..b655523 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -73,7 +73,7 @@ public function getForm($form_arg);
    *   can implement hook_forms(), which maps different $form_id values to the
    *   proper form constructor function. Examples may be found in node_forms(),
    *   and search_forms().
-   * @param $form_state
+   * @param array $form_state
    *   An array which stores information about the form. This is passed as a
    *   reference so that the caller can use it to examine what in the form
    *   changed when the form submission process is complete. Furthermore, it may
@@ -229,7 +229,7 @@ public function getForm($form_arg);
    *
    * @see self::redirectForm()
    */
-  public function buildForm($form_id, &$form_state);
+  public function buildForm($form_id, array &$form_state);
 
   /**
    * Retrieves default values for the $form_state array.
diff --git a/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php b/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php
index 2374046..9943f67 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php
@@ -25,7 +25,7 @@ class FormBuilderTest extends UnitTestCase {
   /**
    * The form builder being tested.
    *
-   * @var \Drupal\Core\Form\FormBuilderInterface
+   * @var \Drupal\Core\Form\FormBuilder
    */
   protected $formBuilder;
 
@@ -354,6 +354,48 @@ public function testGetFormWithObject() {
   }
 
   /**
+   * Tests the getForm() method with a class name based form ID.
+   */
+  public function testgetFormWithClassString() {
+    $form_id = '\Drupal\Tests\Core\Form\TestForm';
+    $object = new TestForm();
+    $form = array();
+    $form_state = array();
+    $expected_form = $object->buildForm($form, $form_state);
+
+    $form = $this->formBuilder->getForm($form_id);
+    $this->assertFormElement($expected_form, $form, 'test');
+    $this->assertSame('test_form', $form['#id']);
+  }
+
+  /**
+   * Tests the buildForm() method with a string based form ID.
+   */
+  public function testBuildFormWithString() {
+    $form_id = 'test_form_id';
+    $expected_form = $form_id();
+
+    $form = $this->formBuilder->getForm($form_id);
+    $this->assertFormElement($expected_form, $form, 'test');
+    $this->assertSame($form_id, $form['#id']);
+  }
+
+  /**
+   * Tests the buildForm() method with a class name based form ID.
+   */
+  public function testBuildFormWithClassString() {
+    $form_id = '\Drupal\Tests\Core\Form\TestForm';
+    $object = new TestForm();
+    $form = array();
+    $form_state = array();
+    $expected_form = $object->buildForm($form, $form_state);
+
+    $form = $this->formBuilder->buildForm($form_id, $form_state);
+    $this->assertFormElement($expected_form, $form, 'test');
+    $this->assertSame('test_form', $form['#id']);
+  }
+
+  /**
    * Tests the buildForm() method with a form object.
    */
   public function testBuildFormWithObject() {
@@ -363,7 +405,6 @@ public function testBuildFormWithObject() {
     $form_arg = $this->getMockForm(NULL, $expected_form);
 
     $form_state['build_info']['callback_object'] = $form_arg;
-    $form_state['build_info']['args'] = array();
 
     $form = $this->formBuilder->buildForm($form_id, $form_state);
     $this->assertFormElement($expected_form, $form, 'test');
@@ -388,8 +429,7 @@ public function testBuildFormWithHookForms() {
         ),
       )));
 
-    $form_state['build_info']['args'] = array();
-
+    $form_state = array();
     $form = $this->formBuilder->buildForm($form_id, $form_state);
     $this->assertFormElement($expected_form, $form, 'test');
     $this->assertSame($form_id, $form_state['build_info']['form_id']);
@@ -410,7 +450,6 @@ public function testRebuildForm() {
     // Do an initial build of the form and track the build ID.
     $form_state = array();
     $form_state['build_info']['callback_object'] = $form_arg;
-    $form_state['build_info']['args'] = array();
     $form = $this->formBuilder->buildForm($form_id, $form_state);
     $original_build_id = $form['#build_id'];
 
@@ -682,7 +721,6 @@ public function testSendResponse() {
     // Do an initial build of the form and track the build ID.
     $form_state = array();
     $form_state['build_info']['callback_object'] = $form_arg;
-    $form_state['build_info']['args'] = array();
     $this->formBuilder->buildForm($form_id, $form_state);
   }
 
@@ -855,7 +893,9 @@ public function getFormId() {
     return 'test_form';
   }
 
-  public function buildForm(array $form, array &$form_state) { }
+  public function buildForm(array $form, array &$form_state) {
+    return test_form_id();
+  }
   public function validateForm(array &$form, array &$form_state) { }
   public function submitForm(array &$form, array &$form_state) { }
 }