class FormTestServiceObject

form_test.services.yml in drupal/core/modules/system/tests/modules/form_test/form_test.services.yml

drupal/core/modules/system/tests/modules/form_test/form_test.services.yml

form_test.form.serviceform in drupal/core/modules/system/tests/modules/form_test/form_test.services.yml

Drupal\form_test\FormTestServiceObject

class FormTestServiceObject implements FormInterface {

  /**
   * Implements \Drupal\Core\Form\FormInterface::getFormID().
   */
  public function getFormID() {
    return 'form_test_form_test_service_object';
  }

  /**
   * Implements \Drupal\Core\Form\FormInterface::buildForm().
   */
  public function buildForm(array $form, array &$form_state) {
    $form['element'] = array(
      '#markup' => 'The FormTestServiceObject::buildForm() method was used for this form.',
    );
    $form['bananas'] = array(
      '#type' => 'textfield',
      '#default_value' => 'brown',
      '#title' => t('Bananas'),
    );
    $form['actions']['#type'] = 'actions';
    $form['actions']['submit'] = array(
      '#type' => 'submit',
      '#value' => t('Save'),
    );
    return $form;
  }

  /**
   * Implements \Drupal\Core\Form\FormInterface::validateForm().
   */
  public function validateForm(array &$form, array &$form_state) {
    drupal_set_message(t('The FormTestServiceObject::validateForm() method was used for this form.'));
  }

  /**
   * Implements \Drupal\Core\Form\FormInterface::submitForm().
   */
  public function submitForm(array &$form, array &$form_state) {
    drupal_set_message(t('The FormTestServiceObject::submitForm() method was used for this form.'));
    config('form_test.object')
      ->set('bananas', $form_state['values']['bananas'])
      ->save();
  }

}