class FormTestArgumentsObject

class FormTestArgumentsObject extends ConfigFormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_test_form_test_arguments_object';
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'form_test.object',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, $arg = NULL) {
    $form['element'] = [
      '#markup' => 'The FormTestArgumentsObject::buildForm() method was used for this form.',
    ];
    $form['bananas'] = [
      '#type' => 'textfield',
      '#default_value' => $arg,
      '#title' => $this->t('Bananas'),
    ];
    $form['actions']['#type'] = 'actions';
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Save'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $this->messenger()
      ->addStatus($this->t('The FormTestArgumentsObject::validateForm() method was used for this form.'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->messenger()
      ->addStatus($this->t('The FormTestArgumentsObject::submitForm() method was used for this form.'));
    $this->config('form_test.object')
      ->set('bananas', $form_state->getValue('bananas'))
      ->save();
  }

}