class OptionsButtonsWidget

class OptionsButtonsWidget extends OptionsWidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);
    $options = $this->getOptions($items->getEntity());
    $selected = $this->getSelectedOptions($items);
    // If required and there is one single option, preselect it.
    if ($this->required && count($options) == 1) {
      reset($options);
      $selected = [
        key($options),
      ];
    }
    if ($this->multiple) {
      $element += [
        '#type' => 'checkboxes',
        '#default_value' => $selected,
        '#options' => $options,
      ];
    }
    else {
      $element += [
        '#type' => 'radios',
        // Radio buttons need a scalar value. Take the first default value, or
        // default to NULL so that the form element is properly recognized as
        // not having a default value.
'#default_value' => $selected ? reset($selected) : NULL,
        '#options' => $options,
      ];
    }
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEmptyLabel() {
    if (!$this->required && !$this->multiple) {
      return t('N/A');
    }
  }

}