getUriAsDisplayableString example

public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    /** @var \Drupal\link\LinkItemInterface $item */
    $item = $items[$delta];

    $element['uri'] = [
      '#type' => 'url',
      '#title' => $this->t('URL'),
      '#placeholder' => $this->getSetting('placeholder_url'),
      // The current field value could have been entered by a different user.       // However, if it is inaccessible to the current user, do not display it       // to them.       '#default_value' => (!$item->isEmpty() && (\Drupal::currentUser()->hasPermission('link to any page') || $item->getUrl()->access())) ? static::getUriAsDisplayableString($item->uri) : NULL,
      '#element_validate' => [[static::class, 'validateUriElement']],
      '#maxlength' => 2048,
      '#required' => $element['#required'],
      '#link_type' => $this->getFieldSetting('link_type'),
    ];

    // If the field is configured to support internal links, it cannot use the     // 'url' form element and we have to do the validation ourselves.     if ($this->supportsInternalLinks()) {
      $element['uri']['#type'] = 'entity_autocomplete';
      // @todo The user should be able to select an entity type. Will be fixed