* expression {@code o -> o.toJavaUtil()} instead.
   *
   * @since 21.0
   */
  public java.util.Optional<T> toJavaUtil() {
    return java.util.Optional.ofNullable(orNull());
  }

  Optional() {}

  /**
   * Returns {@code true} if this holder contains a (non-null) instance.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
   */

   */
  public abstract boolean isPresent();

  /**
   * Returns the contained instance, which must be present. If the instance might be absent, use
   * {@link #or(Object)} or {@link #orNull} instead.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method
   * throws {@link IllegalStateException}, whereas the {@code java.util} counterpart throws {@link