*
   * @since 21.0
   */
  @CheckForNull
  public static <T> Optional<T> fromJavaUtil(@CheckForNull java.util.Optional<T> javaUtilOptional) {
    return (javaUtilOptional == null) ? null : fromNullable(javaUtilOptional.orElse(null));
  }

  /**
   * Returns the equivalent {@code java.util.Optional} value to the given {@code
   * com.google.common.base.Optional}, or {@code null} if the argument is null.
   *

   * Number value = first.or(0.5); // fine
   * }</pre>
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's {@code
   * Optional.orElse}, but will not accept {@code null} as a {@code defaultValue} ({@link #orNull}
   * must be used instead). As a result, the value returned by this method is guaranteed non-null,