public void testToJavaUtil_static() {
    assertThat(Optional.toJavaUtil(null)).isNull();
    assertEquals(java.util.Optional.empty(), Optional.toJavaUtil(Optional.absent()));
    assertEquals(java.util.Optional.of("abc"), Optional.toJavaUtil(Optional.of("abc")));
  }

  public void testToJavaUtil_instance() {
    assertEquals(java.util.Optional.empty(), Optional.absent().toJavaUtil());

   *
   * <p>If {@code googleOptional} is known to be non-null, use {@code googleOptional.toJavaUtil()}
   * instead.
   *
   * <p>Unfortunately, the method reference {@code Optional::toJavaUtil} will not work, because it
   * could refer to either the static or instance version of this method. Write out the lambda
   * expression {@code o -> Optional.toJavaUtil(o)} instead.
   *
   * @since 33.4.0 (but since 21.0 in the JRE flavor)
   */