in bijection-guava/src/main/scala/com/twitter/bijection/guava/GuavaBijections.scala [29:34]
implicit def optional2Option[T, U](implicit
bij: ImplicitBijection[T, U]
): Bijection[Optional[T], Option[U]] =
Bijection.build[Optional[T], Option[U]] { opt =>
if (opt.isPresent) Some(bij(opt.get)) else None
} { opt => if (opt.isDefined) Optional.of[T](bij.invert(opt.get)) else Optional.absent[T] }