lambda - Using Java 8's Optional with Stream::flatMap

Question

The new Java 8 stream framework and friends make for some very concise java code, but I have come across a seemingly-simple situation that is tricky to do concisely.

Consider a List<Thing> things and method Optional<Other> resolve(Thing thing). I want to map the Things to Optional<Other>s and get the first Other. The obvious solution would be to use things.stream().flatMap(this::resolve).findFirst(), but flatMap requires that you return a stream, and Optional doesn't have a stream() method (or is it a Collection or provide a method to convert it to or view it as a Collection).

The best I can come up with is this:

things.stream()
    .map(this::resolve)
    .filter(Optional::isPresent)
    .map(Optional::get)
    .findFirst();

But that seems awfully long-winded for what seems like a very common case. Anyone have a better idea?

Answer 1

Java 9

Optional.stream has been added to JDK 9. This enables you to do the following, without the need of any helper method:

Optional<Other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(Optional::stream)
          .findFirst();

Java 8

Yes, this was a small hole in the API, in that it's somewhat inconvenient to turn an Optional<T> into a zero-or-one length Stream<T>. You could do this:

Optional<Other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
          .findFirst();

Having the ternary operator inside the flatMap is a bit cumbersome, though, so it might be better to write a little helper function to do this:

/**
 * Turns an Optional<T> into a Stream<T> of length zero or one depending upon
 * whether a value is present.
 */
static <T> Stream<T> streamopt(Optional<T> opt) {
    if (opt.isPresent())
        return Stream.of(opt.get());
    else
        return Stream.empty();
}

Optional<Other> result =
    things.stream()
          .flatMap(t -> streamopt(resolve(t)))
          .findFirst();

Here, I've inlined the call to resolve() instead of having a separate map() operation, but this is a matter of taste.