Java 8 Spliterator (or similar) that returns a value iff there's only a single value

Question

I'm a big fan of the singleOrEmpty stream operator. It's not in the std lib, but I find it very useful. If a stream has only a single value, it returns that value in an Optional. If it has no values or more than one value, it returns Optional.empty().

Optional<Int> value = someList.stream().{singleOrEmpty}
[]     -> Optional.empty()
[1]    -> Optional.of(1)
[1, 1] -> Optional.empty()
etc.

I asked a question about it earlier and @ThomasJungblut came up with this great implementation:

public static <T> Optional<T> singleOrEmpty(Stream<T> stream) {
    return stream.limit(2)
        .map(Optional::ofNullable)
        .reduce(Optional.empty(),
             (a, b) -> a.isPresent() ^ b.isPresent() ? b : Optional.empty());
}

The only problem is, you have to put it at the beginning of your call

singleOrEmpty(someList.stream().filter(...).map(...))

rather than sequentially at the end

someList.stream().filter().map().singleOrEmpty()

which makes it harder to read than other stream mechanisms.

So as a newbie to this stream processing stuff, does anybody have any tricks for how to go about putting a short-circuiting singleOrEmpty mechanism at the end of a sequence of stream transformations?

Answer 1

It will not be as fast as the one with limit(2) but you can use it as list.stream().filter(...).map(...).collect(singleOrEmpty())

static <T> Collector<T, ?, Optional<T>> singleOrEmpty() {
    return Collectors.collectingAndThen(
            Collectors.mapping(
                    Optional::of,
                    Collectors.reducing((a, b) -> Optional.empty())
            ),
            o -> o.orElseGet(Optional::empty)
    );
}

Stream.empty().collect(singleOrEmpty());   // Optional.empty
Stream.of(1).collect(singleOrEmpty());     // Optional[1]
Stream.of(1, 1).collect(singleOrEmpty());  // Optional.empty
Stream.of(1, 1).skip(1).collect(singleOrEmpty());  // Optional[1]

For what it's worth, unless this is really performance critical code, I would personally prefer the less clever but much clearer implementation

static<T> Collector<T,?,Optional<T>> singleOrEmpty() {
    return Collectors.collectingAndThen(
            Collectors.toList(),
            lst -> lst.size() == 1
                    ? Optional.of(lst.get(0))
                    : Optional.empty()
    );
}