Java 8 Collector that returns a value if there’s only a single value [duplicate]

This will incur an overhead of creating a set but it’s simple and will work correctly even if you forget to distinct() the stream first.

static<T> Collector<T,?,Optional<T>> singleOrEmpty() {
    return Collectors.collectingAndThen(
            Collectors.toSet(),
            set -> set.size() == 1 
                    ? set.stream().findAny() 
                    : Optional.empty()
    );
}