Collectors.Characteristics.IDENTITY_FINISH Spec Clarification

[Chris Dennis]

Is there a more suitable mailing list for this kind of core-libs spec related enquiry to be directed at?

Thanks,

Chris

> On Oct 9, 2018, at 10:48 AM, Chris Dennis <chris.w.dennis at gmail.com> wrote:
> 
> I’m attempting to clarify the meaning/intent of the language in the specification of the identity-finish characteristic of collectors.
> 
> The javadoc reads: 
> 
>    Indicates that the finisher function is the identity function and
>    can be elided.  If set, it must be the case that an unchecked cast
>    from A to R will succeed.
> 
> In the following code the second sentence clearly indicates that the second cast from A to R must succeed. What I’m trying to confirm however is that the first sentence is intended to require that the finisher is a true identity function, so the result of the collector must be one of the instances created through the supplier, and that therefore the cast from R to A is also safe.
> 
> public static void main(String[] args) {
>  Stream<String> a = Stream.of("foo", "bar", "baz");
>  Stream<String> b = Stream.of("alice", "bob", "eve");
> 
>  System.out.println(collect(Stream.of(a, b), partitioningBy(s -> s.startsWith("b"), toList())));
> }
> 
> public static <T, A, R> R collect(Stream<Stream<T>> streams, Collector<T, A, R> collector) {
>  if (collector.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
>    return streams.map(s -> s.collect(collector))
>        // Indicates that the finisher function is the identity function and can be elided.
>        .map(r -> (A) r)
>        .reduce(collector.combiner())
> 
>        // If set, it must be the case that an unchecked cast from A to R will succeed.
>        .map(a -> (R) a)
>        .orElse(Stream.<T>empty().collect(collector));
>  } else {
>    return streams.flatMap(identity()).collect(collector);
>  }
> }
> 
> Thoughts, opinions?
> 
> Thanks,
> 
> Chris Dennis