Collectors.Characteristics.IDENTITY_FINISH Spec Clarification
Chris Dennis
chris.w.dennis at gmail.com
Tue Oct 9 14:48:02 UTC 2018
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
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