RFR: JDK-8205461 Create Collector which merges results of two other collectors
brian.goetz at oracle.com
Sat Sep 15 15:36:45 UTC 2018
tl;dr: "Duplexing" is an OK name, though I think `teeing` is less likely
to be a name we regret, for reasons outlined below.
The behavior of this Collector is:
- duplicate the stream into two identical streams
- collect the two streams with two collectors, yielding two results
- merge the two results into a single result
Obviously, a name like `duplexingAndCollectingAndThenMerging`, which,
entirely accurate and explanatory, is "a bit" unwieldy. So the
- how much can we drop and still be accurate
- which parts are best to drop.
When we pick names, we are not just trying to pick the best name for
now, but we should imagine all the possible operations one might ever
want to do in the future (names in the JDK are forever) and make a
reasonable attempt to imagine whether this could cause confusion or
regret in the future.
To evaluate "duplexing" here (which seems the most important thing to
keep), I'd ask: is there any other reasonable way to imagine a
`duplexing` collect operation, now or in the future?
One could imagine wanting an operation that takes a stream and produces
two streams whose contents are that of the original stream. And
"duplex" is a good name for that. But, it is not a Collector; it would
be a stream transform, like concat. So that doesn't seem a conflict; a
duplexing collector and a duplexing stream transform are sort of from
Can one imagine a "duplexing" Collector that doesn't do any collection?
I cannot. Something that returns a pair of streams would not be a
Collector, but something else. So dropping AndCollecting seems justified.
What about "AndThenMerging"? The purpose of collect is to reduce the
stream into a summary description. Can we imagine a duplexing operation
that doesn't merge the two results, but instead just returns a tuple of
the results? Yes, I can totally imagine this, especially once we have
value types and records, which makes returning ad-hoc tuples cheaper
(syntactically, heap-wise, CPU-wise.) So I think this is quite a
reasonable possibility. But, I would have no problem with an overload
that didn't take a merger and returned a tuple of the result, and was
still called `duplexing`.
So I'm fine with dropping all the extra AndThisAndThat.
Finally, there's one other obvious direction we might extend this --
more than two collectors. There's no reason why we can only do two; we
could take a (likely homogeneous) varargs of Collectors, and return a
List of results -- which itself could then be streamed into another
collector. This actually sounds pretty useful (though I'm not
suggesting doing this right now.) And, I think it would be silly if this
were not called the same thing as the two-collector version (just as it
would be silly to have separate names for "concat two" and "concat n".)
And, this is where I think "duplexing" runs out of gas -- duplex implies
"two". Pedantic argue-for-the-sake-of-argument folks might observe that
"tee" also has bilateral symmetry, but I don't think you could
reasonably argue that a four-way "tee" is not less of an arity abuse
than a four-way "duplex", and the plumbing industry would agree:
So, for these reasons, I still think "teeing" has a better balance of
being both evocative what it does and likely to stand the test of time.
On 9/14/2018 1:09 PM, Stuart Marks wrote:
> First, naming. I think "duplex" as the root word wins! Using
> "duplexing" to conform to many of other collectors is fine; so,
> "duplexing" is good.
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