Stream.concat

[Remi Forax]

On 07/14/2014 01:37 PM, Paul Sandoz wrote:
> On Jul 14, 2014, at 12:57 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>
>> On 07/14/2014 12:51 PM, Paul Sandoz wrote:
>>> On Jul 12, 2014, at 5:41 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>>>
>>>> I was not able to find the answer to my question in the archive,
>>>> why Stream.concat is not implemented like this ?
>>>>
>>>>   @SafeVarargs
>>>>   public static <T> Stream<T> concat(Stream<T>... streams) {
>>>>     return Arrays.stream(streams).flatMap(Function.identity());
>>>>   }
>>>>
>>> Because the capabilities and characteristics of the streams are then lost e.g. in this case the splitting is governed by the number of streams passed in.
>> it seems to be a limitation of flatMap in that case, no ?
>>
> That would be much harder to optimise since each element gets mapped to a Stream of 0 or more elements when the pipeline is executed. The operation has no "global" view of all the streams to concatenate as all it knows is a mapping function. At the moment flatMap is quite a simple and efficient stateless operation and i think it best it stays that way.
>
> e.g. imagine the case of streaming over the lines of a file and flatMapping each line to one or more words.
>
> Paul.
>

and imagine the case where you know the size of a stream returned by 
flatMap, in that case,
you may want to split before pumping the values of the stream.

Rémi