Stream.concat

[Paul Sandoz]

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.