Stream.limit() - puzzler/bug/feature

[Remi Forax]

On 11/15/2012 06:22 PM, Brian Goetz wrote:
> The best way to think about it is that a Stream is more like an Iterator
> than a data structure.  There is some abstract source of data somewhere
> (it might be in a data structure, or might be generated from a function,
> or read from a network), and a series of transformations applied to the
> data between the source and the consumer.  Streams can additionally
> execute using parallelism, if requested.
>
> Stream constructs like:
>
> Stream<Person> s = people.stream()
>                            .filter(p -> p.getLastName().equals("Smith")))
>
> do not do any filtering on construction.  It simply says "there's a
> stream source, the collection 'people', and when you consume from the
> stream s, you'll get the results of filtering the source values."
>
> The confusion in Dmitry's example is akin to multiple activities reading
> from the same IO channel -- they might interfere with each other over
> who gets the next value, and any buffering that any consumer does may
> confuse other consumers.

Maybe the implementation should protect users to use two aliases of a 
non-replayable stream.
Using the example of Dmitry, if the stream is an IO channel, the second
call to limit() or to any method of 's' should throw an 
IllegalStateException.

Rémi

>
>
>
> On 11/15/2012 12:11 PM, Marcus Thiesen wrote:
>> Hey List,
>>
>> sorry, I haven't followed the whole Stream discussion and without publicliy
>> available JavaDocs (are there?) I'm currently trying to figure out what
>> this comment means, because without all that background I read the code as
>> annotated:
>>
>> 2012/11/15 Dmitry Bessonov <dmitry.bessonov at oracle.com>
>>
>>> While playing with method Sream.limit(int) using a mini-code like
>>>
>>>            final Stream<Integer> s = Arrays.asStream(1, 2, 3, 4, 5);
>>>
>> Give me a "Stream" view of the Array int[] 1, 2, 3, 4, 5.
>>
>>
>>
>>>            final Stream<Integer> to3 = s.limit(3);
>>>
>> Give me a Stream view of s limitted to 3 values.
>>
>>
>>>            final Stream<Integer> to4 = s.limit(4);
>>>
>> Give me a Stream view of s limitted to 4 values.
>>
>>
>>> have to admit that there's no unambiguous answer
>>> to the question of contents  of streams "to3" and "to4".
>>> It depends on which of the streams is consumed first.
>>>
>>>
>> My best guess would be to3 = [ 1, 2, 3 ] and to4 = [ 1, 2, 3, 4 ].
>>
>> Given my naive understanding of the above code the comment does not make
>> sense. The only way the above comment makes sense is that we are not
>> talking of (somehow immutable) views here but that those operations mutate
>>    the backing array on read time of the resulting stream. Am I right?
>>
>> Cheers,
>>     Marcus
>>
>>
>>
>>