Stream.limit() - puzzler/bug/feature

[Sam Pullara]

This was my thinking when I read the example. Not sure if that is practical but it might reduce errors such as the one described.

Sam

On Nov 15, 2012, at 9:36 AM, Remi Forax <forax at univ-mlv.fr> wrote:

> 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
>>> 
>>> 
>>> 
>>> 
> 
>