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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: parallel stream with return
Date: Wed, 30 Sep 2015 13:22:11 -0400
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On 9/30/2015 11:02 AM, Philipp Kraus wrote:
> Am Mittwoch, 30. September 2015 14:01:37 UTC+2 schrieb Eric Sosman:
>> On 9/30/2015 2:35 AM, Philipp Kraus wrote:
>>> Hello,
>>>
>>> I'm new with the Java 1.8 parallel stream, so I have got a problem:
>>>
>>> I have the sequential for loop:
>>>
>>> final Set<myClass> x = new HashSet<>();
>>> for(int i=0; i < value; ++i)
>>>       x.add( this.generate() );
>>>
>>> And I would like to do this with a parallel stream which collects each generate return value into the set.
>>> My first try was to use a synchronized HashSet and
>>>
>>> IntStream.range(0, value).parallel().forEach( i-> { x.add(this.generate()) } );
>>>
>>> After that I try to swap forEach to map, but at this I must return an int value and not a myClass value, so how can I do n generate calls in parallel and collect the results into a hashset?
>>
>>       I think you want java.util.stream.Collectors, either toSet()
>> or toCollection().
>
> but how can I iterate within the range [0,n) in parallel? Can you show me the code of the sequential for loop?

     Untested:

	Set<myClass> x =
	    IntStream.range(0, value)
	    .parallel()
	    .mapToObj(i -> this.generate())
	    .collect(Collectors.toSet());

Note that this won't necessarily produce a HashSet, but is more
likely to produce some other kind of Set.  If you really need an
actual HashSet, you could

     - Use the toCollection() method of Collectors (but I'm not
       100% sure about what kind of synchronization you'll need), or

     - Copy the unknown Set's content into a HashSet after the
       pipeline finishes:

	Set<myClass> x = new HashSet<>(
	    IntStream.range ...

-- 
esosman@comcast-dot-net.invalid
"Don't be afraid of work. Make work afraid of you." -- TLM