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From: Steven Simpson <ss@my.domain.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: parallel stream with return
Date: Wed, 30 Sep 2015 22:25:55 +0100
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On 30/09/15 18:22, Eric Sosman wrote:
> 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

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Slop {
     private static Object generate() {
         return new Object();
     }

     public static void main(String[] args) throws Exception {
         Collection<Object> foo =
             IntStream.range(0, 4096)
             .parallel()
             .mapToObj(i -> generate())
             .collect(Collectors.toCollection(() -> new HashSet<Object>()));
         System.out.println(foo);
     }
}



> (but I'm not
>       100% sure about what kind of synchronization you'll need), or

<http://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#collect-java.util.stream.Collector->
> When executed in parallel, multiple intermediate results may be 
> instantiated, populated, and merged so as to maintain isolation of 
> mutable data structures. Therefore, even when executed in parallel 
> with non-thread-safe data structures (such as |ArrayList|), no 
> additional synchronization is needed for a parallel reduction.

Perhaps it would be interesting to see whether the lambda provided to 
toCollection is invoked multiple times...

import java.util.*;
import java.util.concurrent.atomic.*;
import java.util.function.*;
import java.util.stream.*;

public class Slop {
     private static Object generate() {
         return new Object();
     }

     private static AtomicInteger counter = new AtomicInteger();

     private static class MySet extends HashSet<Object> {
         public MySet() {
             counter.getAndIncrement();
         }
     }

     public static void main(String[] args) throws Exception {
         Collection<Object> foo =
             IntStream.range(0, 4096)
             .parallel()
             .mapToObj(i -> generate())
             .collect(Collectors.toCollection(() -> new MySet()));
         System.out.println(foo);
         System.out.println(counter);
     }
}

I got '16'.


-- 
ss at comp dot lancs dot ac dot uk