uniqueElements in parallel mode - encounter order

[Paul Sandoz]

What revision are you working from?

I don't observe such behaviour from the tip, encounter order is preserved for both sequential and parallel streams.

Example output:

   [testng] [1, 1, 8, 0, 1, 3, 5, 6, 5, 3, 9, 1, 5, 9, 8, 6, 6, 4, 7, 4, 4, 2, 7, 6, 7, 5, 0, 4, 0, 9, 7, 4, 1, 2, 5, 4, 4, 4, 5, 4, 3, 5, 4, 1, 9, 2, 2, 4, 0, 3, 6, 8, 0, 3, 6, 3, 1, 3, 2, 8, 7, 8, 2, 6, 5, 8, 2, 4, 4, 4, 0, 8, 3, 0, 2, 5, 0, 3, 8, 0, 9, 6, 7, 5, 1, 6, 5, 1, 0, 0, 9, 7, 1, 9, 7, 4, 3, 8, 8, 9, 8, 7, 1, 1, 7, 6, 0, 2, 3, 8, 2, 9, 7, 6, 1, 1, 5, 4, 9, 8, 3, 8, 2, 5, 9, 3, 1, 5, 3, 8, 5, 5, 3, 1, 3, 4, 7, 5, 9, 8, 8, 7, 8, 1, 2, 3, 2, 1, 9, 7, 1, 0, 2, 3, 0, 5, 3, 1, 6, 7, 3, 6, 3, 7, 7, 4, 4, 2, 9, 5, 7, 9, 2, 6, 5, 5, 4, 0, 0, 9, 0, 0, 8, 3, 7, 2, 8, 0, 6, 9, 3, 9, 8, 3, 8, 0, 8, 8, 1, 6, 5, 2, 0, 5, 1, 8, 8, 7, 5, 3, 0, 9, 1, 0, 2, 6, 0, 4, 0, 6, 9, 9, 8, 4, 4, 4, 2, 8, 2, 2, 9, 9, 0, 6, 3, 4, 7, 9, 2, 4, 4, 2, 1, 6, 6, 2, 6, 5, 4, 2, 6, 7, 1, 5, 6, 6, 9, 2, 9, 9, 2, 9, 5, 1, 5, 7, 7, 9, 3, 6, 5, 3, 4, 7, 7, 2, 6, 7, 1, 6, 1, 1, 3, 8, 0, 6, 4, 2, 0, 9, 0, 7, 1, 6, 7, 0, 5, 3, 8, 7]
   [testng] [1, 8, 0, 3, 5, 6, 9, 4, 7, 2]

Assuming that the input to the uniqueElements operation is not already sorted and declared as such then the implementation uses a LinkedHashSet or Set (if order is to be preserved or not respectively) to hold unique elements. It never explicitly sorts the input.

Paul.

On Nov 30, 2012, at 3:26 PM, Georgiy Rakov <georgiy.rakov at oracle.com> wrote:

> Hello,
> *
> uniqueElements()* method working in parallel mode does not preserve encounter order now. Consider following code. *result *will always contain sorted numbers in spite of shuffling, i. e. [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]. I could suppose this is the used algorithm artifact.
> 
>   import java.util.ArrayList;
>   import java.util.Arrays;
>   import java.util.Collections;
> 
>   public class UniqueElementsIssue1 {
>        public static void main(String arg[]) {
>            ArrayList<Integer> data = new ArrayList<>();
>            for (int i = 0; i<10; ++i) {
>                for (int k = 0; k<30; ++k) {
>                    data.add(i);
>                }
>            }
>            Collections.shuffle(data);
>            ArrayList<Integer> result = new ArrayList<>();
>            Arrays.parallel(data.toArray(new
>   Integer[0])).uniqueElements().into(result);
>        }
>   }
> 
> 
> While for sequential mode (Arrays.asStream) the encounter order is preserved.
> 
> Could you please tell if this is considered as expected behavior and will be showed in spec; for instance something like following could be there.
> /Having been called on stream in sequential mode, UniqueElements preserve encounter order.
> Having been called on stream in parallel mode, UniqueElements doesn't preserve encounter order.
> /
> The code above is attached for your convenience.
> 
> Thanks,
> Georgiy.
> <UniqueElementsIssue1.java>