StreamOpFlag.* and OutOfMemoryError when going parall

[Christian Mallwitz]

Very well :-)

Leaving the parallel aspect aside, If I just wanted to find the first
n prime numbers based on a possibly infinity stream of numbers (I
don't want to use a Collection with a known size), is there another
'officially' approved way to covert an Iterator to a Stream (instead
of using Streams.stream())?

Regarding a fully lazy, parallel limit() and its future: does 'future'
mean pre- or post-JDK8?

Cheers!
Christian

On Wed, Dec 5, 2012 at 6:34 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> There's a good reason you are struggling with it -- this API is not intended
> for "casual" builders of streams.  The intention is that 99.9% of users will
> never write code to call the Streams.stream() method directly; they will use
> arrays, collections, generators, or other packaged providers of streams.
>
> A safe default is 0.  You can OR together the IS_ version of the flags if
> you know something about the nature of your stream.  You probably want
> IS_ORDERED if you want to represent a stream for which the order of elements
> means something (e.g., a range generator, a List, an array, etc.)
>
> The reason it is failing with OOME is that the parallel implementation of
> limit() computes the entire results rather than operating fully lazily.
> While it would be preferable to operate more lazily, a lazy parallel
> implementation of operations like limit is not trivial.  We hope to improve
> this in the future.
>
> On 12/5/2012 1:18 PM, Christian Mallwitz wrote:
>>
>> Hi,
>>
>> Using lambda-8-b67-linux-i586-03_dec_2012 I'm trying to compute n (not
>> necessarily the first n) prime numbers:
>>
>> import java.util.*;
>> import java.util.function.*;
>> import java.util.stream.*;
>>
>> public class LambdaExample3 {
>>
>>      public static boolean isPrime(long n) {
>>          if (n <= 1) { return false; }
>>          if (n == 2) { return true; }
>>          for (int i = 2; i <= Math.sqrt(n) + 1; i++) { if (n % i == 0)
>> return false; }
>>          return true;
>>      }
>>
>>      public static void main(String[] args) {
>>
>>          Stream<Long> stream =
>> Streams.parallel(Streams.spliteratorUnknownSize(new Iterator<Long>() {
>>                  private long n = 0;
>>                  @Override public boolean hasNext() { return true; }
>>                  @Override public Long next() { return ++n; }
>>                  }),
>>              // fails with OutOfMemoryError
>>              // StreamOpFlag.toStreamFlags(StreamOpFlag.NOT_SIZED,
>> StreamOpFlag.INITIAL_OPS_VALUE)
>>              // fails with OutOfMemoryError
>>              // StreamOpFlag.NOT_SIZED
>>              // works, but no speed-up to non-parallel version
>>              StreamOpFlag.INITIAL_OPS_VALUE
>>              );
>>
>>          stream
>>              .filter(LambdaExample3::isPrime)
>>              .limit(300000)
>>              .forEach(l -> { /*System.out.println(l);*/ });
>>      }
>> }
>>
>> I'm struggling with the StreamOpFlag parameter. What should I pick?
>> INITIAL_OPS_VALUE seems to work but isn't running anything in parallel
>> (at least it is not faster than the serial version). NOT_SIZED isn't
>> working but failing miserably with an  OutOfMemoryError. IS_PARALLEL
>> is not needed because I already use parallel() - should specifying
>> IS_PARALLEL and using Streams.stream() supposed to go parallel as
>> well?
>>
>> Is the OutOfMemoryError caused by a bug? The OutOfMemoryError is
>> reported from a fork/join pool thread so at least it is going
>> parallel? Am I missing something?
>>
>> Thanks
>> Christian
>>
>