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From: markspace <markspace@nospam.nospam>
Newsgroups: comp.lang.java.programmer
Subject: Re: Java 8 Streams and Eratosthenes
Date: Tue, 04 Jun 2013 17:24:30 -0700
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On 6/4/2013 2:54 PM, Sebastian wrote:
> Hello there,
>
> I'd like to start an open discussion on issues involving the Streams
> API. As an exercise, I have looked at generating the first n primes,
> for some given n, using the sieve of Eratosthenes. It seemed a
> plausible enough proposition, seeing that the sieve involves repeatedly
> filtering a sequence of integers.


I don't think it's plausible or a good implementation.  The point of 
this particular sieve is that it relies on easily skipping past values 
and only accessing the one's you want.  For example, in an array, you 
can easily compute the even numbers by adding +2 to the previous offset. 
  Multiples of 3 are found by adding +3 to the previous offset.  Etc.

In order to determine if n is prime, you have to run n filters on all 
previous values of n.  This seems inefficient.

With streams, you have to account for the fact that the previous sieve 
has already removed a lot of values.  (I assume;  I'm no expert on Java 
lambdas.)  How do you even do that?  I don't know.

I think I agree with your conclusion that "Perhaps the lesson is that 
some things are best not done using streams." That's likely the case imo.

Perhaps a better candidate is BigInteger#nextProbablePrime().