kicking the tires on Gatherer mechanism in Java streams.

PrimeStream.java

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

public class PrimeStream {

  private static Gatherer<Long, List<Long>, Long> sieve =
      Gatherer.ofSequential(
          ArrayList::new,
          (soFar, n, downstream) -> {
            if (isPrime(soFar, n)) {
              soFar.add(n);
              downstream.push(n);
            }
            return true;
          });

  // Assumes soFar has all the primes less than n and that n >= 2
  private static boolean isPrime(List<Long> soFar, long n) {
    if (n == 2) return true;
    for (long p : soFar) {
      if (n % p == 0) return false;
      if (p * p > n) break;
    }
    return true;
  }

  public static Stream<Long> primes() {
    return Stream.iterate(2L, n -> n + 1).gather(sieve);
  }

  public static void main(String[] args) {
    int nth = Integer.parseInt(args[0]) - 1;
    System.out.println(primes().skip(nth).findFirst().get());
  }
}