import java.util.stream.Stream;
import java.util.stream.IntStream;
import java.util.stream.Collectors;
import java.util.function.IntSupplier;
import java.util.Arrays;
import java.util.*;

public class Streamer {
  public static void main(String[] args){
    int[] xs = {1,2,3,4,5,6,7,8,9};
    
    // find the primes in the array.
    // there's only one stream operation here.
    
    int[] keepers = Arrays.stream(xs)       // build a stream 
                  . filter(n -> prime(n))   // stream op.: only keep primes
                  . toArray();              // collector: put back in an array
    
    System.out.println("the primes: "+Arrays.toString(keepers));
    
    
    // let's sum the primes we were just under:
    
    int ans = Arrays.stream(xs)               // stream builder
                    .filter(n -> prime(n))    // stream operation
                    .sum();                   // collector: add them
    
    System.out.println("\nsum of primes in the original array:\n"+ans);
  }
  
  public static boolean prime(int n){
    // base cases still handled directly.
    if (n<2){ return false; }
    
    // build a thing that generates ints from 2 and up
    IntSupplier twoAndUp = new IntSupplier(){
      int x=2;
      @Override public int getAsInt(){ return x++;}
    };
    
    // gratuitous use of streams here.
    return IntStream
           .generate(twoAndUp)        // start with ints 2,3,4,...
           .limit(Math.max(0,n-2))    // only keep enough through (n-1)
           .filter(i-> n%i==0)        // only keep ones that divide n evenly
           .count() == 0;             // are there any left?
  }
  
}