Issue with Java IntStream whilst trying to solve a codewars problem (Prime Streaming PG-13)

Question

I signed up on codewars in March and I have finished a number of challenges. I am encountering difficulty with the following. The name is Prime Streaming (PG-13) and is highlighted below. Create an endless stream of prime numbers - a bit like IntStream.of(2, 3, 5, 7, 11, 13, 17), but infinite. The stream must be able to produce a million primes in a few seconds.

This is the code that I am using in an attempt to solve the problem.

import java.util.stream.IntStream;
     import java.util.ArrayList;
     import java.util.Arrays;
     public class Primes {
     private static ArrayList<Integer> primes = new ArrayList<Integer>();
     public static IntStream stream() {
      return IntStream.iterate(2,i->i+1).filter(x->isPrime(x)).limit(1000000); 
     }
     private static boolean isPrime(int n) {
      if(n<2) return false;
      for (int i = 0; i < primes.size();i++){
       int x = primes . get( i );
       if ( n % x ==0 ) return false;
      }
      primes.add(n);
      return true;
     }
     }

These are the tests that the code must pass in order for the challenge to be completed.

import org.junit.Test;
   import static org.junit.Assert.assertArrayEquals;
   public class PrimesTest {
    @Test
    public void test_0_10() {
     test(0, 10, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
    }
    @Test
    public void test_10_10() {
     test(10, 10, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71);
    }
    @Test
    public void test_100_10() {
     test(100, 10, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601);
    }
    @Test
    public void test_1000_10() {
     test(1000, 10, 7927, 7933, 7937, 7949, 7951, 7963, 7993, 8009, 8011, 8017); 
    }
    private void test(int skip, int limit, int... expect) {
     int[] found = Primes.stream().skip(skip).limit(limit).toArray();
     assertArrayEquals(expect, found);
    }
    }

My issue is this. The code will pass the first test but not the others. I have noticed that the number produced is a prime but it is one further down in the series. Apparently , everytime Primes.stream() is called, instead of starting from scratch and creating a new one, it continues from the prime number in the last test. For each further test, using the first set of primes , it skips a 10 extra and calls a number . The first test called is test_0_10 and it correctly returns 29. The second test called is test_1000_10 and it returns 8039 instead of 7927. The third test called is test_10_10 and it calls 8221 instead of 31. I know streams are lazily evaluated and suspect that this is the problem. Can someone please help?

See my description above for what I tried and the results

Answer 1

You might have to create the ArrayList for every call to stream to prevent its previous state interfering with new result. Something like below should fix your issue:

  private static ArrayList<Integer> primes;
  public static IntStream stream() {
    primes = new ArrayList<Integer>();
    return IntStream.rangeClosed(2,1000000).filter(x->isPrime(x));
  }