Count Primes with a Thread Pool

/*
 * Problem : Count primes.
 * Input   : n.
 * Output  : The count.
 * Approach: Thread pool.
 * Time    : O(n sqrt n / 4)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Count the primes from low to high (both included) with trial division.
        private static long countPrimes(long low, long high) {
            long count = 0;
            for (long x = Math.max(low, 2); x <= high; x++) {
                boolean prime = true;
                for (long d = 2; d * d <= x; d++) {
                    if (x % d == 0) {
                        prime = false;
                        break;
                    }
                }
                if (prime) {
                    count++;
                }
            }
            return count;
        }

    // This method solves the task. It receives the input and returns the answer.
    static long solve(long n) {
        // Step 1: A pool of 4 worker threads. Each part of the range becomes one task.
        int parts = 4;
        java.util.concurrent.ExecutorService pool = java.util.concurrent.Executors.newFixedThreadPool(parts);
        try {
            java.util.List<java.util.concurrent.Future<Long>> futures = new java.util.ArrayList<java.util.concurrent.Future<Long>>();
            for (int p = 0; p < parts; p++) {
                // Step 2: Part p covers the numbers from low to high. Together the four parts cover 2..n once.
                final long low = 2 + (n - 1) * p / parts;
                final long high = 2 + (n - 1) * (p + 1) / parts - 1;
                futures.add(pool.submit(() -> countPrimes(low, high)));
            }
            // Step 3: get() waits for each task. We add the counts of the four parts.
            long total = 0;
            for (java.util.concurrent.Future<Long> future : futures) {
                total += future.get();
            }
            return total;
        } catch (Exception e) {
            throw new RuntimeException(e);
        } finally {
            pool.shutdown();
        }
    }

    public static void main(String[] args) {
        long n = new Scanner(System.in).nextLong();
        System.out.println(solve(n));
    }
}