PrimeCalculatorEnhanced.java

package com.koltsa;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.*;

/**
 * Enhanced version of PrimeCalculator implementation.
 */
public class PrimeCalculatorEnhanced {

    /**
     * Returns list of prime numbers from range [2; maxPrime]
     * @param maxPrime - upper bound of prime number.
     * @return list of prime numbers.
     * @throws InterruptedException if the thread that calls the method is interrupted.
     */
    public static List<Integer> getPrimes(int maxPrime) throws InterruptedException {
        if (maxPrime < 2) {
            // no prime numbers => return empty list
            return new LinkedList<>();
        }

        final int cores = Runtime.getRuntime().availableProcessors();
        ExecutorService executors = Executors.newWorkStealingPool(cores);

        ConcurrentLinkedQueue<Integer> primeNumbersQueue = new ConcurrentLinkedQueue<>();
        // Subtract 2 from "maxPrime" since we cover the range [2; maxPrime]
        CountDownLatch latch = new CountDownLatch(maxPrime - 2);
        for (int i = 2; i <= maxPrime; i++) {
            // final efficiency requirement
            final int candidate = i;
            // Possible "java.lang.OutOfMemoryError: unable to create new native thread"
            executors.submit(() -> {
                if (isPrime(candidate)) {
                    primeNumbersQueue.add(candidate);
                }
                latch.countDown();
            });
        }

        // wait until all threads are completed
        latch.await();
        // shut down executors
        executors.shutdownNow();
        return Arrays.asList(primeNumbersQueue.toArray(new Integer[0]));
    }

    /**
     * Check if number is a prime number, e.g.: 2, 3, 5, 7, 11, ...
     * @param number - candidate for prime number.
     * @return true if number is prime (could be decided only by itself)
     */
    private static boolean isPrime(int number) {
        // Check number in bounds of primes and not an even
        if (number < 3) {
            // prime numbers start with "2"
            return number > 1;
        }

        if (number % 2 == 0) {
            // omit even numbers, thus not include '2' into for-loop
            return true;
        }

        // sequentially check for other numbers. Odd numbers are omitted ...
        // ... given the check (num % 2) above.
        for (int i = 3; i < number; i+= 2) {
            if (number % i == 0) {
                return false;
            }
        }
        return true;
    }
}