Intention was to solve it with multi-threading, the first one is without any multi-threading, the second was an attempt at doing the same problem multi-threaded way. Yes, I know it will not yield correct result but I thought it should at least run faster compared to first one. Why is that ?

Problem003

/*
 * The prime factors of 13195 are 5, 7, 13 and 29.
 * What is the largest prime factor of the number 600851475143 ?
 */
package problem.original;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Problem003 {

    static final long val = 600851475143L;
    static long current = (val / 2 + 1);
    static long count = 0;
    static boolean flag = true;

    public static void main(String[] args) {
        final ExecutorService es = Executors.newFixedThreadPool(8);
        while (flag) {
            es.submit(new Runnable() {
                @Override
                public void run() {
                    count++;
                    if (isFactorOf(val, current) && isPrime(current)) {
                        System.out.println("Value is : " + current);
                        es.shutdownNow();
                    }
                    current--;
                }
            });
        }
        es.shutdown();
    }

    private static boolean isPrime(long val) {
        System.out.println("Prime Check for : " + val);
        long sqrt = (long) Math.sqrt(val);
        for (long i = 2; i < sqrt; i++) {
            if (val % i == 0) {
                return false;
            }
        }
        System.out.println(val + " is Prime");
        return true;
    }

    private static boolean isFactorOf(long num, long candidate) {
        System.out.println("Factor Check for : " + candidate + " ====> Thread : " + Thread.currentThread().getId() + " || Count : " + count);
        return num % candidate == 0l;
    }
}

Problem003_2

/*
 * The prime factors of 13195 are 5, 7, 13 and 29.
 * What is the largest prime factor of the number 600851475143 ?
 */
package problem.original;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Problem003_2 {

    static final long val = 600851475143L;
    static long current = (val / 2 + 1);
    static long count = 0;
    static boolean flag = true;

    public static void main(String[] args) {
        final ExecutorService es = Executors.newFixedThreadPool(8);
        while (flag) {
//            es.submit(new Runnable() {
//                @Override
//                public void run() {
                    count++;
                    if (isFactorOf(val, current) && isPrime(current)) {
                        System.out.println("Value is : " + current);
                        es.shutdownNow();
                    }
                    current--;
//                }
//            });
        }
        es.shutdown();
    }

    private static boolean isPrime(long val) {
        System.out.println("Prime Check for : " + val);
        long sqrt = (long) Math.sqrt(val);
        for (long i = 2; i < sqrt; i++) {
            if (val % i == 0) {
                return false;
            }
        }
        System.out.println(val + " is Prime");
        return true;
    }

    private static boolean isFactorOf(long num, long candidate) {
        System.out.println("Factor Check for : " + candidate + " ====> Thread : " + Thread.currentThread().getId() + " || Count : " + count);
        return num % candidate == 0l;
    }
}