Java Concurrency - RecursiveAction Examples

package com.logicbig.example;

import java.math.BigInteger;
import java.time.LocalTime;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import java.util.concurrent.TimeUnit;


public class RecursiveActionExample {

    public static void main (String[] args) {
        List<BigInteger> list = new ArrayList<>();
        for (int i = 3; i < 20; i++) {
            list.add(new BigInteger(Integer.toString(i)));
        }
        ForkJoinPool.commonPool().invoke(new FactorialTask(list));
    }

    private static class FactorialTask extends RecursiveAction {
        private static int SEQUENTIAL_THRESHOLD = 5;
        private List<BigInteger> integerList;

        private FactorialTask (List<BigInteger> integerList) {
            this.integerList = integerList;
        }

        @Override
        protected void compute () {
            if (integerList.size() <= SEQUENTIAL_THRESHOLD) {
                showFactorials();
            } else {
                int middle = integerList.size() / 2;
                List<BigInteger> newList = integerList.subList(middle, integerList.size());
                integerList = integerList.subList(0, middle);
                FactorialTask task = new FactorialTask(newList);
                task.fork();
                this.compute();
            }
        }

        private void showFactorials () {
            for (BigInteger i : integerList) {
                System.out.printf("[%s] : %s! = %s, thread = %s %n",
                                    LocalTime.now(), i,
                                    CalcUtil.calculateFactorial(i),
                                    Thread.currentThread().getName());
                try { //sleep to simulate long running task
                    TimeUnit.MILLISECONDS.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

package com.logicbig.example;

import java.math.BigInteger;

public class CalcUtil {

    public static BigInteger calculateFactorial (BigInteger input) {
        BigInteger factorial = BigInteger.ONE;
        for (BigInteger i = BigInteger.ONE;
             i.compareTo(input) <= 0;
             i = i.add(BigInteger.ONE)) {
            factorial = factorial.multiply(i);
        }
        return factorial;
    }
}