Java Concurrency - RecursiveTask Examples

package com.logicbig.example;

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

public class RecursiveTaskExample {

    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)));
        }

        BigInteger sum = ForkJoinPool.commonPool().
                            invoke(new FactorialTask(list));
        System.out.println("Sum of the factorials = " + sum);
    }

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

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

        @Override
        protected BigInteger compute () {
            if (integerList.size() <= SEQUENTIAL_THRESHOLD) {
                return sumFactorials();
            } 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();
                BigInteger thisSum = this.compute();
                BigInteger thatSum = task.join();
                return thisSum.add(thatSum);
            }
        }

        private BigInteger sumFactorials () {
            BigInteger sum = BigInteger.ZERO;
            for (BigInteger i : integerList) {
                sum = sum.add(CalcUtil.calculateFactorial(i));
            }
            return sum;
        }
    }
}