Gaus формула и BigDeclima Просчет суммы чисел от входа

Solution.java

package com.javarush.task.task28.task2808;


import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

/* 
Осваиваем Callable
*/
public class Solution {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        List<Future<String>> futures = new ArrayList<>();
        ExecutorService executor = Executors.newFixedThreadPool(5);
        for (int i = 1000; i <= 1010; i++) {
            futures.add(executor.submit(getTask(i)));
        }

        futures.add(executor.submit(getTask(10000000)));

        for(Future<String> future : futures) {
            System.out.println(future.get());
        }

        executor.shutdown();
        executor.awaitTermination(10, TimeUnit.SECONDS);

/* output
500500
501501
502503
503506
504510
505515
506521
507528
508536
509545
510555
50000005000000
*/
    }

    public static Callable<String> getTask(final int i) {
       /* Callable<String> callable = new Callable<String>() {
            @Override
            public String call() throws Exception {
                BigDecimal s= new BigDecimal("0");
                for (int j=1; j<=i; j++){
                    s = s.add(BigDecimal.valueOf(j));
                }
                return s.toString();
            }
        };
        return callable;*/
       return () -> ((long)i*(i+1))/2+"";
    }
}