我试图了解ExecutorService，我想对数组的所有元素求和。我做了两种方法，一种使序列求和（只是一个for循环，将所有元素求和），另一种与池并发。我的问题是，使用ExecutorService的总金额几乎总是与序列总金额不符，有时甚至是一个。

package sumArregloConc;

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class SumaArregloMain {

    private final static int cantElem = 1000;
    private static ExecutorService tpool = Executors.newCachedThreadPool();



    public static void main(String[] args)
    {

        int[] arreglo = generarArreglo(cantElem); //generate the array with random numbers

        System.out.println(sumaSerial(arreglo));
        System.out.println(sumaConcurrente(arreglo));
    }

    public static int sumaSerial(int[] arreglo)
    {
        int suma = 0;
        for(int i =0; i< arreglo.length; i++)
        {
            suma += arreglo[i];
        }

        return suma;

    }

    public static int sumaConcurrente(int[] arreglo)
    {
        AtomicInteger total = new AtomicInteger(0);

        for(int i = 1 ; i < cantElem; i++){

            int a = arreglo[i];
                Thread thread = new Thread(new Runnable(){
                    public void run() {
                        int res = a;
                        total.addAndGet(res);
                        return;
                }});
                tpool.submit(thread);
        }

        tpool.shutdown(); //wait for everything to finish
        return total.get();
    }

    public static int[] generarArreglo(int cantElem)
    {
        int[] arreglo = new int[cantElem];
        Random rand = new Random();

        for(int i = 0; i < cantElem; i++)
        {
            arreglo[i] = rand.nextInt(10);
        }

        return arreglo;
    }
}

有人可以说出什么问题吗？