一千万个数，高效求和

private class SumTask extends RecursiveTask<Long> {

    private static final int THRESHOLD = 1250000;

    private long arr[];
    private int start;
    private int end;

    public SumTask(long[] arr, int start, int end) {
        this.arr = arr;
        this.start = start;
        this.end = end;
    }

    /**
     * 小计
     */
    private long subtotal() {
        long sum = 0;
        for (int i = start; i < end; i++) {
            sum += arr[i];
        }

        return sum;
    }

    @Override
    protected Long compute() {

        if ((end - start) <= THRESHOLD) {
            return subtotal();
        }else {
            int middle = (start + end) / 2;
            SumTask left = new SumTask(arr, start, middle);
            SumTask right = new SumTask(arr, middle, end);
            left.fork();
            right.fork();

            return left.join() + right.join();
        }
    }
}

public static void main(String[] args) throws ExecutionException, InterruptedException{
    int num = Runtime.getRuntime().availableProcessors();
    long time = System.currentTimeMillis();
    long[] arr = new long[10000000];
    for (int i =0 ; i < 10000000; i++) {
        arr[i] =i+1;
    }

    ForkJoinPool pool = new ForkJoinPool();
    ForkJoinTask<Long> result = pool.submit(new ForkJoinTest().new SumTask(arr, 0, arr.length));
    System.out.println("最终计算结果: " + result.invoke()+"耗时："+(System.currentTimeMillis() - time)+"，毫秒");
    pool.shutdown();
}