// Callableタスク(複数)を実行する
        final ExecutorService executor = Executors.newFixedThreadPool(numTasks);
        try {
            final List<Future<Integer>> futures = executor.invokeAll(tasks);
            for (final Future<Integer> future : futures) {
                future.get();
            }
        } catch (final InterruptedException e) {
            // no thing to do