import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.Callable;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Future;

public class App {

    public static void main(String[] args) {

        try {

            List<String> list = new ArrayList<String>();

            for (int i = 0; i < 1000; i++) {

                list.add(i + ",");

            }

            System.out.println(list2Str(list, 5));

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

    public static String list2Str(List<String> list, final int nThreads)

            throws Exception {

        if (list == null || list.isEmpty()) {

            return null;

        }

        StringBuffer ret = new StringBuffer();

        int size = list.size();

        ExecutorService executorService = Executors.newFixedThreadPool(nThreads);

        List<Future<String>> futures = new ArrayList<Future<String>>(nThreads);

        for (int i = 0; i < nThreads; i++) {

            final List<String> subList = list.subList(size / nThreads * i, size/ nThreads * (i + 1));

            Callable<String> task = new Callable<String>() {

                public String call() throws Exception {

                    StringBuffer sb = new StringBuffer();

                    for (String str : subList) {

                        sb.append(str);

                    }

                    return sb.toString();

                }

            };

            futures.add(executorService.submit(task));

        }

        for (Future<String> future : futures) {

            ret.append(future.get());

        }

        executorService.shutdown();

        return ret.toString();

    }

}