java.util.concurrent.CopyOnWriteArrayList

 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.CopyOnWriteArrayList;

 public class TestCalc {
     /**
      *read
      */
     private static class ReadTask implements Runnable {
         List<String> list;

         public ReadTask(List<String> list) {
             this.list = list;
         }

         public void run() {
             for (String str : list) {
                 System.out.println(str);
             }
         }
     }
     /**
      *write
      *
      */
     private static class WriteTask implements Runnable {
         List<String> list;
         int index;

         public WriteTask(List<String> list, int index) {
             this.list = list;
             this.index = index;
         }

         public void run() {
             list.remove(index);
             list.add(index, "write_" + index);
         }
     }

     public void run() {
         final int NUM = ;
          //List<String> list = new ArrayList<String>(); //java.util.ConcurrentModificationException
          CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<String>();
         for (int i = ; i < NUM; i++) {
             list.add("main_" + i);
         }
         ExecutorService executorService = Executors.newFixedThreadPool(NUM);
         for (int i = ; i < NUM; i++) {
             executorService.execute(new ReadTask(list));
             executorService.execute(new WriteTask(list, i));
         }
         executorService.shutdown();
     }

     public static void main(String[] args) {
         new TestCalc().run();
     }
 }

 /*

  //List<String> list = new ArrayList<String>();
 Exception in thread "pool-1-thread-11" java.util.ConcurrentModificationException

         at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
         at java.util.ArrayList$Itr.next(Unknown Source)
         at TestCalc$ReadTask.run(TestCalc.java:19)
         at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
         at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
         at java.lang.Thread.run(Unknown Source)
 */