迭代列表時碰到java.util.ConcurrentModificationException

Question

我試圖將記錄列表拆分成記錄的子列表。我成功地將列表拆分爲子列表，我想查看子列表的內容，但不知何故我仍然遇到此ConcurrentModificationException。

我的拆分方法：

/** 
    * @param list - list of results 
    * @param size - how many sublists 
    * @return ret - returns a list containing the sublists 
    * */ 
    public static <T> List<List<T>> split(List<T> list, int size) throws NullPointerException, IllegalArgumentException { 
     if (list == null) { 
      throw new NullPointerException("The list parameter is null."); 
     } 
     if (size <= 0) { 
      throw new IllegalArgumentException("The size parameter must be more than 0."); 
     } 

     int recordsPerSubList = list.size()/size; // how many records per sublist 
     List<List<T>> sublists = new ArrayList<List<T>>(size); // init capacity of sublists 

     // add the records to each sublist 
     for (int i=0; i<size; i++) { 
      sublists.add(i, list.subList(i * recordsPerSubList, (i + 1) * recordsPerSubList)); 
     } 

     // for the remainder records, just add them to the last sublist 
     int mod = list.size() % recordsPerSubList; 
     if (mod > 0) { 
      int remainderIndex = list.size() - mod; 
      sublists.get(size - 1).addAll(list.subList(remainderIndex, list.size())); 
     } 

     return sublists; 
    } 

我在這裏把它叫做：

List<List<QuoteSearchInfo>> ret = Util.split(quoteSearchInfoList, 5); 

      int fileCounter = 0; 
      for (List<QuoteSearchInfo> sublist : ret) { 
       fileCounter++; 

       String sublistJson = new Gson().toJson(sublist); 
       filename = JSON_FILE_NAME + fileCounter + JSON_FILE_END; 
       saveToFile(filename, sublistJson); 
       AWSManager.getInstance().uploadQuoteSearchJson(filename); 
      } 

^在這裏，我想列表分成子列表，這樣我就可以將它們上傳到S3。

和堆棧跟蹤：

java.util.ConcurrentModificationException 
at java.util.SubList.checkForComodification(AbstractList.java:752) 
at java.util.SubList.listIterator(AbstractList.java:682) 
at java.util.AbstractList.listIterator(AbstractList.java:284) 
at java.util.SubList.iterator(AbstractList.java:678) 
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:95) 
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60) 
at com.google.gson.Gson.toJson(Gson.java:546) 
at com.google.gson.Gson.toJson(Gson.java:525) 
at com.google.gson.Gson.toJson(Gson.java:480) 
at com.google.gson.Gson.toJson(Gson.java:460) 
at com.crover.QuoteSearchRover.execute(QuoteSearchRover.java:41) 
at com.crover.CroverMain.execute(CroverMain.java:85) 
at com.crover.CroverMain.main(CroverMain.java:35) 

Answer 1

sublists.get(size - 1).addAll(list.subList(remainderIndex, list.size())); 

sublists.get(size - 1)是列表的一個子區域的實時取景，所以當你添加元素到它，你將元素添加到原始列表以及，同時你試圖從list.subList獲取元素。一般來說，當你迭代它時你不能修改列表。

最簡單的解決方案是不元素添加到sublists.get(size - 1)，但只是爲了更新：

sublists.set(size - 1, 
    list.subList(remainderIndex - recordsPerSublist, list.size()));