Running into java.util.ConcurrentModificationException while iterating list

Question

I'm trying to split a list of records into sublists of records. I successfully split the list into sublists, I wanted to see the contents of the sublists, but somehow I keep running into this ConcurrentModificationException.

My split method:

/**
     * @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;
    }

I call it here:

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);
            }

^ Here I am trying to split the list into sublists so I can upload them to S3.

and the stack trace:

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)

@Pangea added, it looks like something went wrong when i was trying to gson the list. — iCodeLikeImDrunk
– iCodeLikeImDrunk, Commented May 17, 2013 at 21:33
This exception normally means that the list is modified while being iterated over. What does GSON do under the hood? Also you might want to change the company name to something different. — zw324
– zw324, Commented May 17, 2013 at 21:35

OldCurmudgeon · Accepted Answer · 2013-05-17 21:57:05Z

6

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

sublists.get(size - 1) is a live view of a subrange of the list, so when you add elements to it, you're adding elements to the original list as well, at the same time that you're trying to get elements out from list.subList. In general, you can't modify a list while you're iterating over it.

The simplest solution is not to add the elements to sublists.get(size - 1), but just to update it:

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

edited May 17, 2013 at 21:57

OldCurmudgeon

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answered May 17, 2013 at 21:36

Louis Wasserman

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2 Comments

iCodeLikeImDrunk Over a year ago

currently in my code i would have 5 lists, where the first 4 lists would have equal lengths. the last(5th) list would contain the length whatever other lists has PLUS any remainder records. wouldn't that code replace what the 5th list is?

zw324 Over a year ago

This looks like the answer, but the stacktrace says the exception does not happen in the split method; any comments?

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