ConcurrentModificationException when adding elements to another list [duplicate]

Question

Suddenly a project I have been working on is throwing a ConcurrentModificationException when adding elements to a list and after trying to see what's going on for quite a while I am as confused as when this error first happened.

I have a Tournament class and it is composed of Event instances. Events have a list of timeslots List<Timeslot> timeslots (when the matches are to be played). When creating an instance of a tournament, the tournament's timeslots are all the event's timeslots combined. Timeslots shared by different events are only added once.

See how I am instantiating a tournament (I am omitting the irrelevant arguments):

List<Localization> localizations = TournamentUtils.buildGenericLocalizations(1, "Court");
List<Timeslot> timeslots = TournamentUtils.buildSimpleTimeslots(8);
Event firstEvent = new Event(/*...*/, timeslots.subList(0, 2)););
Event secondEvent = new Event(/*...*/, timeslots);

Tournament tournament = new Tournament("Tournament", firstEvent, secondEvent);

And this is how the relevant part of the tournament constructor looks like:

public Tournament(String name, List<Event> categories) {
    // ...

    allPlayers = new ArrayList<>();
    allTimeslots = new ArrayList<>();
    allLocalizations = new ArrayList<>();

    events = new ArrayList<>(categories);

    for (Event event : events) {
        event.getPlayers().stream().filter(p -> !allPlayers.contains(p)).forEach(allPlayers::add);
        event.getTimeslots().stream().filter(t -> !allTimeslots.contains(t)).forEach(allTimeslots::add);
        event.getLocalizations().stream().filter(l -> !allLocalizations.contains(l)).forEach(allLocalizations::add);
        event.setTournament(this);
    }

    // ...
}

public Tournament(String name, Event... categories) {
    this(name, new ArrayList<>(Arrays.asList(categories)));
}

The exception happens at the event.getTimeslots().stream().filter(t -> !allTimeslots.contains(t)).forEach(allTimeslots::add); line, I have tried doing the same with a traditional foreach instead of using streams, but the issue was exactly the same.

I honestly have no clue of why this is happening, since I am not modifying the list that's being iterated in any moment.

Edit. This is the other approach I tried, but I still get the same exception.

for (Event event : events) {
    for (Player player : event.getPlayers())
        if (!allPlayers.contains(player))
            allPlayers.add(player);

    for (Localization localization : event.getLocalizations())
        if (!allLocalizations.contains(localization))
            allLocalizations.add(localization);

    for (Timeslot timeslot : event.getTimeslots())
        if (!allTimeslots.contains(timeslot))
            allTimeslots.add(timeslot);

    event.setTournament(this);
}

You are modifying the list while iterating. Use a for loop. Related/dup: stackoverflow.com/questions/25832790/… — Idos
– Idos, Commented Jun 21, 2016 at 12:51
I can see that now. I am filtering based on whether the element is contained in the allTimeslots list and at the same time adding to that list. However, see my edit. I am still getting the error using the traditional approach. — dabadaba
– dabadaba, Commented Jun 21, 2016 at 12:56
You are still using foreach my good man, use a traditional for. — Idos
– Idos, Commented Jun 21, 2016 at 12:57
I still fail to see why that produces a ConcurrentModificationException. I must be missing something obvious. — dabadaba
– dabadaba, Commented Jun 21, 2016 at 12:59
Sorry but I don't see it. If I am iterating and NOT modifying list A, and not iterating but modifying (adding) to list B, where is the problem? I don't understand the issue, and I still can't reach a solution (how should I use a for loop according to you? Because I also tried using indices and the problem remains) — dabadaba
– dabadaba, Commented Jun 21, 2016 at 13:04

Jaiprakash · Accepted Answer · 2016-06-21 13:07:58Z

1

The issue is not with iterating, but with the same list being passed to different events.

Event firstEvent = new Event(/*...*/, timeslots.subList(0, 2)););
Event secondEvent = new Event(/*...*/, timeslots);

Here firstEvent and secondEvent both contain the same list as Java sublist does not create a new list, but instead returns a view of the original list.

For the list timeslots used above, the same list is being passed different values of events, and later is used to iterate over the events, to fetch (albeit different views of the same list) the timeslots.

Due to this difference in the list view, a ConcurrentModificationException occurs.

Use new Arraylist(timeslots.sublist(0,2)) instead.

edited Jun 21, 2016 at 13:07

answered Jun 21, 2016 at 12:55

Jaiprakash

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8 Comments

dabadaba Over a year ago

Wrapping the list in an ArrayList solves the problem. However, I still don't get why the ConcurrentModificationException happens, despite your explanation.

Antoniossss Over a year ago

@dabadaba CME happens when one thread interates over collection and another modifies(adds or removes elements) the same list. times.sublist(0,2) represents the same list as timesLots so modifying sublist fallsback to modication of orignal list thus CME. new ArrayList(sublist) solves the issue as this will create totally new, separate collection (array list in this case) that will contain only some of the elements of original list - but still it will be separate collection and CME will not be thrown.

dabadaba Over a year ago

Quoting you: "times.sublist(0,2) represents the same list as timesLots so modifying sublist fallsback to modication of orignal list thus CME" My response to this: as I said before several times, that sublist you're talking about is not being modified. It is just being read.

Tomasz Stanczak Over a year ago

I also think you're wrong, your answer is actually potentially harmful since it suggests that iterating over different views of the same list can cause ConcurrentModificationException - not true. You can iterate as many times over as many views of the same list as you like without any Exception. Iterating doesn't change anything so there is no reason for a MODIFICATION Exception (upper cased to stress the meaning). @Antoniossss there was no mention of threads, but even then, reading from the same list on multiple threads without modifying it shouldn't cause an exception.

Antoniossss Over a year ago

@TomaszStanczak So I as far as I understand, your stance here is that this is a bug in JRE as CME means exactly what I and jaiprakash wrote in the answer. Besides neither I no Jaiprakash wrote that parallel reading would cause CME, I dont know where did you read this.

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Tomasz Stanczak · Accepted Answer · 2016-06-22 14:11:03Z

0

First let's sum up the facts: you're experiencing ConcurrentModificationException which can happen in the context of your question if a "fail-fast" iterator recognizes its collection being modified while iterating.

(see ConcurrentModificationException)

Your code contains a list of events, each of them supplying lists of players, timeslots and localizations. They all are being iterated over, collecting unique objects into ArrayList instances being created just before the iteration.

The code doesn't modify any of the events' collections; it just adds elements to the newly created lists. No problems here, no reason for an exception. I've even built a small proof of concept containing just the relevant parts: two lists, one being a subview of the other and similar iteration with ArrayList collecting the elements - works as expected without any problems.

Still an exception happens – so something must be happening behind the back.

I promised my guess – since all of the lists are disconnected, all of the objects cannot modify its own collection neither in the iteration nor being added to separate lists (no code called except contains and add), I suppose either the Event class modifies its collections on a separate thread (yes I know you didn't mention threads) or the Tournament class does it (the events are being assigned to the event instance variable just before the iteration starts).

What actually happens if you change the code to iterate over the categories parameter and assign it after the for loop?

answered Jun 22, 2016 at 14:11

Tomasz Stanczak

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8 Comments

dabadaba Over a year ago

The Event class is never modifying any of the lists. These are declared as final actually, so we can be sure of that. If I iterate categories instead of events and assign it after the loop, I still get the ConcurrentModificationException.

Tomasz Stanczak Over a year ago

final is not enough - the variable pointing to the list is protected from modification, its content is not.

dabadaba Over a year ago

Well, I am not modifying any of the lists anyway.

Antoniossss Over a year ago

@dabadaba final List someList =new ArrayList() means that you will not be able to assign new value to someField so only calls like someList=new LinkedList futrher on will fail , but list itself is still mutable

Antoniossss Over a year ago

@dabadaba Then wrap lists you are passing into unmodifable collections and we will se what will happen.... docs.oracle.com/javase/1.5.0/docs/api/java/util/…

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