java.util.ConcurrentModificationException with custom ArrayList

Question

Of course i know what does this error mean, but i don't know how to remove this. Now i'm trying with

 private void removeFriendFromList() {
    List<Friend> copy = new ArrayList<Friend>(globalSearchFriends);
    for (Friend friend : globalSearchFriends) {
        if (friend.equals(remove)) {
            copy.remove(friend);
        }
    }
}

But this doesn't work. This is my globallist

 private List<Friend> globalSearchFriends = new ArrayList<>();

I'm trying iterating too, but it didn't work or i did something bad.

Also i need to use it here: where i search for a friend in api, this is working like when i input text in EditText then in my adapter i see that user, but this work only for few members, always when i search like "andrew" and then i search "youko" i get the error.

private void serachFriend(final String query) {

    etGlobalSearch.addTextChangedListener(new TextWatcherAdapter() {
        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            FindFriend request = new FindFriend();
            request.query = query;
            request.query = s.toString().toLowerCase().trim();
            backend.findFriend(request).enqueue(new Callback<ResponseFindFriend>() {
                @Override
                public void onResponse(Call<ResponseFindFriend> call, Response<ResponseFindFriend> response) {
                    synchronized (globalSearchFriends) {
                        globalSearchFriends.clear();
                        removeFriendFromList();
                        try {
                            if (response == null)
                                throw new Exception();
                            if (!response.isSuccessful())
                                throw new Exception();
                            if (response.body() == null)
                                throw new Exception();
                            if (response.body().results == null)
                                throw new Exception();
                            globalSearchFriends = response.body().results;
                        } catch (Exception e) {
                            Log.d("Blad", "sobie");
                        } finally {
                            gatherResults();
                        }
                    }
                }

                @Override
                public void onFailure(Call<ResponseFindFriend> call, Throwable t) {
                    synchronized (globalSearchFriends) {
                        globalSearchFriends.clear();
                        removeFriendFromList();
                        gatherResults();
                    }
                }
            });
        }
    });
}

private void removeFriendFromList() {
    List<Friend> copy = new ArrayList<Friend>(globalSearchFriends);
    for (Friend friend : globalSearchFriends) {
        if (friend.equals(remove)) {
            copy.remove(friend);
        }
    }
}

private void gatherResults() {
    removeFriendFromList();
    for (Friend f : globalSearchFriends)
        globalSearchFriends.add(f);
    findedFriendsAdapter.setFriendList(globalSearchFriends);
}

Any kind of help associated, Have a nice day! :)

Edit I got error on this case.

java.util.ConcurrentModificationException
   for (Friend f : globalSearchFriends)
        globalSearchFriends.add(f);
    findedFriendsAdapter.setFriendList(globalSearchFriends);

And on log i have:

   at java.util.ArrayList$ArrayListIterator.next

You should use iterator when modifying a list while iterating it. — jitinsharma, Jan 24 '17 at 07:53
Please provide a **real** [mcve] with a matching stack trace. When iterating one list, removing elements from another list should just work. So I assume that your code is only showing parts of the truth. And just for the record: you did **override** equals in your Friends class? — GhostCat, Jan 24 '17 at 07:57
@jitinsharma He is **not** modifying the list while iterating. He is manipulating a **copy** of that list! — GhostCat, Jan 24 '17 at 07:58
`globalSearchFriends.clear(); removeFriendFromList();` The list is already empty, which you are iterating. — Nizam, Jan 24 '17 at 07:59
I don't understand `removeFriendFromList` function. And meaning of calling `globalSearchFriends.clear(); removeFriendFromList();` — Paresh P., Jan 24 '17 at 08:01
@Nizam But then the **iteration** in his helper method would iterate an **empty** list nothing; and thus no iteration at all, and no call to remove(). I think we are wasting our time explaining an observation ... that can not be explained with the code he is swowing. — GhostCat, Jan 24 '17 at 08:05
Well, he is updating the list, but in the other method ;) this is obvious with the exception... that why it should always be present. You iterate a list then add each element in this same list. — AxelH, Jan 24 '17 at 08:06
@jitinsharma Wrong. It is more error prone to modify a list while iterating it with `Iterator`. See this http://stackoverflow.com/a/6866271/2709580 — fluffyBatman, Jan 24 '17 at 08:08

Nicolas Filotto · Accepted Answer · 2017-01-24T10:26:40.377

2

This sounds suspicious:

for (Friend f : globalSearchFriends)
    globalSearchFriends.add(f);

You try to add the content of globalSearchFriends to itself while iterating which is not allowed with an ArrayList thus it leads to a ConcurrentModificationException. Indeed ArrayList#iterator() returns a fail-fast iterator which means that:

If the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.

It doesn't sound like a normal/expected behavior but if you really wand to duplicate the content of the list simply use addAll(Collection<? extends E> c) instead of iterating as next:

globalSearchFriends.addAll(globalSearchFriends);
// or globalSearchFriends.addAll(new ArrayList<>(globalSearchFriends)); for safety
findedFriendsAdapter.setFriendList(globalSearchFriends);

NB: An ArrayList is not thread-safe so ensure to call addAll(Collection<? extends E> c) on the list if and only if the list is not shared or under the protection of an explicit or intrinsic lock otherwise you will get an unpredictable behavior.

edited Jan 24 '17 at 10:26

answered Jan 24 '17 at 08:09

Nicolas Filotto

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AddAll is not guaranteed to work: "the behavior of this call is undefined if the specified collection is this list, and this list is nonempty." – Joni Jan 24 '17 at 08:14
@Joni of course since `ArrayList` is not thread safe but here it is not the issue as it is done within a synchronized block – Nicolas Filotto Jan 24 '17 at 08:19
Yea this helped me. problem is that i use one list to everything, now when i create new list for findedFrieds im now have a problem with that. Thanks u helped me a lot :) – Rodriquez Jan 24 '17 at 08:22
@Joni I added an NB for this – Nicolas Filotto Jan 24 '17 at 08:30
Read the documentation of the addAll method: Threads and locks have nothing to do with it. `list.addAll(list)` is not guaranteed to work, even though with the current implementation it does. (Maybe this is a documentation bug?) – Joni Jan 24 '17 at 08:57
@Joni `addAll` of an `ArrayList` get the content of the collection as an array then does an array copy so only a concurrent modification of the list between the `toArray` and the `System.arraycopy` will cause an unexpected behavior which will not happen if concurrent modifications are prevented – Nicolas Filotto Jan 24 '17 at 10:25

score 0 · Answer 2 · answered Jan 24 '17 at 08:29

how about CopyOnWrite Collection ?

public static void main(String[] args) {
    CopyOnWriteArrayList<Integer> list = new CopyOnWriteArrayList();
    for (int i = 0; i < 10; i++) {
        list.add(i);
    }

    for (Integer num : list) {
        if (num % 2 == 0) {
            list.add(num * 100);
        }
    }
    System.out.println("list = " + list);
}

output list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 200, 400, 600, 800]

score 0 · Answer 3 · answered Jan 24 '17 at 10:00

Try below

   private void removeFriendFromList() {
     List<Friend> copy = new ArrayList<Friend>(globalSearchFriends);
     Friend targetToRemove = null;
     for (Friend friend : globalSearchFriends) {
       if (friend.equals(remove)) {
         targetToRemove = friend;
       }
     }
     if (targetToRemove != null) {
       copy.remove(targetToRemove);
     }
   }

java.util.ConcurrentModificationException with custom ArrayList

3 Answers3