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

Answer 1

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.