Question

My Android app has an IntentService where it requests a list of MessageThreads objects from Facebook, parses the JSON response to build an ArrayList of the objects:

ArrayList<MessageThread> mMessageThreads = new ArrayList<MessageThread>();

Then it calls FB again in the same service, get the names for the MessageThread ids and matches them with the MessageThread objects. At this point I have an ArrayList with complete MessageThread objects and I insert them into an SQLite db.

// save to db and broadcast
for (MessageThread message : mMessageThreads) {
    ((FBClientApplication)getApplication()).getMessagesData().insertOrIgnore(message.toContentValues();
}

where:

public void insertOrIgnore(ContentValues values) {
    SQLiteDatabase db = this.dbHelper.getWritableDatabase();
    db.insertWithOnConflict(TABLE, null, values, SQLiteDatabase.CONFLICT_IGNORE);
}

Via ACRA reports I see that intermittently the line

for (MessageThread message : mMessageThreads) 

throws an ConcurrentModificationException and the app forcloses. I haven’t been able to isolate under what conditions. I read about this Exception and as I understand it it happens when we remove items from an ArrayList while iterating over it, but I’m not removing items from the list. Any pointers to help with this problem are greatly appreciated.

Answer 1

It also happens when you add items to an ArrayList while iterating over it, which it looks like you might do in this code.

In general, it’s any “structural modification” that occurs to the ArrayList that can cause a CME while iterating.