Hi, I noticed a possible bug in ParseRelations when trying to add or remove an object from a relation. I've found the crash consistently happening when preforming the following:

1: add an object to the relation.
2: add a different object to the same relation.
3: removing the first object from the relation.

Most of the time i get a crash first try, sometimes it doesn't but performing the same list of actions repeatedly will cause it eventually. Here is the full crash:

 java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1441) at java.util.HashMap$KeyIterator.next(HashMap.java:1465) at com.parse.ParseRelationOperation.removeParseObjectFromSet(ParseRelationOperation.java:124) at com.parse.ParseRelationOperation.removeAllParseObjectsFromSet(ParseRelationOperation.java:136) at com.parse.ParseRelationOperation.mergeWithPrevious(ParseRelationOperation.java:243) at com.parse.ParseObject.performOperation(ParseObject.java:2998) at com.parse.ParseRelation.remove(ParseRelation.java:149) 

My Android app currently allows an admin user to set permissions for other users, and they do this using a check boxes to allow or deny a user from reading or writing objects. I use a ParseRole and add the users to the "users" ParseRelation.

The admin users are shown a RecyclerView that holds a list of users and a check boxes allowing them to be given permission to read and write. The RecyclerViewAdapter holds a reference to the user ParseRelation for read and write, and the user is added or removed when the checkbox value changes. (Below is the a picture of the UI and my snippet of RecyclerViewAdapter code)

 mReadCheckListener = (compoundButton, checked) -> { int position = getBindingAdapterPosition(); if (position != RecyclerView.NO_POSITION) { ParseUser user = getItem(position); if (checked) { mReadRelation.add(user); } else { mReadRelation.remove(user); } } }; mWriteCheckListener = (compoundButton, checked) -> { int position = getBindingAdapterPosition(); if (position != RecyclerView.NO_POSITION) { ParseUser user = getItem(position); if (checked) { mWriteRelation.add(user); } else { mWriteRelation.remove(user); } } }; 

Any help or feedback is appreciated! In the meantime, i've moved over my ParseRelation logic to Cloud Code as a temporary solution to the problem.