ConcurrentModificationException in unmodifiable collection [duplicate]

Answer 1

You must be updating the map in another thread while you are iterating through col. Both map#values and Collections.unmodifiableCollection return views of existing data structures, so what you are iterating over (and this is witnessed by your stacktrace) is the entry set of your map.

Answer 2

Be aware, that Collections.unmodifiable* is not copying collection data, but only wrapping the original collection in a special wrapper. So if you modify the original collection, you can get this error.

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If you want to create really independent unmodifiable collection instance:

Collections.unmodifiableCollection(new ArrayList<>(map.values()));