A number of methods of CopyOnWriteArrayList take other collections as arguments.
They assume that those collections are not being concurrently mutated.
In particular, they trust that the size() of the other collection is invariant.
This means, in particular, that "binary" operations on pairs of CopyOnWriteArrayLists
are unsafe.

This is one part of the fix for
6347106: (coll) methods taking concurrently modified collection arguments fail sporadically
If all you have is a Collection, then both size() and iteration are unsafe!