Removes all elements of this set that are not elements in elements.
Checks for each element of elements whether there is an element in this
set that is equal to it (according to this.contains), and if so, the
equal element in this set is retained, and elements that are not equal
to any element in elements are removed.
Source
void retainAll(Iterable<Object> elements) {
// Create a copy of the set, remove all of elements from the copy,
// then remove all remaining elements in copy from this.
Set<E> toRemove = toSet();
for (Object o in elements) {
toRemove.remove(o);
}
removeAll(toRemove);
}