@Override
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }
Returns an iterator over the elements in this set. The elements are
returned in no particular order.

@return an Iterator over the elements in this set
@see java.util.ConcurrentModificationException
core/common/src/main/java/com/alipay/sofa/rpc/common/struct/ConcurrentHashSet.java