public Iterator<K> iterator() {
            return newKeyIterator();
        }
Returns a {@link Set} view of the keys contained in this map. The set is
backed by the map, so changes to the map are reflected in the set, and
vice-versa. If the map is modified while an iteration over the set is in
progress (except through the iterator's own <tt>remove</tt> operation),
the results of the iteration are undefined. The set supports element
removal, which removes the corresponding mapping from the map, via the
<tt>Iterator.remove</tt>, <tt>Set.remove</tt>, <tt>removeAll</tt>,
<tt>retainAll</tt>, and <tt>clear</tt> operations. It does not support
the <tt>add</tt> or <tt>addAll</tt> operations.
src/main/java/com/alibaba/fastjson/util/AntiCollisionHashMap.java