public MapIterator iterator() {
        return new MapIterator(numElements, loc, false);
    }
Returns an iterator for iterating over the entries of this map.

For efficiency, all calls to `next()` will return the same {@link Location} object.

If any other lookups or operations are performed on this map while iterating over it, including
`lookup()`, the behavior of the returned iterator is undefined.
indexr-query-opt/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java