code: @Override
     public boolean isEmpty() {
        Set<K> keySet = keySet();
        Iterator<K> iter = keySet.iterator();
        return !iter.hasNext();
     }
docs: Checks if the provided map is empty.  This is done by iterating over all of the keys
  until it can find a key that maps to a given segment.
  <p>
  This method should always be preferred over checking the size to see if it is empty.
  <p>
  This time complexity for this method between O(1) to O(N).