code: @Override
     public int size() {
        Iterator<E> iter = iterator();
        int count = 0;
        while (iter.hasNext()) {
           iter.next();
           count++;
        }
        return count;
     }
docs: Returns the size of the read only set.  This is done by iterating over all of the
  values counting all that are in the segments.
  <p>
  If you are using this method to verify if the set is empty, you should instead use
  the {@link ReadOnlySegmentAwareEntryCollection#isEmpty()} as it will perform better if the
  size is only used for this purpose.
  <p>
  This time complexity for this method is always O(N).