code: public SortedMap tailMap(Object startKey) {
          // Check for errors
          if (comparator == null)
              ((Comparable) startKey).compareTo(startKey);
          else
              comparator.compare(startKey, startKey);
          return makeSubMap(startKey, null);
      }
docs: Answers a SortedMap of the specified portion of this TreeMap which
  contains keys greater or equal to the start key. The returned SortedMap
  is backed by this TreeMap so changes to one are reflected by the other.
  
  @param startKey the start key
  
  @return a submap where the keys are greater or equal to
  <code>startKey</code>
  
  @exception ClassCastException
  when the start key cannot be compared with the keys in
  this TreeMap
  @exception NullPointerException
  when the start key is null and the comparator cannot
  handle null