@Override public Set<K> keySet(){
  HashSet<K> set=new HashSet<K>();
  for (  Segment<K,V> s : segments) {
    set.addAll(s.keySet());
  }
  return set;
}

@ Override public Set < K > keySet ( ) { HashSet < K > set = new HashSet < K > ( ) ; for ( Segment < K , V > s : segments ) { set . addAll ( s . keySet ( ) ) ; } return set ; }
get the set of keys for resident entries .
