public void delete(Key key){
  if (key == null)   throw new NullPointerException("called delete() with a null key");
  set.remove(key);
}

public void delete ( Key key ) { if ( key == null ) throw new NullPointerException ( "called delete() with a null key" ) ; set . remove ( key ) ; }
removes the specified key from this set ( if the set contains the specified key ) .
