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

public void add ( Key key ) { if ( key == null ) throw new NullPointerException ( "called add() with a null key" ) ; set . add ( key ) ; }
adds the key to this set ( if it is not already present ) .
