public void shuffle(Random rand){
  for (int i=_pos; i-- > 1; ) {
    swap(i,rand.nextInt(i));
  }
}

public void shuffle ( Random rand ) { for ( int i = _pos ; i -- > 1 ; ) { swap ( i , rand . nextInt ( i ) ) ; } }
shuffle the elements of the list using the specified random number generator .
