Take n random elements from a List?

Question

How can I take n random elements from an ArrayList? Ideally, I\'d like to be able to make successive calls to the take() method to get an

Answer 1

As noted in other answers, Collections.shuffle is not very efficient when the source list is big, because of the copying. Here is a Java 8 one-liner that:

• efficient enough on random access lists like ArrayList if you don't need many elements from the source
• doesn't modify the source
• DOES NOT guarantee uniqueness, if it's not super important for you. If you pick 5 from a hundred, there's a very good chance the elements will be unique.

Code:

private static  List pickRandom(List list, int n) {
  return new Random().ints(n, 0, list.size()).mapToObj(list::get).collect(Collectors.toList());
}

However, for a list with no quick random access (like LinkedList) the complexity would be n*O(list_size).