Pick multiple random elements from a list in Java

Question

So say I have

List<String> teamList = new LinkedList<String>()
teamList.add("team1");
teamList.add("team2");
teamList.add("team3");
teamList.add("team4");
teamList.add("team5");
teamList.add("team6");

Is there a simple way of picking... say 3 out the 6 elements in this list in a randomized way without picking the same element twice (or more times)?

Answer 1

You can also use reservoir sampling.

It has the advantage that you do not need to know the size of the source list in advance (e.g. if you are given an Iterable instead of a List.) Also it is efficient even when the source list is not random-access, like the LinkedList in your example.

Answer 2

Try this:

public static List<String> pickNRandom(List<String> lst, int n) {
    List<String> copy = new LinkedList<String>(lst);
    Collections.shuffle(copy);
    return copy.subList(0, n);
}

I'm assuming that there are no repeated elements in the input list, also I take the precaution of shuffling a copy for leaving the original list undisturbed. It's called like this:

List<String> randomPicks = pickNRandom(teamList, 3);