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javaalgorithmrandomsampling

Take n random elements from a List<E>?

How can I take n random elements from an ArrayList<E>? Ideally, I'd like to be able to make successive calls to the take() method to get another x elements, without replacement.

Solution

  • Two main ways.

    1. Use Random#nextInt(int):

      List<Foo> list = createItSomehow();
Random random = new Random();
Foo foo = list.get(random.nextInt(list.size()));

      It's however not guaranteed that successive n calls returns unique elements.

    2. Use Collections#shuffle():

      List<Foo> list = createItSomehow();
Collections.shuffle(list);
Foo foo = list.get(0);

      It enables you to get n unique elements by an incremented index (assuming that the list itself contains unique elements).

    In case you're wondering if there's a Java 8 Stream approach; no, there isn't a built-in one. There's no such thing as Comparator#randomOrder() in standard API (yet?). You could try something like below while still satisfying the strict Comparator contract (although the distribution is pretty terrible):

    List<Foo> list = createItSomehow();
int random = new Random().nextInt();
Foo foo = list.stream().sorted(Comparator.comparingInt(o -> System.identityHashCode(o) ^ random)).findFirst().get();

    Better use Collections#shuffle() instead.