Take n random elements from a List?

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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.

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