Two main ways.
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.
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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