How to shuffle a list

Short answer

Collections.shuffle() puts the list in random order:

void main() {
    var cards = new ArrayList<>(List.of("A", "K", "Q", "J", "10"));
    Collections.shuffle(cards);
 
    IO.println(cards);
}

Output:

[A, Q, K, 10, J]

The order is different on every run.

A repeatable result

Pass a Random with a fixed seed — the shuffle will be the same:

void main() {
    var numbers = new ArrayList<>(List.of(1, 2, 3, 4, 5));
    Collections.shuffle(numbers, new Random(42));
 
    IO.println(numbers);
}

Output:

[2, 3, 4, 5, 1]

A random element

To pick one element, you do not need to shuffle the whole list:

void main() {
    var prizes = List.of("book", "mug", "t-shirt");
    String prize = prizes.get(new Random().nextInt(prizes.size()));
 
    IO.println(prize);
}

Output:

mug

Common mistake

Shuffling an unmodifiable list:

void main() {
    Collections.shuffle(List.of(1, 2, 3));
}

Output:

Exception in thread "main" java.lang.UnsupportedOperationException

Copy it into new ArrayList<>(...).

Thanks for reading!

Short answer

A repeatable result

A random element

Common mistake

Functions used in this recipe

Thanks for reading!