[20210619] (Java) Collections.shuffle()

[JuHyun419]

Collections.shuffle()

• List<?> 타입 랜덤으로 섞기

    public static void shuffle(List<?> list) {
        Random rnd = r;
        if (rnd == null)
            r = rnd = new Random(); // harmless race.
        shuffle(list, rnd);
    }


    private static Random r;


    @SuppressWarnings({"rawtypes", "unchecked"})
    public static void shuffle(List<?> list, Random rnd) {
        int size = list.size();
        if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
            for (int i=size; i>1; i--)
                swap(list, i-1, rnd.nextInt(i));
        } else {
            Object[] arr = list.toArray();

            // Shuffle array
            for (int i=size; i>1; i--)
                swap(arr, i-1, rnd.nextInt(i));

            // Dump array back into list
            // instead of using a raw type here, it's possible to capture
            // the wildcard but it will require a call to a supplementary
            // private method
            ListIterator it = list.listIterator();
            for (Object e : arr) {
                it.next();
                it.set(e);
            }
        }
    }

    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();

        for (int i = 0; i < 10; i++) {
            list.add(i);
        }
        System.out.println(list);

        Collections.shuffle(list);
        System.out.println(list);

        Collections.shuffle(list);
        System.out.println(list);

    }