Fisher–Yates shuffle

https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

public void shuffle() {

Random rd = new Random();

for (i = cards.size() - 1; i > 0; i--) {

int j = rd.nextInt(i + 1);

swap(i, j);

}

}

private static void swap(int i, int j) {

Card temp = cards.get(i);

cards.set(i, cards.get(i));

cards.set(j, temp);

}