Take n random elements from a List?

Question

How can I take n random elements from an ArrayList? Ideally, I\'d like to be able to make successive calls to the take() method to get an

Answer 1

This solution doesn't modify the original list or otherwise scale in complexity with the list size.

To get a sample of 4 from a list of 7, we just select a random element out of all 7, then select a random element out of the remaining 6, and so on. If we've already selected indices 4, 0, 3, next we generate a random number out of 0, 1, 2, 3, respectively representing index 1, 2, 5, 6.

static Random rand = new Random();

static  List randomSample(List list, int size) {
    List sample = new ArrayList<>();

    for (int sortedSampleIndices[] = new int[size], i = 0; i < size; i++) {
        int index = rand.nextInt(list.size() - i);

        int j = 0;
        for (; j < i && index >= sortedSampleIndices[j]; j++)
            index++;
        sample.add(list.get(index));

        for (; j <= i; j++) {
            int temp = sortedSampleIndices[j];
            sortedSampleIndices[j] = index;
            index = temp;
        }
    }

    return sample;
}