Generate a set of M elements from an array of size N

I'm trying to understand solution for the following task: Randomly generate a set of M elements from an array of size N. Each element must have equal probability of being chosen.

I found the following solution (I've already read this question, but it does not answer my question):

char[] generateSet(char[] original, int subsetSize) {
    char[] subset = new char[subsetSize];

    for (int i = 0; i < subsetSize; i++) {
        subset[i] = original[i];
    }
    for (int i = subsetSize; i < original.length; i++) {
        int r = rand(0, i);
        boolean takeIthElement = r < m;
        if (takeIthElement) {
            subset[r] = original[i];
        }
    }

    return subset;
}

I think that function does not work correctly. Reasons:

1. For example we have an array {'1', '2', 'a', 'b'} and m = 2
2. Therefore, we should have qual probabilities to generate the following sets:

{1, 2}; {2, 1}; {1, a}; {a, 1}; {1, b}; {b, 1}; {a, 2}; {2, a}; {b, 2}; {2, b}; {a, b}; {b, a}

My concern here is that function will never generate the following sets: {2, 1}; {2, a}; {2, b}

So, it means that it is incorrect.