Randomly pick k bits out of n from a Java BitSet

Question

How to pick exactly k bits from a Java BitSet of length m with n bits turned on, where k≤n≤m?

Example input: m=2

Answer 1

If n is much larger than k, you can just pare down the Fisher-Yates shuffle algorithm to stop after you've chosen as many as you need:

private static Random rand = new Random();
public static BitSet chooseBits(BitSet b, int k) {
    int n = b.cardinality();
    int[] indices = new int[n];
    // collect indices:
    for (int i = 0, j = 0; i < n; i++) {
        j=b.nextSetBit(j);
        indices[i] =j++;
    }
    // create returning set:
    BitSet ret = new BitSet(b.size());
    // choose k bits:
    for (int i = 0; i