In Java, how to get positions of ones in reversed binary form of an integer?

Just check the bits in turn:

List<Integer> bits(int num) {
  List<Integer> setBits = new ArrayList<>();
  for (int i = 1; num != 0; ++i, num >>>= 1) {
    if ((num & 1) != 0) setBits.add(i);
  }
  return setBits;
}

Online Demo

6 [2, 3]
7 [1, 2, 3]
8 [4]