java - How can I get the position of bits

Question

I have a decimal number which I need to convert to binary and then find the position of one's in that binary representation.

Input is 5 whose binary is 101 and Output should be

1
3

Below is my code which only provides output as 2 instead I want to provide the position of one's in binary representation. How can I also get position of set bits starting from 1?

public static void main(String args[]) throws Exception {
    System.out.println(countBits(5));
}

private static int countBits(int number) {
    boolean flag = false;

    if (number < 0) {
        flag = true;
        number = ~number;
    }
    int result = 0;
    while (number != 0) {
        result += number & 1;
        number = number >> 1;
    }
    return flag ? (32 - result) : result;
}

Answer 1

Your idea of having countBits return the result, instead of putting a System.out.println inside the method, is generally the best approach. If you want it to return a list of bit positions, the analogue would be to have your method return an array or some kind of List, like:

private static List<Integer> bitPositions(int number) {

As I mentioned in my comments, you will make life a lot easier for yourself if you use >>> and get rid of the special code to check for negatives. Doing this, and adapting the code you already have, gives you something like

private static List<Integer> bitPositions(int number) {
    List<Integer> positions = new ArrayList<>();
    int position = 1;
    while (number != 0) {
        if (number & 1 != 0) {
            positions.add(position);
        }
        position++;
        number = number >>> 1;
    }
    return positions;
}

Now the caller can do what it wants to print the positions out. If you use System.out.println on it, the output will be [1, 3]. If you want each output on a separate line:

for (Integer position : bitPositions(5)) {
     System.out.println(position);
}

In any case, the decision about how to print the positions (or whatever else you want to do with them) is kept separate from the logic that computes the positions, because the method returns the whole list and doesn't have its own println.

(By the way, as Alex said, it's most common to think of the lower-order bit as "bit 0" instead of "bit 1", although I've seen hardware manuals that call the low-order bit "bit 31" and the high-order bit "bit 0". The advantage of calling it "bit 0" is that a 1 bit in position N represents the value 2^N, making things simple. My code example calls it "bit 1" as you requested in your question; but if you want to change it to 0, just change the initial value of position.)