如何求二进制数中1的个数？

数据结构与算法

public class Solution {
    public int countOne(int num) {
        int a = 1;
        int counter = 0;
        for (int i = 0; i < 31; i++) {
            if ((a & num) != 0) {
                counter++;
            }
            a <<= 1;
        }
        return counter;
    }
    public static void main(String[] args) {
        Solution solution = new Solution();
        System.out.println(solution.countOne(15));
    }
}