Look up method\ Brian kerningam's method and a naive method for counting set bits GFG

 package com.company;

    class CWR_BitMagic_Gfg{

        static int[] table = new int[256];

        // recursive function to count set bits
        public static void initialize() {

            // To initially generate the table algorithmically:
            table[0] = 0;
            for (int i = 0; i < 256; i++) {
                table[i] = (i & 1) + table[i / 2];
            }
        }

        public static int countSetBits(int n) {
            return table[n & 0xff] +
                    table[(n >> 8) & 0xff] +
                    table[(n >> 16) & 0xff] +
                    table[n >> 24];
        }

        // Driver function
        public static void main(String[] args) {
            initialize(); // important point********************************
            int n = 9;
            System.out.println(countSetBits(n));
        }
    }

//    static int [] table = new int [256];
//    //        My method
//    public static int countBits(int n) {
//        int count = 0;
//        for (int i = 1; i <= 32; i++) {
//            if ((n & (1 << (i - 1))) != 0) {
//                count++;
//            }
//        }
//        return count;
//    }
//
//    //     Naive solution
//    public static int count1(int n) {
//        int res = 0;
//        while (n > 0) {
//            if ((n & 1) == 1) {
//                res++;
//                n = n / 2;
//            }
//        }
//        return res;
//    }
//
//    //    Brian kerningam's algorithm
//    public static int count2(int n) {
//        int res = 0;
//        while (n > 0) {
//            n = (n & (n - 1));
//            res++;
//        }
//        return res;
//    }
//
////    Lookup table method
//    public static void initialize(){
//        table[0] = 0;
//        for(int i = 0; i<256; i++){
//            table[i] = (i&1) + table[i/2];
//        }
////        for (int e:table) {
////            System.out.println(e);
//        }
//
//
//    public static int count3(int n){
//
//        return table[n & 0xff] +
//                table[(n >> 8) & 0xff] +
//                table[(n >> 16) & 0xff] +
//                table[n >> 24];
//    }

//    public static void main(String[] args) {
//
////        System.out.println(countBits(40));
////        System.out.println(count1(7));
////        System.out.println(count3(7));
////        initialize();
//    }
//}