code: public static int bitCount(final byte[] data) {
          int count = 0;
          for (final byte element : data) {
              byte bits = element;
              for (int j = 0; j < 8; j++) {
                  if ((bits & 1) == 1) {
                      count++;
                  }
                  bits >>= 1;
              }
          }
          return count;
      }
docs: Count the number of true bits.
  
  @param data a target data
  @return the number of true bits