code: private static int popCount(int unit)
      {
          unit = ((unit & 0xAAAAAAAA) >>> 1) + (unit & 0x55555555);
          unit = ((unit & 0xCCCCCCCC) >>> 2) + (unit & 0x33333333);
          unit = ((unit >>> 4) + unit) & 0x0F0F0F0F;
          unit += unit >>> 8;
          unit += unit >>> 16;
          return unit & 0xFF;
      }
docs: 1的数量
  @param unit
  @return