code: public static int multiply(double num1, double num2) {
          BigDecimal num1Bd = new BigDecimal(Double.toString(num1));
          BigDecimal num2Bd = new BigDecimal(Double.toString(num2));
          MathContext mathContext = new MathContext(num1Bd.precision(), RoundingMode.HALF_UP);
          return num1Bd.multiply(num2Bd, mathContext).intValue();
      }
docs: 将double与小数相乘，计算结四舍五入保留整数位。
  
  @param num1 数字1
  @param num2 数字2
  @return 数字相乘计算结果