BigDecimal add(BigDecimal val) //BigDecimal 加法
BigDecimal subtract (BigDecimal val) //BigDecimal 减法
BigDecimal multiply (BigDecimal val)
BigDecimal divide(BigDecimal divisor, int scale, int roundingMode)//BigDecimal除法
注意
if divisor is zero, roundingMode==ROUND_UNNECESSARY and the specified scale is
insufficient to represent the result of
the division exactly
所以应该指定scale和roundingMode,保证对于无限小数有足够的范围来表示结果。