商业应用中Java浮点数的精确计算及表示(三)

BigDecimal 舍入模式（Rounding mode）介绍：

在 java中， 四舍五入通过 BigDecimal 来实现。一定要注意：BigDecimal is Immutable。也就是跟String一样，对前一个的修改，比如setScale(), add（）等都会返回一个新的ＢigDecimal.四舍五入舍入模式是　BigDecimal.ROUND_HALF_UP　

BigDecimal定义了一下舍入模式，只有在作除法运算或四舍五入时才用到舍入模式，下面简单介绍，详细请查阅J2se API文档

static int	ROUND_CEILING           Rounding mode to round towards positive infinity. 向正无穷方向舍入
static int	ROUND_DOWN           Rounding mode to round towards zero. 向零方向舍入
static int	ROUND_FLOOR           Rounding mode to round towards negative infinity. 向负无穷方向舍入
static int	ROUND_HALF_DOWN           Rounding mode to round towards "nearest neighbor" unless both neighbors are equidistant, in which case round down. 向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，向下舍入, 例如1.55 保留一位小数结果为1.5
static int	ROUND_HALF_EVEN           Rounding mode to round towards the "nearest neighbor" unless both neighbors are equidistant, in which case, round towards the even neighbor. 向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，如果保留位数是奇数，使用ROUND_HALF_UP ，如果是偶数，使用ROUND_HALF_DOWN
static int	ROUND_HALF_UP           Rounding mode to round towards "nearest neighbor" unless both neighbors are equidistant, in which case round up. 向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，向上舍入, 1.55保留一位小数结果为1.6
static int	ROUND_UNNECESSARY           Rounding mode to assert that the requested operation has an exact result, hence no rounding is necessary. 计算结果是精确的，不需要舍入模式
static int	ROUND_UP           Rounding mode to round away from zero. 向远离0的方向舍入