BigDecimal舍入模式(Rounding mode)

ROUND_CEILING
Rounding mode to round towards positive infinity.
向正无穷方向舍入

ROUND_DOWN
Rounding mode to round towards zero.
向零方向舍入

ROUND_FLOOR
Rounding mode to round towards negative infinity.
向负无穷方向舍入

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

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


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


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


ROUND_UP
Rounding mode to round away from zero.
向远离0的方向舍入

例子

BigDecimal n3  = new BigDecimal(1.2345678901234567890E+9);
System.out.println(n3)

这样输出的结果是:1234567890.1234567165374755859375
当到了后面17位之后就开始不准确了
因Double类型本身只能精确到17位

但是如果这样

BigDecimal n4  = new BigDecimal("1.23456789012345678901234567890E+9");

结果是1234567891.12345678901234567890
这样就能绝对精确