java中使用BigDecimal

在java中使用BigDecimal

BigDecimal处理一些精确度较高的数字

首先引入包java.math.BigDecimal;

定义
BigDecimal a = new BigDecimal(0);

//乘 a*a
a.multiply(a)
//除 b/a
b.divide(a)
//减 b-a
b.subtract(a)


/**引入*/
**java.math.RoundingMode**;
//除的重名方法
//含义 y1/y2, 结果保留10位小数   并四舍五入 
y1.divide(y2,10,RoundingMode.DOWN);

RoundingMode管理下面的规则
RoundingMode的使用方法 RoundingMode.DOWN 替代ROUND_DOWN
四舍五入规则：

    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，如果保留位上的数字是奇数，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