BigDecimal 小数位的处理（RoundingMode）

前言：

在日常项目开发中，接触小数是常有的事情，而产品对于小数的处理，在不同的业务中有不同的定义，比如四舍五入、截取、向上舍位、向下舍位等等场景，面对如此多的场景，你是否会束手无策？或者说自己去研究怎么实现？其实 Java 已经给我们提供了一个枚举类 RoundingMode，RoundingMode 共有 8 种舍位方式，本篇我们来逐个分析。

RoundingMode 枚举类源码如下：

package java.math;

/**
 *
 * @see     BigDecimal
 * @see     MathContext
 * @author  Josh Bloch
 * @author  Mike Cowlishaw
 * @author  Joseph D. Darcy
 * @since 1.5
 */
public enum RoundingMode {
   

        /**
         * Rounding mode to round away from zero.  Always increments the
         * digit prior to a non-zero discarded fraction.  Note that this
         * rounding mode never decreases the magnitude of the calculated
         * value.
         *
         *<p>Example:
         *<table border>
         * <caption><b>Rounding mode UP Examples</b></caption>
         *<tr valign=top><th>Input Number</th>
         *    <th>Input rounded to one digit<br> with {@code UP} rounding
         *<tr align=right><td>5.5</td>  <td>6</td>
         *<tr align=right><td>2.5</td>  <td>3</td>
         *<tr align=right><td>1.6</td>  <td>2</td>
         *<tr align=right><td>1.1</td>  <td>2</td>
         *<tr align=right><td>1.0</td>  <td>1</td>
         *<tr align=right><td>-1.0</td> <td>-1</td>
         *<tr align=right><td>-1.1</td> <td>-2</td>
         *<tr align=right><td>-1.6</td> <td>-2</td>
         *<tr align=right><td>-2.5</td> <td>-3</td>
         *<tr align=right><td>-5.5</td> <td>-6</td>
         *</table>
         */
    UP(BigDecimal.ROUND_UP),

        /**
         * Rounding mode to round towards zero.  Never increments the digit
         * prior to a discarded fraction (i.e., truncates).  Note that this
         * rounding mode never increases the magnitude of the calculated value.
         *
         *<p>Example:
         *<table border>
         * <caption><b>Rounding mode DOWN Examples</b></caption>
         *<tr valign=top><th>Input Number</th>
         *    <th>Input rounded to one digit<br> with {@code DOWN} rounding
         *<tr align=right><td>5.5</td>  <td>5</td>
         *<tr align=right><td>2.5</td>  <td>2</td>
         *<tr align=right><td>1.6</td>  <td>1</td>
         *<tr align=right><td>1.1</td>  <td>1</td>
         *<tr align=right><td>1.0</td>  <td>1</td>
         *<tr align=right><td>-1.0</td> <td>-1</td>
         *<tr align=right><td>-1.1</td> <td>-1</td>
         *<tr align=right><td>-1.6</td> <td>-1</td>
         *<tr align=right><td>-2.5</td> <td>-2</td>
         *<tr align=right><td>-5.5</td> <td>-5</td>
         *</table>
         */
    DOWN(BigDecimal.ROUND_DOWN),

        /**
         * Rounding mode to round towards positive infinity.  If the
         * result is positive, behaves as for {@code RoundingMode.UP};
         * if negative, behaves as for {@code RoundingMode.DOWN}.  Note
         * that this rounding mode never decreases the calculated value.
         *
         *<p>Example:
         *<table border>
         * <caption><b>Rounding mode CEILING Examples</b></caption>
         *<tr valign=top><th>Input Number</th>
         *    <th>Input rounded to one digit<br> with {@code CEILING} rounding
         *<tr align=right><td>5.5</td>  <td>6</td>
         *<tr align=right><td>2.5</td>  <td>3</td>
         *<tr align=right><td>1.6</td>  <td>2</td>
         *<tr align=right><td>1.1</td>  <td>2</td>
         *<tr align=right><td>1.0</td>  <td>1</td>
         *<tr align=right><td>-1.0</td> <td>-1</td>
         *<tr align=right><td>-1.1</td> <td>-1</td>
         *<tr align=right><td>-1.6</td> <td>-1</td>
         *<tr align=right><td>-2.5</td> <td>-2</td>
         *<tr align=right><td>-5.5</td> <td>-5</td>
         *</table>
         */
    CEILING(BigDecimal.ROUND_CEILING),

        /**
         * Rounding mode to round towards negative infinity.  If the
         * result is positive, behave as for {@code RoundingMode.DOWN};
         * if negative, behave as for {@code RoundingMode.UP}.  Note that
         * this rounding mode never increases the calculated value.
         *
         *<p>Example:
         *<table border>
         * <caption><b>Rounding mode FLOOR Examples</b></caption>
         *<tr valign=top><th>Input Number</th>
         *    <th>Input rounded to one digit<br> with {@code FLOOR} rounding
         *<tr align=right><td>5.5</td>  <td>5</td>
         *<tr align=right><td>2.5</td>  <td>2</td>
         *<tr align=right><td>1.6</td>  <td>1</td>
         *<tr align=right><td>1.1</td>  <td>1</td>
         *<tr align=right><td>1.0</td>  <td>1</td>
         *<tr align=right><td>-1.0</td> <td>-1</td>
         *<tr align=right><td>-1.1</td> <td>-2</td>
         *<tr align=right><td>-1.6</td> <td>-2</td>
         *<tr align=right><td>-2.5</td> <td>-3</td>
         *<tr align=right><td>-5.5</td> <td>-6</td>
         *</table>
         */
    FLOOR(BigDecimal.ROUND_FLOOR),

        /**
         * Rounding mode to round towards {@literal "nearest neighbor"}
         * unless both neighbors are equidistant, in which case round up.
         * Behaves as for {@code RoundingMode.UP} if the discarded
         * fraction is &ge; 0.5; otherwise, behaves as for
         * {@code RoundingMode.DOWN}.  Note that this is the rounding
         * mode commonly taught at school.
         *
         *<p>Example:
         *<table border>
         * <caption><b>Rounding mode HALF_UP Examples</b></caption>
         *<tr valign=top><th>Input Number</th>
         *    <th>Input rounded to one digit<br> with {@code HALF_UP} rounding
         *<tr align=right><td>5.5</td>  <td>6</td>
         *<tr align=right><td>2.5</td>  <td>3</td>
         *<tr align=right><td>1.6</td>  <td>2</td>
         *<tr align=right><td>1.1</td>  <td>1</td>
         *<tr align=right><td>1.0</td>  <td>1</td>
         *<t