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>*<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>-2</td>*<tr align=right><td>-2.5</td> <td>-3</td>*<tr align=right><td>-5.5</td> <td>-6</td>*</table>*/HALF_UP(BigDecimal.ROUND_HALF_UP),/*** Rounding mode to round towards {@literal "nearest neighbor"}* unless both neighbors are equidistant, in which case round* down.  Behaves as for {@code RoundingMode.UP} if the discarded* fraction is &gt; 0.5; otherwise, behaves as for* {@code RoundingMode.DOWN}.**<p>Example:*<table border>* <caption><b>Rounding mode HALF_DOWN Examples</b></caption>*<tr valign=top><th>Input Number</th>*    <th>Input rounded to one digit<br> with {@code HALF_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>2</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>-2</td>*<tr align=right><td>-2.5</td> <td>-2</td>*<tr align=right><td>-5.5</td> <td>-5</td>*</table>*/HALF_DOWN(BigDecimal.ROUND_HALF_DOWN),/*** Rounding mode to round towards the {@literal "nearest neighbor"}* unless both neighbors are equidistant, in which case, round* towards the even neighbor.  Behaves as for* {@code RoundingMode.HALF_UP} if the digit to the left of the* discarded fraction is odd; behaves as for* {@code RoundingMode.HALF_DOWN} if it's even.  Note that this* is the rounding mode that statistically minimizes cumulative* error when applied repeatedly over a sequence of calculations.* It is sometimes known as {@literal "Banker's rounding,"} and is* chiefly used in the USA.  This rounding mode is analogous to* the rounding policy used for {@code float} and {@code double}* arithmetic in Java.**<p>Example:*<table border>* <caption><b>Rounding mode HALF_EVEN Examples</b></caption>*<tr valign=top><th>Input Number</th>*    <th>Input rounded to one digit<br> with {@code HALF_EVEN} rounding*<tr align=right><td>5.5</td>  <td>6</td>*<tr align=right><td>2.5</td>  <td>2</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>*<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>-2</td>*<tr align=right><td>-2.5</td> <td>-2</td>*<tr align=right><td>-5.5</td> <td>-6</td>*</table>*/HALF_EVEN(BigDecimal.ROUND_HALF_EVEN),/*** Rounding mode to assert that the requested operation has an exact* result, hence no rounding is necessary.  If this rounding mode is* specified on an operation that yields an inexact result, an* {@code ArithmeticException} is thrown.*<p>Example:*<table border>* <caption><b>Rounding mode UNNECESSARY Examples</b></caption>*<tr valign=top><th>Input Number</th>*    <th>Input rounded to one digit<br> with {@code UNNECESSARY} rounding*<tr align=right><td>5.5</td>  <td>throw {@code ArithmeticException}</td>*<tr align=right><td>2.5</td>  <td>throw {@code ArithmeticException}</td>*<tr align=right><td>1.6</td>  <td>throw {@code ArithmeticException}</td>*<tr align=right><td>1.1</td>  <td>throw {@code ArithmeticException}</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>throw {@code ArithmeticException}</td>*<tr align=right><td>-1.6</td> <td>throw {@code ArithmeticException}</td>*<tr align=right><td>-2.5</td> <td>throw {@code ArithmeticException}</td>*<tr align=right><td>-5.5</td> <td>throw {@code ArithmeticException}</td>*</table>*/UNNECESSARY(BigDecimal.ROUND_UNNECESSARY);// Corresponding BigDecimal rounding constantfinal int oldMode;/*** Constructor** @param oldMode The {@code BigDecimal} constant corresponding to*        this mode*/private RoundingMode(int oldMode) {this.oldMode = oldMode;}/*** Returns the {@code RoundingMode} object corresponding to a* legacy integer rounding mode constant in {@link BigDecimal}.** @param  rm legacy integer rounding mode to convert* @return {@code RoundingMode} corresponding to the given integer.* @throws IllegalArgumentException integer is out of range*/public static RoundingMode valueOf(int rm) {switch(rm) {case BigDecimal.ROUND_UP:return UP;case BigDecimal.ROUND_DOWN:return DOWN;case BigDecimal.ROUND_CEILING:return CEILING;case BigDecimal.ROUND_FLOOR:return FLOOR;case BigDecimal.ROUND_HALF_UP:return HALF_UP;case BigDecimal.ROUND_HALF_DOWN:return HALF_DOWN;case BigDecimal.ROUND_HALF_EVEN:return HALF_EVEN;case BigDecimal.ROUND_UNNECESSARY:return UNNECESSARY;default:throw new IllegalArgumentException("argument out of range");}}
}

8种舍位模式含义解析：

RoundingMode.UP：正数向上舍位，负数向下舍位。
RoundingMode.DOWN：直接保留指定位数数字，可以理解为截取。
RoundingMode.CEILING：直译是天花板的意思，正数的效果如同 RoundingMode.UP，负数的效果如同 RoundingMode.DOWN。
RoundingMode.FLOOR：直译是地板的意思，和 RoundingMode.CEILING 刚好相反，正数的效果如同 RoundingMode.DOWN，负数的效果如同 RoundingMode.UP。
RoundingMode.HALF_UP：就是比较常见的四舍五入。
RoundingMode.HALF_DOWN：五舍六入。
RoundingMode.HALF_EVEN：指定位数后面的小数是5，且5后面有数字且这些数字不全部为0，则进一位，否则看5前面的一位，如果是奇数就进一位，如果是偶数则不进位，其他情况四舍五入。
RoundingMode.UNNECESSARY：指定位数后没有数字或者全部是0，否则会抛出异常。

RoundingMode.UP 验证：

BigDecimal bigDecimal1 = new BigDecimal("1.235");
BigDecimal up1 = bigDecimal1.setScale(2, RoundingMode.UP);
BigDecimal bigDecimal2 = new BigDecimal("-1.233");
BigDecimal up2 = bigDecimal2.setScale(2, RoundingMode.UP);
System.out.println("up1:" + up1);
System.out.println("up2:" + up2);

执行结果：

up1:1.24
up2:-1.24

正数向上舍位，负数向下舍位，符合预期。

RoundingMode.HALF_DOWN 验证：

BigDecimal down = new BigDecimal("1.235").setScale(2, RoundingMode.DOWN);
System.out.println("down:" + down);

执行结果：

down:1.23

截取直接保留指定位数数字，符合预期。

RoundingMode.CEILING 验证：

BigDecimal ceilingOne = new BigDecimal("1.235").setScale(2, RoundingMode.CEILING);
BigDecimal ceilingTwo = new BigDecimal("-1.235").setScale(2, RoundingMode.CEILING);
System.out.println("ceilingOne:" + ceilingOne);
System.out.println("ceilingTwo:" + ceilingTwo);

执行结果：

ceilingOne:1.24
ceilingTwo:-1.23

正数向上舍位，负数直接截取，符合预期。

RoundingMode.FLOOR 验证：

BigDecimal floorOne = new BigDecimal("1.235").setScale(2, RoundingMode.FLOOR);
BigDecimal floorTwo = new BigDecimal("-1.235").setScale(2, RoundingMode.FLOOR);
System.out.println("floorOne:" + floorOne);
System.out.println("floorTwo:" + floorTwo);

执行结果：

floorOne:1.23
floorTwo:-1.24

正数截取，负数向下舍位，符合预期。

RoundingMode.HALF_UP 验证：

BigDecimal halfUpOne = new BigDecimal("1.235").setScale(2, RoundingMode.HALF_UP);
BigDecimal halfUpTwo = new BigDecimal("1.234").setScale(2, RoundingMode.HALF_UP);
System.out.println("halfUpOne:" + halfUpOne);
System.out.println("halfUpTwo:" + halfUpTwo);

执行结果：

halfUpOne:1.24
halfUpTwo:1.23

四舍五入，符合预期。

RoundingMode.HALF_EVEN 验证：

BigDecimal halfEvenOne = new BigDecimal("1.2351").setScale(2, RoundingMode.HALF_EVEN);
BigDecimal halfEvenTwo = new BigDecimal("1.2250").setScale(2, RoundingMode.HALF_EVEN);
BigDecimal halfEvenThree = new BigDecimal("1.2350").setScale(2, RoundingMode.HALF_EVEN);
BigDecimal halfEvenFour = new BigDecimal("1.2450").setScale(2, RoundingMode.HALF_EVEN);
BigDecimal halfEvenFive = new BigDecimal("1.244").setScale(2, RoundingMode.HALF_EVEN);
BigDecimal halfEvenSix = new BigDecimal("1.246").setScale(2, RoundingMode.HALF_EVEN);
System.out.println("halfEvenOne:" + halfEvenOne);
System.out.println("halfEvenTwo:" + halfEvenTwo);
System.out.println("halfEvenThree:" + halfEvenThree);
System.out.println("halfEvenFour:" + halfEvenFour);
System.out.println("halfEvenFive:" + halfEvenFive);
System.out.println("halfEvenSix:" + halfEvenSix);

执行结果：

halfEvenOne:1.24
halfEvenTwo:1.22
halfEvenThree:1.24
halfEvenFour:1.24
halfEvenFive:1.24
halfEvenSix:1.25

符合预期。

RoundingMode.HALF_EVEN 验证：

BigDecimal unnecessaryOne = new BigDecimal("1.240").setScale(2, RoundingMode.UNNECESSARY);
System.out.println("unnecessaryOne:" + unnecessaryOne);
BigDecimal unnecessaryTwo = new BigDecimal("1.2401").setScale(2, RoundingMode.UNNECESSARY);
System.out.println("unnecessaryTwo:" + unnecessaryTwo);

执行结果：

unnecessaryOne:1.24
Exception in thread "main" java.lang.ArithmeticException: Rounding necessaryat java.math.BigDecimal.commonNeedIncrement(BigDecimal.java:4148)at java.math.BigDecimal.needIncrement(BigDecimal.java:4204)at java.math.BigDecimal.divideAndRound(BigDecimal.java:4112)at java.math.BigDecimal.setScale(BigDecimal.java:2452)at java.math.BigDecimal.setScale(BigDecimal.java:2386)at com.zt.dc.portal.admin.web.service.impl.satisfaction.SatisfactionEvaluationServiceImpl.main(SatisfactionEvaluationServiceImpl.java:262)