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Java DecimalFormat 保留小数位及四舍五入的陷阱介绍

作者：智鼎在线IT团队　　发布时间：2023-11-09 04:49:33　

标签：Java,DecimalFormat,小数位,四舍五入

需求

业务需要导出的Excel的数字内容保留两位小数,并且四舍五入

代码实现

百度一圈所抄袭的代码


DecimalFormat dfScale2 = new DecimalFormat("###.##");
dfScale2.format(1.125D);

发现问题

导出数据很诡异.不是所有数据都是如所想的四舍五入.

经过排查最终发现是RoundingMode的问题,应该使用HALF_UP,

DecimalFormat 默认使用的是HALF_EVEN


DecimalFormat dfScale2 = new DecimalFormat("###.##");
System.out.println("dfScale2.getRoundingMode()=" + dfScale2.getRoundingMode());
//输出结果
dfScale2.getRoundingMode()=HALF_EVEN
//

RoundingMode.HALF_EVEN

想了解HALF_EVEN，去官网API看了下

HALF_EVEN 被舍位是5(如保留两位小数的2.115),后面还有非0值进1(如保留两位小数的2.11500001 格式化为2.12),5后面没有数字或者都是0时,前面是偶数则舍,是奇数则进1,目标是让被舍前一位变为偶数.

  • CEILING 向更大的值靠近

  • Rounding mode to round towards positive infinity.

  • DOWN向下取整

  • Rounding mode to round towards zero.

  • FLOOR 向更小的值靠近

  • Rounding mode to round towards negative infinity.

  • HALF_DOWN 五舍六入

  • Rounding mode to round towards “nearest neighbor” unless both neighbors are equidistant, in which case round down.

  • HALF_EVEN

  • Rounding mode to round towards the “nearest neighbor” unless both neighbors are equidistant, in which case, round towards the even neighbor.

  • HALF_UP 四舍五入

  • Rounding mode to round towards “nearest neighbor” unless both neighbors are equidistant, in which case round up.

  • UNNECESSARY 设置这个模式,对于精确值格式化会抛出异常

  • Rounding mode to assert that the requested operation has an exact result, hence no rounding is necessary.

  • UP 向远离数字0进行进位.

  • Rounding mode to round away from zero.

错误的代码测试RoundingMode.HALF_EVEN

为了更好的理解HALF_EVEN,写了些测试代码但是发现自己更迷惘了…搞不清楚到底HALF_EVEN是什么机制进舍…输出结果的尾数很不规律.


import java.math.BigDecimal;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.*;
public class LocalTest {
//定义一个保留两位小数格式的 DecimalFormat 的变量 dfScale2
@Test
 public void testDecimalFormat() {
   DecimalFormat dfScale2 = new DecimalFormat("###.##");
   System.out.println("dfScale2.getRoundingMode()=" + dfScale2.getRoundingMode());
   System.out.println("dfScale2.format(1.125D)=" + dfScale2.format(1.125D));
   System.out.println("dfScale2.format(1.135D)=" + dfScale2.format(1.135D));
   System.out.println("dfScale2.format(1.145D)=" + dfScale2.format(1.145D));
   System.out.println("dfScale2.format(1.225D)=" + dfScale2.format(1.225D));
   System.out.println("dfScale2.format(1.235D)=" + dfScale2.format(1.235D));
   System.out.println("dfScale2.format(1.245D)=" + dfScale2.format(1.245D));
   System.out.println();
   System.out.println("dfScale2.format(2.125D)=" + dfScale2.format(2.125D));
   System.out.println("dfScale2.format(2.135D)=" + dfScale2.format(2.135D));
   System.out.println("dfScale2.format(2.145D)=" + dfScale2.format(2.145D));
   System.out.println("dfScale2.format(2.225D)=" + dfScale2.format(2.225D));
   System.out.println("dfScale2.format(2.235D)=" + dfScale2.format(2.235D));
   System.out.println("dfScale2.format(2.245D)=" + dfScale2.format(2.245D));
   System.out.println();
   System.out.println("dfScale2.format(3.125D)=" + dfScale2.format(3.125D));
   System.out.println("dfScale2.format(3.135D)=" + dfScale2.format(3.135D));
   System.out.println("dfScale2.format(3.145D)=" + dfScale2.format(3.145D));
   System.out.println("dfScale2.format(3.225D)=" + dfScale2.format(3.225D));
   System.out.println("dfScale2.format(3.235D)=" + dfScale2.format(3.235D));
   System.out.println("dfScale2.format(3.245D)=" + dfScale2.format(3.245D));
   System.out.println();
   System.out.println("dfScale2.format(4.125D)=" + dfScale2.format(4.125D));
   System.out.println("dfScale2.format(4.135D)=" + dfScale2.format(4.135D));
   System.out.println("dfScale2.format(4.145D)=" + dfScale2.format(4.145D));
   System.out.println("dfScale2.format(4.225D)=" + dfScale2.format(4.225D));
   System.out.println("dfScale2.format(4.235D)=" + dfScale2.format(4.235D));
   System.out.println("dfScale2.format(4.245D)=" + dfScale2.format(4.245D));
 }
}

dfScale2.getRoundingMode()=HALF_EVEN
dfScale2.format(1.125D)=1.12
dfScale2.format(1.135D)=1.14
dfScale2.format(1.145D)=1.15
dfScale2.format(1.225D)=1.23
dfScale2.format(1.235D)=1.24
dfScale2.format(1.245D)=1.25
dfScale2.format(2.125D)=2.12
dfScale2.format(2.135D)=2.13
dfScale2.format(2.145D)=2.15
dfScale2.format(2.225D)=2.23
dfScale2.format(2.235D)=2.23
dfScale2.format(2.245D)=2.25
dfScale2.format(3.125D)=3.12
dfScale2.format(3.135D)=3.13
dfScale2.format(3.145D)=3.15
dfScale2.format(3.225D)=3.23
dfScale2.format(3.235D)=3.23
dfScale2.format(3.245D)=3.25
dfScale2.format(4.125D)=4.12
dfScale2.format(4.135D)=4.13
dfScale2.format(4.145D)=4.14
dfScale2.format(4.225D)=4.22
dfScale2.format(4.235D)=4.24
dfScale2.format(4.245D)=4.25

正确的代码测试RoundingMode.HALF_EVEN

突然发现自己忽略了一个事情,测试的参数都是用的double类型.想起来double类型不精准.但是侥幸心理以及知识不牢靠以为 3位小数应该影响不大吧.改了下代码,把参数改为BigDecimal类型

使用BigDecimal时,参数尽量传入字符串,要比传入double精准.


new BigDecimal("1.125")

 @Test
 public void testDecimalFormat() {
   DecimalFormat dfScale2 = new DecimalFormat("###.##");
     dfScale2.setRoundingMode(RoundingMode.HALF_EVEN);
   System.out.println("dfScale2.getRoundingMode()=" + dfScale2.getRoundingMode());
       System.out.println("dfScale2.format(new BigDecimal(\"1.1251\"))=" + dfScale2.format(new BigDecimal("1.1251")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.1351\"))=" + dfScale2.format(new BigDecimal("1.1351")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.1451\"))=" + dfScale2.format(new BigDecimal("1.1451")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.2250\"))=" + dfScale2.format(new BigDecimal("1.2250")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.2350\"))=" + dfScale2.format(new BigDecimal("1.2350")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.2450\"))=" + dfScale2.format(new BigDecimal("1.2450")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.22501\"))=" + dfScale2.format(new BigDecimal("1.22501")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.23505\"))=" + dfScale2.format(new BigDecimal("1.23505")));
   System.out.println("dfScale2.format(new BigDecimal(\"1.24508\"))=" + dfScale2.format(new BigDecimal("1.24508")));


dfScale2.getRoundingMode()=HALF_EVEN
dfScale2.format(new BigDecimal("1.1251"))=1.13
dfScale2.format(new BigDecimal("1.1351"))=1.14
dfScale2.format(new BigDecimal("1.1451"))=1.15
dfScale2.format(new BigDecimal("1.2250"))=1.22
dfScale2.format(new BigDecimal("1.2350"))=1.24
dfScale2.format(new BigDecimal("1.2450"))=1.24
dfScale2.format(new BigDecimal("1.22501"))=1.23
dfScale2.format(new BigDecimal("1.23505"))=1.24
dfScale2.format(new BigDecimal("1.24508"))=1.25

结论

1、警觉doulbe的不精确所引起RoundingMode结果不稳定的问题,即使是四舍五入的模式,对double类型参数使用也会有不满足预期的情况.

2、使用数字格式化时,要注意默认RoundingMode模式是否是自己需要的.如果不是记得手动设置下.

来源：https://blog.csdn.net/baixf/article/details/88792219

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