Java DecimalFormat 保留小數(shù)位及四舍五入的陷阱介紹

更新時間：2021年10月12日 11:07:52 作者：智鼎在線IT團(tuán)隊

這篇文章主要介紹了Java DecimalFormat 保留小數(shù)位及四舍五入的陷阱，具有很好的參考價值，希望對大家有所幫助。如有錯誤或未考慮完全的地方，望不吝賜教

需求

業(yè)務(wù)需要導(dǎo)出的Excel的數(shù)字內(nèi)容保留兩位小數(shù),并且四舍五入

代碼實現(xiàn)

百度一圈所抄襲的代碼

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

發(fā)現(xiàn)問題

導(dǎo)出數(shù)據(jù)很詭異.不是所有數(shù)據(jù)都是如所想的四舍五入.

經(jīng)過排查最終發(fā)現(xiàn)是RoundingMode的問題,應(yīng)該使用HALF_UP,

DecimalFormat 默認(rèn)使用的是HALF_EVEN

DecimalFormat dfScale2 = new DecimalFormat("###.##");
System.out.println("dfScale2.getRoundingMode()=" + dfScale2.getRoundingMode());
//輸出結(jié)果
dfScale2.getRoundingMode()=HALF_EVEN
//

RoundingMode.HALF_EVEN

想了解HALF_EVEN，去官網(wǎng)API看了下

HALF_EVEN 被舍位是5(如保留兩位小數(shù)的2.115),后面還有非0值進(jìn)1(如保留兩位小數(shù)的2.11500001 格式化為2.12),5后面沒有數(shù)字或者都是0時,前面是偶數(shù)則舍,是奇數(shù)則進(jìn)1,目標(biāo)是讓被舍前一位變?yōu)榕紨?shù).

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 設(shè)置這個模式,對于精確值格式化會拋出異常
Rounding mode to assert that the requested operation has an exact result, hence no rounding is necessary.
UP 向遠(yuǎn)離數(shù)字0進(jìn)行進(jìn)位.
Rounding mode to round away from zero.

錯誤的代碼測試RoundingMode.HALF_EVEN

為了更好的理解HALF_EVEN,寫了些測試代碼但是發(fā)現(xiàn)自己更迷惘了…搞不清楚到底HALF_EVEN是什么機(jī)制進(jìn)舍…輸出結(jié)果的尾數(shù)很不規(guī)律.

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.*;
public class LocalTest {
//定義一個保留兩位小數(shù)格式的 DecimalFormat 的變量 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

正確的代碼測試RoundingMode.HALF_EVEN

突然發(fā)現(xiàn)自己忽略了一個事情,測試的參數(shù)都是用的double類型.想起來double類型不精準(zhǔn).但是僥幸心理以及知識不牢靠以為 3位小數(shù)應(yīng)該影響不大吧.改了下代碼,把參數(shù)改為BigDecimal類型

使用BigDecimal時,參數(shù)盡量傳入字符串,要比傳入double精準(zhǔn).

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