Java中的 BigDecimal，80%的人都用錯(cuò)了....

Java在java.math包中提供的API類BigDecimal，用來(lái)對(duì)超過(guò)16位有效位的數(shù)進(jìn)行精確的運(yùn)算。雙精度浮點(diǎn)型變量double可以處理16位有效數(shù)，但在實(shí)際應(yīng)用中，可能需要對(duì)更大或者更小的數(shù)進(jìn)行運(yùn)算和處理。

一般情況下，對(duì)于那些不需要準(zhǔn)確計(jì)算精度的數(shù)字，我們可以直接使用Float和Double處理，但是Double.valueOf(String)和Float.valueOf(String)會(huì)丟失精度。所以開發(fā)中，如果我們需要精確計(jì)算的結(jié)果，則必須使用BigDecimal類來(lái)操作。

BigDecimal所創(chuàng)建的是對(duì)象，故我們不能使用傳統(tǒng)的+、-、*、/等算術(shù)運(yùn)算符直接對(duì)其對(duì)象進(jìn)行數(shù)學(xué)運(yùn)算，而必須調(diào)用其相對(duì)應(yīng)的方法。方法中的參數(shù)也必須是BigDecimal的對(duì)象。構(gòu)造器是類的特殊方法，專門用來(lái)創(chuàng)建對(duì)象，特別是帶有參數(shù)的對(duì)象。

二、BigDecimal常用構(gòu)造函數(shù)

2.1、常用構(gòu)造函數(shù)

• BigDecimal(int)

創(chuàng)建一個(gè)具有參數(shù)所指定整數(shù)值的對(duì)象

• BigDecimal(double)

創(chuàng)建一個(gè)具有參數(shù)所指定雙精度值的對(duì)象

• BigDecimal(long)

創(chuàng)建一個(gè)具有參數(shù)所指定長(zhǎng)整數(shù)值的對(duì)象

• BigDecimal(String)

創(chuàng)建一個(gè)具有參數(shù)所指定以字符串表示的數(shù)值的對(duì)象

2.2、使用問(wèn)題分析

使用示例：

BigDecimal?a?=new?BigDecimal(0.1);??
System.out.println("a?values?is:"+a);??
System.out.println("=====================");??
BigDecimal?b?=new?BigDecimal("0.1");??
System.out.println("b?values?is:"+b);??

結(jié)果示例：

a?values?is:0.1000000000000000055511151231257827021181583404541015625??
=====================??
b?values?is:0.1??

原因分析：

1）參數(shù)類型為double的構(gòu)造方法的結(jié)果有一定的不可預(yù)知性。有人可能認(rèn)為在Java中寫入newBigDecimal(0.1)所創(chuàng)建的BigDecimal正好等于 0.1（非標(biāo)度值 1，其標(biāo)度為 1），但是它實(shí)際上等于0.1000000000000000055511151231257827021181583404541015625。這是因?yàn)?.1無(wú)法準(zhǔn)確地表示為 double（或者說(shuō)對(duì)于該情況，不能表示為任何有限長(zhǎng)度的二進(jìn)制小數(shù)）。這樣，傳入到構(gòu)造方法的值不會(huì)正好等于 0.1（雖然表面上等于該值）。

2）String 構(gòu)造方法是完全可預(yù)知的：寫入newBigDecimal(“0.1”)將創(chuàng)建一個(gè) BigDecimal，它正好等于預(yù)期的 0.1。因此，比較而言， 通常建議優(yōu)先使用String構(gòu)造方法。

3）當(dāng)double必須用作BigDecimal的源時(shí)，請(qǐng)注意，此構(gòu)造方法提供了一個(gè)準(zhǔn)確轉(zhuǎn)換；它不提供與以下操作相同的結(jié)果：先使用Double.toString(double)方法，然后使用BigDecimal(String)構(gòu)造方法，將double轉(zhuǎn)換為String。要獲取該結(jié)果，請(qǐng)使用static valueOf(double)方法。

三、BigDecimal常用方法詳解

3.1、常用方法

• add(BigDecimal)

BigDecimal對(duì)象中的值相加，返回BigDecimal對(duì)象

• subtract(BigDecimal)

BigDecimal對(duì)象中的值相減，返回BigDecimal對(duì)象

• multiply(BigDecimal)

BigDecimal對(duì)象中的值相乘，返回BigDecimal對(duì)象

• divide(BigDecimal)

BigDecimal對(duì)象中的值相除，返回BigDecimal對(duì)象

• toString()

將BigDecimal對(duì)象中的值轉(zhuǎn)換成字符串

• doubleValue()

將BigDecimal對(duì)象中的值轉(zhuǎn)換成雙精度數(shù)

• floatValue()

將BigDecimal對(duì)象中的值轉(zhuǎn)換成單精度數(shù)

• longValue()

將BigDecimal對(duì)象中的值轉(zhuǎn)換成長(zhǎng)整數(shù)

• intValue()

將BigDecimal對(duì)象中的值轉(zhuǎn)換成整數(shù)

3.2、BigDecimal大小比較

java中對(duì)BigDecimal比較大小一般用的是bigdemical的compareTo方法

int?a?=?bigdemical.compareTo(bigdemical2)??

返回結(jié)果分析：

a =?-1,表示bigdemical小于bigdemical2；??
a =?0,表示bigdemical等于bigdemical2；??
a = 1,表示bigdemical大于bigdemical2；??

舉例：a大于等于b

new?bigdemica(a).compareTo(new?bigdemical(b))?>=?0??

四、BigDecimal格式化

由于NumberFormat類的format()方法可以使用BigDecimal對(duì)象作為其參數(shù)，可以利用BigDecimal對(duì)超出16位有效數(shù)字的貨幣值，百分值，以及一般數(shù)值進(jìn)行格式化控制。

以利用BigDecimal對(duì)貨幣和百分比格式化為例。首先，創(chuàng)建BigDecimal對(duì)象，進(jìn)行BigDecimal的算術(shù)運(yùn)算后，分別建立對(duì)貨幣和百分比格式化的引用，最后利用BigDecimal對(duì)象作為format()方法的參數(shù)，輸出其格式化的貨幣值和百分比。

NumberFormat?currency?=?NumberFormat.getCurrencyInstance();?//建立貨幣格式化引用???
NumberFormat?percent?=?NumberFormat.getPercentInstance();??//建立百分比格式化引用???
percent.setMaximumFractionDigits(3);?//百分比小數(shù)點(diǎn)最多3位???
??
BigDecimal?loanAmount?=?new?BigDecimal("15000.48");?//貸款金額??
BigDecimal?interestRate?=?new?BigDecimal("0.008");?//利率?????
BigDecimal?interest?=?loanAmount.multiply(interestRate);?//相乘??
??
System.out.println("貸款金額:\t"?+?currency.format(loanAmount));???
System.out.println("利率:\t"?+?percent.format(interestRate));???
System.out.println("利息:\t"?+?currency.format(interest));???

結(jié)果：

貸款金額:?￥15,000.48?利率:?0.8%?利息:?￥120.00??

BigDecimal格式化保留2為小數(shù)，不足則補(bǔ)0：

public?class?NumberFormat?{??
??
????public?static?void?main(String[]?s){??
????????System.out.println(formatToNumber(new?BigDecimal("3.435")));??
????????System.out.println(formatToNumber(new?BigDecimal(0)));??
????????System.out.println(formatToNumber(new?BigDecimal("0.00")));??
????????System.out.println(formatToNumber(new?BigDecimal("0.001")));??
????????System.out.println(formatToNumber(new?BigDecimal("0.006")));??
????????System.out.println(formatToNumber(new?BigDecimal("0.206")));??
????}??
????/**??
?????*?@desc?1.0~1之間的BigDecimal小數(shù)，格式化后失去前面的0,則前面直接加上0。??
?????*?2.傳入的參數(shù)等于0，則直接返回字符串"0.00"??
?????*?3.大于1的小數(shù)，直接格式化返回字符串??
?????*?@param?obj傳入的小數(shù)??
?????*?@return??
?????*/??
????public?static?String?formatToNumber(BigDecimal?obj)?{??
????????DecimalFormat?df?=?new?DecimalFormat("#.00");??
????????if(obj.compareTo(BigDecimal.ZERO)==0)?{??
????????????return?"0.00";??
????????}else?if(obj.compareTo(BigDecimal.ZERO)>0&&obj.compareTo(new?BigDecimal(1))<0){??
????????????return?"0"+df.format(obj).toString();??
????????}else?{??
????????????return?df.format(obj).toString();??
????????}??
????}??
}??

結(jié)果為：

3.44??
0.00??
0.00??
0.00??
0.01??
0.21??

五、BigDecimal常見異常

5.1、除法的時(shí)候出現(xiàn)異常

java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result

原因分析：

通過(guò)BigDecimal的divide方法進(jìn)行除法時(shí)當(dāng)不整除，出現(xiàn)無(wú)限循環(huán)小數(shù)時(shí)，就會(huì)拋異常：java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.

解決方法：

divide方法設(shè)置精確的小數(shù)點(diǎn)，如：divide(xxxxx,2)

六、BigDecimal總結(jié)

6.1、總結(jié)

在需要精確的小數(shù)計(jì)算時(shí)再使用BigDecimal，BigDecimal的性能比double和float差，在處理龐大，復(fù)雜的運(yùn)算時(shí)尤為明顯。故一般精度的計(jì)算沒必要使用BigDecimal。
盡量使用參數(shù)類型為String的構(gòu)造函數(shù)。

BigDecimal都是不可變的（immutable）的， 在進(jìn)行每一次四則運(yùn)算時(shí)，都會(huì)產(chǎn)生一個(gè)新的對(duì)象 ，所以在做加減乘除運(yùn)算時(shí)要記得要保存操作后的值。

6.2、工具類推薦

package?com.vivo.ars.util;??
import?java.math.BigDecimal;??
??
/**??
?*?用于高精確處理常用的數(shù)學(xué)運(yùn)算??
?*/??
public?class?ArithmeticUtils?{??
????//默認(rèn)除法運(yùn)算精度??
????private?static?final?int?DEF_DIV_SCALE?=?10;??
??
????/**??
?????*?提供精確的加法運(yùn)算??
?????*??
?????*?@param?v1?被加數(shù)??
?????*?@param?v2?加數(shù)??
?????*?@return?兩個(gè)參數(shù)的和??
?????*/??
??
????public?static?double?add(double?v1,?double?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(Double.toString(v1));??
????????BigDecimal?b2?=?new?BigDecimal(Double.toString(v2));??
????????return?b1.add(b2).doubleValue();??
????}??
??
????/**??
?????*?提供精確的加法運(yùn)算??
?????*??
?????*?@param?v1?被加數(shù)??
?????*?@param?v2?加數(shù)??
?????*?@return?兩個(gè)參數(shù)的和??
?????*/??
????public?static?BigDecimal?add(String?v1,?String?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.add(b2);??
????}??
??
????/**??
?????*?提供精確的加法運(yùn)算??
?????*??
?????*?@param?v1????被加數(shù)??
?????*?@param?v2????加數(shù)??
?????*?@param?scale?保留scale?位小數(shù)??
?????*?@return?兩個(gè)參數(shù)的和??
?????*/??
????public?static?String?add(String?v1,?String?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException(??
????????????????????"The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.add(b2).setScale(scale,?BigDecimal.ROUND_HALF_UP).toString();??
????}??
??
????/**??
?????*?提供精確的減法運(yùn)算??
?????*??
?????*?@param?v1?被減數(shù)??
?????*?@param?v2?減數(shù)??
?????*?@return?兩個(gè)參數(shù)的差??
?????*/??
????public?static?double?sub(double?v1,?double?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(Double.toString(v1));??
????????BigDecimal?b2?=?new?BigDecimal(Double.toString(v2));??
????????return?b1.subtract(b2).doubleValue();??
????}??
??
????/**??
?????*?提供精確的減法運(yùn)算。??
?????*??
?????*?@param?v1?被減數(shù)??
?????*?@param?v2?減數(shù)??
?????*?@return?兩個(gè)參數(shù)的差??
?????*/??
????public?static?BigDecimal?sub(String?v1,?String?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.subtract(b2);??
????}??
??
????/**??
?????*?提供精確的減法運(yùn)算??
?????*??
?????*?@param?v1????被減數(shù)??
?????*?@param?v2????減數(shù)??
?????*?@param?scale?保留scale?位小數(shù)??
?????*?@return?兩個(gè)參數(shù)的差??
?????*/??
????public?static?String?sub(String?v1,?String?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException(??
????????????????????"The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.subtract(b2).setScale(scale,?BigDecimal.ROUND_HALF_UP).toString();??
????}??
??
????/**??
?????*?提供精確的乘法運(yùn)算??
?????*??
?????*?@param?v1?被乘數(shù)??
?????*?@param?v2?乘數(shù)??
?????*?@return?兩個(gè)參數(shù)的積??
?????*/??
????public?static?double?mul(double?v1,?double?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(Double.toString(v1));??
????????BigDecimal?b2?=?new?BigDecimal(Double.toString(v2));??
????????return?b1.multiply(b2).doubleValue();??
????}??
??
????/**??
?????*?提供精確的乘法運(yùn)算??
?????*??
?????*?@param?v1?被乘數(shù)??
?????*?@param?v2?乘數(shù)??
?????*?@return?兩個(gè)參數(shù)的積??
?????*/??
????public?static?BigDecimal?mul(String?v1,?String?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.multiply(b2);??
????}??
??
????/**??
?????*?提供精確的乘法運(yùn)算??
?????*??
?????*?@param?v1????被乘數(shù)??
?????*?@param?v2????乘數(shù)??
?????*?@param?scale?保留scale?位小數(shù)??
?????*?@return?兩個(gè)參數(shù)的積??
?????*/??
????public?static?double?mul(double?v1,?double?v2,?int?scale)?{??
????????BigDecimal?b1?=?new?BigDecimal(Double.toString(v1));??
????????BigDecimal?b2?=?new?BigDecimal(Double.toString(v2));??
????????return?round(b1.multiply(b2).doubleValue(),?scale);??
????}??
??
????/**??
?????*?提供精確的乘法運(yùn)算??
?????*??
?????*?@param?v1????被乘數(shù)??
?????*?@param?v2????乘數(shù)??
?????*?@param?scale?保留scale?位小數(shù)??
?????*?@return?兩個(gè)參數(shù)的積??
?????*/??
????public?static?String?mul(String?v1,?String?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException(??
????????????????????"The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.multiply(b2).setScale(scale,?BigDecimal.ROUND_HALF_UP).toString();??
????}??
??
????/**??
?????*?提供（相對(duì)）精確的除法運(yùn)算，當(dāng)發(fā)生除不盡的情況時(shí)，精確到??
?????*?小數(shù)點(diǎn)以后10位，以后的數(shù)字四舍五入??
?????*??
?????*?@param?v1?被除數(shù)??
?????*?@param?v2?除數(shù)??
?????*?@return?兩個(gè)參數(shù)的商??
?????*/??
??
????public?static?double?div(double?v1,?double?v2)?{??
????????return?div(v1,?v2,?DEF_DIV_SCALE);??
????}??
??
????/**??
?????*?提供（相對(duì)）精確的除法運(yùn)算。當(dāng)發(fā)生除不盡的情況時(shí)，由scale參數(shù)指??
?????*?定精度，以后的數(shù)字四舍五入??
?????*??
?????*?@param?v1????被除數(shù)??
?????*?@param?v2????除數(shù)??
?????*?@param?scale 表示表示需要精確到小數(shù)點(diǎn)以后幾位。??
?????*?@return?兩個(gè)參數(shù)的商??
?????*/??
????public?static?double?div(double?v1,?double?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException("The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b1?=?new?BigDecimal(Double.toString(v1));??
????????BigDecimal?b2?=?new?BigDecimal(Double.toString(v2));??
????????return?b1.divide(b2,?scale,?BigDecimal.ROUND_HALF_UP).doubleValue();??
????}??
??
????/**??
?????*?提供（相對(duì)）精確的除法運(yùn)算。當(dāng)發(fā)生除不盡的情況時(shí)，由scale參數(shù)指??
?????*?定精度，以后的數(shù)字四舍五入??
?????*??
?????*?@param?v1????被除數(shù)??
?????*?@param?v2????除數(shù)??
?????*?@param?scale?表示需要精確到小數(shù)點(diǎn)以后幾位??
?????*?@return?兩個(gè)參數(shù)的商??
?????*/??
????public?static?String?div(String?v1,?String?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException("The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v1);??
????????return?b1.divide(b2,?scale,?BigDecimal.ROUND_HALF_UP).toString();??
????}??
??
????/**??
?????*?提供精確的小數(shù)位四舍五入處理??
?????*??
?????*?@param?v?????需要四舍五入的數(shù)字??
?????*?@param?scale?小數(shù)點(diǎn)后保留幾位??
?????*?@return?四舍五入后的結(jié)果??
?????*/??
????public?static?double?round(double?v,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException("The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b?=?new?BigDecimal(Double.toString(v));??
????????return?b.setScale(scale,?BigDecimal.ROUND_HALF_UP).doubleValue();??
????}??
??
????/**??
?????*?提供精確的小數(shù)位四舍五入處理??
?????*??
?????*?@param?v?????需要四舍五入的數(shù)字??
?????*?@param?scale?小數(shù)點(diǎn)后保留幾位??
?????*?@return?四舍五入后的結(jié)果??
?????*/??
????public?static?String?round(String?v,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException(??
????????????????????"The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b?=?new?BigDecimal(v);??
????????return?b.setScale(scale,?BigDecimal.ROUND_HALF_UP).toString();??
????}??
??
????/**??
?????*?取余數(shù)??
?????*??
?????*?@param?v1????被除數(shù)??
?????*?@param?v2????除數(shù)??
?????*?@param?scale?小數(shù)點(diǎn)后保留幾位??
?????*?@return?余數(shù)??
?????*/??
????public?static?String?remainder(String?v1,?String?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException(??
????????????????????"The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????return?b1.remainder(b2).setScale(scale,?BigDecimal.ROUND_HALF_UP).toString();??
????}??
??
????/**??
?????*?取余數(shù)??BigDecimal??
?????*??
?????*?@param?v1????被除數(shù)??
?????*?@param?v2????除數(shù)??
?????*?@param?scale?小數(shù)點(diǎn)后保留幾位??
?????*?@return?余數(shù)??
?????*/??
????public?static?BigDecimal?remainder(BigDecimal?v1,?BigDecimal?v2,?int?scale)?{??
????????if?(scale?0)?{??
????????????throw?new?IllegalArgumentException(??
????????????????????"The?scale?must?be?a?positive?integer?or?zero");??
????????}??
????????return?v1.remainder(v2).setScale(scale,?BigDecimal.ROUND_HALF_UP);??
????}??
??
????/**??
?????*?比較大小??
?????*??
?????*?@param?v1?被比較數(shù)??
?????*?@param?v2?比較數(shù)??
?????*?@return?如果v1?大于v2?則?返回true?否則false??
?????*/??
????public?static?boolean?compare(String?v1,?String?v2)?{??
????????BigDecimal?b1?=?new?BigDecimal(v1);??
????????BigDecimal?b2?=?new?BigDecimal(v2);??
????????int?bj?=?b1.compareTo(b2);??
????????boolean?res;??
????????if?(bj?>?0)??
????????????res?=?true;??
????????else??
????????????res?=?false;??
????????return?res;??
????}??
}