在Java Numeric Formatting一文中 ，我描述并演示了NumberFormat靜態(tài)方法提供的一些有用實(shí)例，例如NumberFormat.getNumberInstance（Locale） ， NumberFormat.getPercentInstance（Locale） ， NumberFormat.getCurrencyInstance（Locale）和NumberFormat.getIntegerInstance（Locale） ） 。 事實(shí)證明，所有這些抽象NumberFormat實(shí)例實(shí)際上都是DecimalFormat實(shí)例，它擴(kuò)展了NumberFormat 。

下一個(gè)代碼清單和相關(guān)的輸出展示了NumberFormat的“ getInstance”方法返回的所有實(shí)例實(shí)際上都是DecimalFormat實(shí)例。 區(qū)分同一DecimalFormat類(lèi)的這些實(shí)例的是它們的屬性的設(shè)置，例如最小和最大整數(shù)數(shù)字（小數(shù)點(diǎn)左邊的數(shù)字）以及小數(shù)位數(shù)的最小和最大數(shù)目（小數(shù)點(diǎn)右邊的數(shù)字） 。 它們都共享相同的舍入模式和貨幣設(shè)置。

NumberFormat.getInstance（）提供的實(shí)例是DecimalFormat實(shí)例

/*** Write characteristics of provided Currency object to* standard output.** @param currency Instance of Currency whose attributes* are to be written to standard output.*/ public void printCurrencyCharacteristics(final Currency currency) {out.print("\tCurrency: " + currency.getCurrencyCode() + "(ISO 4217 Code: " + currency.getNumericCode() + "), ");out.println(currency.getSymbol() + ", (" + currency.getDisplayName() + ")"); }/*** Writes characteristics of provided NumberFormat instance* to standard output under a heading that includes the provided* description.** @param numberFormat Instance of NumberFormat whose key* characteristics are to be written to standard output.* @param description Description to be included in standard* output.*/ public void printNumberFormatCharacteristics(final NumberFormat numberFormat, final String description) {out.println(description + ": " + numberFormat.getClass().getCanonicalName());out.println("\tRounding Mode: " + numberFormat.getRoundingMode());out.println("\tMinimum Fraction Digits: " + numberFormat.getMinimumFractionDigits());out.println("\tMaximum Fraction Digits: " + numberFormat.getMaximumFractionDigits());out.println("\tMinimum Integer Digits: " + numberFormat.getMinimumIntegerDigits());out.println("\tMaximum Integer Digits: " + numberFormat.getMaximumIntegerDigits());printCurrencyCharacteristics(numberFormat.getCurrency());if (numberFormat instanceof DecimalFormat){final DecimalFormat decimalFormat = (DecimalFormat) numberFormat;out.println("\tPattern: " + decimalFormat.toPattern());} }/*** Display key characteristics of the "standard"* NumberFormat/DecimalFormat instances returned by the static* NumberFormat methods getIntegerInstance(), getCurrencyInstance(),* getPercentInstance(), and getNumberInstance().*/ public void demonstrateDecimalFormatInstancesFromStaticNumberFormatMethods() {final NumberFormat integerInstance = NumberFormat.getIntegerInstance();printNumberFormatCharacteristics(integerInstance, "IntegerInstance");final NumberFormat currencyInstance = NumberFormat.getCurrencyInstance();printNumberFormatCharacteristics(currencyInstance, "CurrencyInstance");final NumberFormat percentInstance = NumberFormat.getPercentInstance();printNumberFormatCharacteristics(percentInstance, "PercentInstance");final NumberFormat numberInstance = NumberFormat.getNumberInstance();printNumberFormatCharacteristics(numberInstance, "NumberInstance"); }

盡管我的上一篇和到目前為止的文章已經(jīng)演示了通過(guò)靜態(tài)NumberFormat訪問(wèn)方法獲取DecimalFormat實(shí)例，但是DecimalFormat還具有三個(gè)重載的構(gòu)造函數(shù)DecimalFormat（） ， DecimalFormat（String）和DecimalFormat（String，DecimalFormatSymbols） 。 但是請(qǐng)注意， DecimalFormat的Javadoc文檔中有一條警告，指出“通常，請(qǐng)勿直接調(diào)用DecimalFormat構(gòu)造函數(shù)，因?yàn)镹umberFormat工廠方法可能會(huì)返回DecimalFormat以外的子類(lèi)。” 盡管有Javadoc警告，但我接下來(lái)的幾個(gè)示例仍使用其直接構(gòu)造函數(shù)實(shí)例化DecimalFormat實(shí)例，因?yàn)檫@樣做不會(huì)造成任何危害。

DecimalFormat實(shí)例在很大程度上控制十進(jìn)制數(shù)的表示格式。 下面的代碼針對(duì)各種不同的自定義模式運(yùn)行在先前示例中使用的標(biāo)準(zhǔn)數(shù)字集。 代碼清單后的屏幕快照顯示了應(yīng)用這些模式時(shí)如何呈現(xiàn)這些數(shù)字。

/*** Apply provided pattern to DecimalFormat instance and write* output of application of that DecimalFormat instance to* standard output along with the provided description.** @param pattern Pattern to be applied to DecimalFormat instance.* @param description Description of pattern being applied.*/ private void applyPatternToStandardSample(final String pattern, final String description) {final DecimalFormat decimalFormat = new DecimalFormat(pattern);printHeader(description + " - Applying Pattern '" + pattern + "'");for (final double theDouble : ourStandardSample){out.println(theDouble + ": " + decimalFormat.format(theDouble));} }/*** Demonstrate various String-based patters applied to* instances of DecimalFormat.*/ public void demonstrateDecimalFormatPatternStringConstructor() {final String sixFixedDigitsPattern = "000000";applyPatternToStandardSample(sixFixedDigitsPattern, "Six Fixed Digits");final String sixDigitsPattern = "###000";applyPatternToStandardSample(sixDigitsPattern, "Six Digits Leading Zeros Not Displayed");final String percentagePattern = "";applyPatternToStandardSample(percentagePattern, "Percentage");final String millePattern = "\u203000";applyPatternToStandardSample(millePattern, "Mille");final String currencyPattern = "\u00A4";applyPatternToStandardSample(currencyPattern, "Currency");final String internationalCurrencyPattern = "\u00A4";applyPatternToStandardSample(internationalCurrencyPattern, "Double Currency");final String scientificNotationPattern = "0.###E0";applyPatternToStandardSample(scientificNotationPattern, "Scientific Notation"); }================================================================== = Six Fixed Digits - Applying Pattern '000000' ================================================================== NaN: 0.25: 000000 0.4: 000000 0.567: 000001 1.0: 000001 10.0: 000010 100.0: 000100 1000.0: 001000 10000.0: 010000 100000.0: 100000 1000000.0: 1000000 1.0E7: 10000000 Infinity: ∞ ================================================================== = Six Digits Leading Zeros Not Displayed - Applying Pattern '###000' ================================================================== NaN: 0.25: 000 0.4: 000 0.567: 001 1.0: 001 10.0: 010 100.0: 100 1000.0: 1000 10000.0: 10000 100000.0: 100000 1000000.0: 1000000 1.0E7: 10000000 Infinity: ∞ ================================================================== = Percentage - Applying Pattern '' ================================================================== NaN: 0.25: %25 0.4: %40 0.567: %57 1.0: %100 10.0: %1000 100.0: %10000 1000.0: %100000 10000.0: %1000000 100000.0: %10000000 1000000.0: %100000000 1.0E7: %1000000000 Infinity: %∞ ================================================================== = Mille - Applying Pattern '‰00' ================================================================== NaN: 0.25: ‰250 0.4: ‰400 0.567: ‰567 1.0: ‰1000 10.0: ‰10000 100.0: ‰100000 1000.0: ‰1000000 10000.0: ‰10000000 100000.0: ‰100000000 1000000.0: ‰1000000000 1.0E7: ‰10000000000 Infinity: ‰∞ ================================================================== = Currency - Applying Pattern '¤' ================================================================== NaN: 0.25: $0 0.4: $0 0.567: $1 1.0: $1 10.0: $10 100.0: $100 1000.0: $1000 10000.0: $10000 100000.0: $100000 1000000.0: $1000000 1.0E7: $10000000 Infinity: $∞ ================================================================== = Double Currency - Applying Pattern '¤' ================================================================== NaN: 0.25: $0 0.4: $0 0.567: $1 1.0: $1 10.0: $10 100.0: $100 1000.0: $1000 10000.0: $10000 100000.0: $100000 1000000.0: $1000000 1.0E7: $10000000 Infinity: $∞ ================================================================== = Scientific Notation - Applying Pattern '0.###E0' ================================================================== NaN: 0.25: 2.5E-1 0.4: 4E-1 0.567: 5.67E-1 1.0: 1E0 10.0: 1E1 100.0: 1E2 1000.0: 1E3 10000.0: 1E4 100000.0: 1E5 1000000.0: 1E6 1.0E7: 1E7 Infinity: ∞

對(duì)于最后兩個(gè)應(yīng)用DecimalFormat示例，我將通過(guò)使用NumberFormat.getInstance（Locale）的首選方法來(lái)獲取DecimalFormat的實(shí)例。 第一個(gè)代碼清單演示了應(yīng)用于同一double的不同語(yǔ)言環(huán)境，然后演示了每種語(yǔ)言的輸出格式。

/*** Provides an instance of DecimalFormat based on the provided instance* of Locale.** @param locale Locale to be associated with provided instance of* DecimalFormat.* @return Instance of DecimalFormat associated with provided Locale.* @throws ClassCastException Thrown if the object provided to me* by NumberFormat.getCurrencyInstance(Locale) is NOT an instance* of class {@link java.text.DecimalFormat}.*/ private DecimalFormat getDecimalFormatWithSpecifiedLocale(final Locale locale) {final NumberFormat numberFormat = NumberFormat.getCurrencyInstance(locale);if (!(numberFormat instanceof DecimalFormat)){throw new ClassCastException("NumberFormat.getCurrencyInstance(Locale) returned an object of type "+ numberFormat.getClass().getCanonicalName() + " instead of DecimalFormat.");}return (DecimalFormat) numberFormat; }/*** Demonstrate formatting of double with various Locales.*/ public void demonstrateDifferentLocalesCurrencies() {final double monetaryAmount = 14.99;out.println("Locale-specific currency representations of " + monetaryAmount + ":");out.println("\tLocale.US: "+ getDecimalFormatWithSpecifiedLocale(Locale.US).format(monetaryAmount));out.println("\tLocale.UK: "+ getDecimalFormatWithSpecifiedLocale(Locale.UK).format(monetaryAmount));out.println("\tLocale.ENGLISH: "+ getDecimalFormatWithSpecifiedLocale(Locale.ENGLISH).format(monetaryAmount));out.println("\tLocale.JAPAN: "+ getDecimalFormatWithSpecifiedLocale(Locale.JAPAN).format(monetaryAmount));out.println("\tLocale.GERMANY: "+ getDecimalFormatWithSpecifiedLocale(Locale.GERMANY).format(monetaryAmount));out.println("\tLocale.CANADA: "+ getDecimalFormatWithSpecifiedLocale(Locale.CANADA).format(monetaryAmount));out.println("\tLocale.CANADA_FRENCH: "+ getDecimalFormatWithSpecifiedLocale(Locale.CANADA_FRENCH).format(monetaryAmount));out.println("\tLocale.ITALY: "+ getDecimalFormatWithSpecifiedLocale(Locale.ITALY).format(monetaryAmount)); }Locale-specific currency representations of 14.99:Locale.US: $14.99Locale.UK: ￡14.99Locale.ENGLISH: ¤14.99Locale.JAPAN: ￥15Locale.GERMANY: 14,99 €Locale.CANADA: $14.99Locale.CANADA_FRENCH: 14,99 $Locale.ITALY: € 14,99

到目前為止，我的DecimalFormat示例專(zhuān)注于格式化表示的數(shù)字。 最后一個(gè)示例是另一個(gè)方向，并從字符串表示形式解析值。

/*** Demonstrate parsing.*/ public void demonstrateParsing() {final NumberFormat numberFormat = NumberFormat.getCurrencyInstance(Locale.US);final double value = 23.23;final String currencyRepresentation = numberFormat.format(value);out.println("Currency representation of " + value + " is " + currencyRepresentation);try{final Number parsedValue = numberFormat.parse(currencyRepresentation);out.println("Parsed value of currency representation " + currencyRepresentation + " is " + parsedValue);}catch (ParseException parseException){out.println("Exception parsing " + currencyRepresentation + parseException);} }Currency representation of 23.23 is $23.23 Parsed value of currency representation $23.23 is 23.23

所示的最后一個(gè)示例實(shí)際上不需要訪問(wèn)具體的DecimalNumber方法，并且能夠僅使用NumberFormat通告的方法。 本示例使用NumberFormat.format（double）格式化貨幣表示形式，然后解析提供的貨幣表示形式以使用NumberFormat.parse（String）返回原始值。

NumberFormat ，尤其是DoubleFormat ，“格式化和解析任何語(yǔ)言環(huán)境的數(shù)字”。

翻譯自: https://www.javacodegeeks.com/2015/08/java-numeric-formatting-decimalformat.html

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