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在C中可以使用類(lèi)似printf(“%d %8.2f\n”, 1001, 52.335)的方法實(shí)現(xiàn)格式化輸出，可是Java中的System.out.println()并沒(méi)有對(duì)應(yīng)的功能。要格式化輸出，必須使用java.text包中的類(lèi)來(lái)實(shí)現(xiàn)類(lèi)似的操作（要不怎么體現(xiàn)面向?qū)ο蟮膬?yōu)越性呢，不過(guò)據(jù)說(shuō)jdk1.5準(zhǔn)備又補(bǔ)上）。當(dāng)然了，java.text包的功能還是很強(qiáng)大的，奇怪的是很多書(shū)中都沒(méi)有涉及，而一般誰(shuí)又有工夫整天去看API？
注意：由于這里說(shuō)得很簡(jiǎn)略，因此請(qǐng)參照下面的Demo程序。?
格式化數(shù)字
在NumberFormat類(lèi)中為我們提供了格式化4種數(shù)字的方法：整數(shù)、小數(shù)、貨幣和百分比，通過(guò)工廠(chǎng)方法getNumberInstance, getNumberIntance, getCurrencyInstance, getPercentInstance方法獲得相應(yīng)的實(shí)例對(duì)象就行。例如我們要以字符串表示人民幣88888.88元，這樣來(lái)寫(xiě)就行：
NumberFormat nf = NumberFormat.getCurrencyInstance();
System.out.println(nf.format(88888.88));
定制格式化數(shù)字
可是對(duì)于稍微復(fù)雜一點(diǎn)的需求，NumberFormat就滿(mǎn)足不了了，幸好java還提供了DecimalFormat實(shí)現(xiàn)定制的格式化。要使用DecimalFormat對(duì)象，必須提供給它提供一個(gè)格式化的模式(pattern)：
String pattern = …
DecimalFormat df = new DecimalFormat(pattern);
或者：
DecimalFormat df = new DecimalFormat();
df. applyPattern(pattern);
然后就調(diào)用它的format方法就行了。
所以關(guān)鍵就是這個(gè)模式怎么定義。在DecimalFormat類(lèi)的JavaDoc中有模式的語(yǔ)法表示，不過(guò)很難說(shuō)清楚（是我說(shuō)不清楚，呵呵），請(qǐng)看看Demo自己多試試吧。下面是模式中某些字符的含義表：
字符???????? ?含義
?
0???????????? ?一位數(shù)字，這一位缺失顯示為0。用來(lái)補(bǔ)零
?
#??????????? ?一位數(shù)字, 這一位缺失就不顯示
?
.?????????????? ?小數(shù)點(diǎn)，不用多說(shuō)了吧
?
,?????????????? ?千位分隔符
?
E?????????????? ?科學(xué)計(jì)數(shù)法
?
%????????????? ?百分比
?
格式化日期
把日期轉(zhuǎn)化為字符串最簡(jiǎn)單的方法就是調(diào)用Date類(lèi)的toString或者toLocaleString方法：
System.out.println(new Date());???
輸出：2004-8-7 8:16:14。可是如果我們想把月和日補(bǔ)成2位不要時(shí)分秒2004-08-07，這就不靈了。java.text.DateFormat提供了大量的工廠(chǎng)方法：getDateInstance(int style), getTimeInstance(int style), getDateTimeInstance(int dateStyle, int timeStyle)等等。其中style必須是DateFormat.LONG, DateFormat.MEDIUM, DateFormat.SHORT之一。Demo中的defaultDateFormat方法作了一個(gè)簡(jiǎn)單的實(shí)驗(yàn)。
定制格式化日期：
同樣，java.text.SimpleDateFormat可以通過(guò)模式(pattern)實(shí)現(xiàn)定制格式化：
String pattern = …
SimpleDateFormat df = new SimpleDateFormat(pattern);
或者：
SimpleDateFormat df = new SimpleDateFormat();
df. applyPattern(pattern);
下面是SimpleDateFormat的javadoc中的一個(gè)模式符號(hào)簡(jiǎn)表：
符號(hào)????? 意義??? ?????????????????????? 合法數(shù)值???? 示例
?
y????????? Year??????????????????????????? Year????????? 1996; 96
?
M?????? Month in year??????????????? Month???????? July; Jul; 07
?
d??????? ?Day in month????????????? ?Number???? ?10
?
a??????? ?Am/pm marker??????????? ?Text??????????? ?PM
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H??????? ?Hour in day (0-23)????? ?Number?????? ?0
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h??????? ?Hour in am/pm (1-12)? ?Number????? ?12
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m?????? ?Minute in hour???????????? ?Number?????? ?30
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s??????? ?Second in minute???????? ?Number????? ?55
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S????????? ?Millisecond??????????????? ?Number?????? ?978
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z??????? ?Time zone????????? ?General time zone??? ?Pacific Standard Time; PST; GMT-8:00
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Z???????? ?Time zone????????? ?RFC 822 time zone??-0800
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注意的是，符號(hào)的大小寫(xiě)意義是不同的，符號(hào)的個(gè)數(shù)也會(huì)導(dǎo)致輸出不一樣。例如用MM就會(huì)把1月份顯示成01，而用M則不會(huì)補(bǔ)零。對(duì)于年份，兩個(gè)yy會(huì)只輸出兩位年份，yyyy則會(huì)輸出4位年份。
?實(shí)際上，上面的類(lèi)還提供了很多其他方法，特別是用于本地化(Locale)定制格式化的方法，以及從字符串表示轉(zhuǎn)化為相應(yīng)對(duì)象的parse方法，還有把格式化結(jié)果附加到一個(gè)StringBuffer的方法（應(yīng)該是用來(lái)提高性能）。
?最后是一個(gè)小Demo和輸出結(jié)果：
TestFormat.java：
import java.text.*;
import java.util.*;
?public class TestFormat {
???? public static void main(String[] args) {
?????? defaultNumberFormat();
?????? System.out.println();
???????customNumberFormat();
?????? System.out.println();
???????defaultDateFormat();
?????? System.out.println();
?????? customDateFormat();
?????? System.out.println();
???}
???? public static void defaultNumberFormat() {
?????? int i = 123456;
?????? double x = 882323.23523;
?????? double p = 0.528;
?????? double c = 52.83;
?????? NumberFormat nf = NumberFormat.getInstance();
?????? System.out.println("Integer " + i + " is displayed as " + nf.format(i));
?????? System.out.println("Double " + x + " is displayed as " + nf.format(x));
?????? NumberFormat nfInt = NumberFormat.getIntegerInstance();
?????? System.out.println("Integer " + i + " is displayed as " + nfInt.format(i));
?????? NumberFormat nfNumber = NumberFormat.getNumberInstance();
?????? System.out.println("Double " + x + " is displayed as " + nfNumber.format(x));
?????? NumberFormat nfPercent = NumberFormat.getPercentInstance();
?????? System.out.println("Percent " + p + " is displayed as " + nfPercent.format(p));
?????? NumberFormat nfCurrency = NumberFormat.getCurrencyInstance();
?????? System.out.println("Currency " + p + " is displayed as " + nfCurrency.format(c));??????
?????? //這里沒(méi)有涉及相應(yīng)的parse方法
??? }
??? public static void customNumberFormat() {
?????? double x = 1000.0 / 3;
?????? System.out.println("default output is " + x);
?????? patternPrint("###,###.##", x);
?????? patternPrint("####.##", x);
?????? patternPrint("####.00", x);
?????? patternPrint("####.0#", x);
?????? patternPrint("00000.##", x);
?????? patternPrint("$###,###.##", x);
??????? patternPrint("0.###E0", x);
?????? patternPrint("00.##%", x);
?????? double y = 23.0012;
?????? System.out.println("default output is " + y);
?????? patternPrint("###,###.##", y);
?????? patternPrint("####.##", y);
?????? patternPrint("####.00", y);
?????? patternPrint("####.0#", y);
?????? patternPrint("00000.##", y);
?????? patternPrint("$###,###.##", y);
?????? patternPrint("0.###E0", y);
?????? patternPrint("00.##%", y);
??? }
??? public static void patternPrint(String pattern, double x) {
?????? DecimalFormat df = new DecimalFormat(pattern);
?????? System.out.println("output for pattern " + pattern + " is " + df.format(x));
??? }
??? public static void defaultDateFormat(){
?????? Date date = new Date();
?????? System.out.println("simple date " + date.toLocaleString());
?????? DateFormat df = DateFormat.getDateTimeInstance();
?????? System.out.println(df.format(date));
?????? DateFormat dfLong = DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.LONG);
?????? System.out.println(dfLong.format(date));
?????? DateFormat dfMedium = DateFormat.getDateTimeInstance(DateFormat.MEDIUM,
?????????? DateFormat.MEDIUM);
?????? System.out.println(dfMedium.format(date));
?????? DateFormat dfShort = DateFormat.getDateTimeInstance(DateFormat.SHORT, DateFormat.SHORT);
?????? System.out.println(dfShort.format(date));
??? }
??? public static void customDateFormat() {
?????? Date date = new Date();
?????? patternPrint("yyyy.MM.dd HH:mm:ss z", date);? //兩個(gè)MM, dd會(huì)導(dǎo)致補(bǔ)零
?????? patternPrint("yy年M月d日 HH時(shí)mm分", date);? //兩個(gè)yy就顯示為兩位年份
?????? patternPrint("EEE, MMM d, ''yy", date);
?????? patternPrint("h:mm a", date);
?????? patternPrint("hh 'o''clock' a, zzzz", date);
?????? patternPrint("yyyyy.MMMMM.dd GGG hh:mm aaa", date);
?????? patternPrint("EEE, d MMM yyyy HH:mm:ss Z", date);
?????? patternPrint("yyMMddHHmmssZ", date);
??? }
??? public static void patternPrint(String pattern, Date date){
?????? SimpleDateFormat df = new SimpleDateFormat(pattern);
?????? System.out.println(df.format(date));
??? }
}
輸出：
Integer 123456 is displayed as 123,456
Double 882323.23523 is displayed as 882,323.235
Integer 123456 is displayed as 123,456
Double 882323.23523 is displayed as 882,323.235
Percent 0.528 is displayed as 53%
Currency 0.528 is displayed as ￥52.83
?
default output is 333.3333333333333
output for pattern ###,###.## is 333.33
output for pattern ####.## is 333.33
output for pattern ####.00 is 333.33
output for pattern ####.0# is 333.33
output for pattern 00000.## is 00333.33
output for pattern $###,###.## is $333.33
output for pattern 0.###E0 is 3.333E2
output for pattern 00.##% is 33333.33%
default output is 23.0012
output for pattern ###,###.## is 23
output for pattern ####.## is 23
output for pattern ####.00 is 23.00
output for pattern ####.0# is 23.0
output for pattern 00000.## is 00023
output for pattern $###,###.## is $23
output for pattern 0.###E0 is 2.3E1
output for pattern 00.##% is 2300.12%
?
simple date 2004-8-7 8:16:14
2004-8-7 8:16:14
2004年8月7日 上午08時(shí)16分14秒
2004-8-7 8:16:14
04-8-7 上午8:16
?
2004.08.07 08:16:14 GMT+08:00
04年8月7日 08時(shí)16分
星期六, 八月 7, '04
8:16 上午
08 o'clock 上午, GMT+08:00
02004.八月.07 公元 08:16 上午
星期六, 7 八月 2004 08:16:14 +0800
040807081614+0800
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