在NumberFormat類中爲我們提供了格式化4種數字的方法:整數、小數、貨幣和百分比,通過工廠方法getNumberInstance, getNumberIntance, getCurrencyInstance, getPercentInstance方法獲得相應的實例對象就行。例如我們要以字符串表示人民幣88888.88元,這樣來寫就行:
NumberFormat nf = NumberFormat.getCurrencyInstance();
System.out.println(nf.format(88888.88));
定製格式化數字
可是對於稍微複雜一點的需求,NumberFormat就滿足不了了,幸好java還提供了DecimalFormat實現定製的格式化。要使用DecimalFormat對象,必須提供給它提供一個格式化的模式(pattern):
String pattern = …
DecimalFormat df = new DecimalFormat(pattern);
或者:
DecimalFormat df = new DecimalFormat();
df. applyPattern(pattern);
然後就調用它的format方法就行了。
所以關鍵就是這個模式怎麼定義。在DecimalFormat類的JavaDoc中有模式的語法表示,不過很難說清楚(是我說不清楚,呵呵),請看看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 一位數字,這一位缺失顯示爲0。用來補零
# 一位數字, 這一位缺失就不顯示
. 小數點,不用多說了吧
, 千位分隔符
E 科學計數法
% 百分比
格式化日期
把日期轉化爲字符串最簡單的方法就是調用Date類的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成2位不要時分秒2004-08-07,這就不靈了。java.text.DateFormat提供了大量的工廠方法: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方法作了一個簡單的實驗。
定製格式化日期:
同樣,java.text.SimpleDateFormat可以通過模式(pattern)實現定製格式化:
String pattern = …
SimpleDateFormat df = new SimpleDateFormat(pattern);
或者:
SimpleDateFormat df = new SimpleDateFormat();
df. applyPattern(pattern);
下面是SimpleDateFormat的javadoc中的一個模式符號簡表:
符號 意義 合法數值 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
H Hour in day (0-23) Number 0
h Hour in am/pm (1-12) Number 12
m Minute in hour Number 30
s Second in minute Number 55
S Millisecond Number 978
z Time zone General time zone Pacific Standard Time; PST; GMT-8:00
Z Time zone RFC 822 time zone -0800
注意的是,符號的大小寫意義是不同的,符號的個數也會導致輸出不一樣。例如用MM就會把1月份顯示成01,而用M則不會補零。對於年份,兩個yy會只輸出兩位年份,yyyy則會輸出4位年份。
實際上,上面的類還提供了很多其他方法,特別是用於本地化(Locale)定製格式化的方法,以及從字符串表示轉化爲相應對象的parse方法,還有把格式化結果附加到一個StringBuffer的方法(應該是用來提高性能)。
最後是一個小Demo和輸出結果:
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));
//這裏沒有涉及相應的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MM, dd會導致補零
patternPrint("yy年M月d日 HH時mm分", date); //兩個yy就顯示爲兩位年份
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