Android日期格式化

 今天整理一下关于日期格式化的内容，开发中时间总要根据我们的需要去显示不同的格式，我们可以利用java.text.SimpleDateFormat，所谓格式转换，肯定是根据模型转化出我们想要的时间格式，首先我们先看一下模型中各个参数的含义吧，胸API文档中看到这样一个表格

Symbol	Meaning	Presentation	Example
G	era designator	(Text)	AD
y	year	(Number)	1996
M	month in year	(Text & Number)	July & 07
d	day in month	(Number)	10
h	hour in am/pm (1˜12)	(Number)	12
H	hour in day (0˜23)	(Number)	0
m	minute in hour	(Number)	30
s	second in minute	(Number)	55
S	fractional second	(Number)	978
E	day of week	(Text)	Tuesday
D	day in year	(Number)	189
F	day of week in month	(Number)	2 (2nd Wed in July)
w	week in year	(Number)	27
W	week in month	(Number)	2
a	am/pm marker	(Text)	PM
k	hour in day (1˜24)	(Number)	24
K	hour in am/pm (0˜11)	(Number)	0
z	time zone	(Text)	Pacific Standard Time
Z	time zone (RFC 822)	(Number)	-0800
v	time zone (generic)	(Text)	Pacific Time
V	time zone (location)	(Text)	United States (Los Angeles)
'	escape for text	(Delimiter)	'Date='
''	single quote	(Literal)	'o''clock'

简单的理解一下吧，y代表年，M代表月份，d代表天，h代表12进制的小时，H代表24进制的小时，m代表分钟，s代表秒，S代表毫秒，E代表星期，D代表一年中的第几天，各种含义大家可以直接从表格中看到，了解了这些，我们学习日期格式化就轻松很多了。

接下来，我们再看下面这段话

The count of pattern letters determines the format:
(Text): 4 or more pattern letters → use the full form, less than 4 pattern letters → use a short or abbreviated form if one exists.
(Number): the minimum number of digits. Shorter numbers are zero-padded to this amount. Year is handled specially; that is, if the count of 'y' is 2, the year
will be truncated to 2 digits. (if "yyyy" produces "1997", "yy" produces "97".) Unlike other fields, fractional seconds are padded on the right with zero.
(Text & Number): 3 or over, use text, otherwise use number.
Any characters in the pattern that are not in the ranges of ['a'..'z'] and ['A'..'Z'] will be treated as quoted text. For instance, characters like ':', '.', ' ', '#' and '@' will appear in the resulting time
text even they are not embraced within single quotes.
A pattern containing any invalid pattern letter will result in an exception thrown during formatting or parsing.
什么意思，总结起来很简单，就是y代表年，yy代表96年格式，yyyy代表1996年格式。我们用文档中的例子来看，

Format Pattern                       Result
--------------                       -------
"yyyy.MM.dd G 'at' HH:mm:ss vvvv" →  1996.07.10 AD at 15:08:56 Pacific Time
"EEE, MMM d, ''yy"                →  Wed, July 10, '96
"h:mm a"                          →  12:08 PM
"hh 'o''clock' a, zzzz"           →  12 o'clock PM, Pacific Daylight Time
"K:mm a, vvv"                     →  0:00 PM, PT
"yyyyy.MMMMM.dd GGG hh:mm aaa"    →  01996.July.10 AD 12:08 PM

          接下来我们看一下SimpleDateFormat的使用,将Date转换成指定格式的时间字符串，将字符串形式的时间转成Date。

/**
* 默认格式输出
*/
SimpleDateFormat f = new SimpleDateFormat();
String format = f.format(new Date());
Log.i("默认格式", format);
/**
* yyyy-MM-dd HH:mm
*/
SimpleDateFormat f2 = new SimpleDateFormat("yyyy-MM-dd HH:mm");
String format2 = f2.format(new Date());
Log.i("yyyy-MM-dd HH:mm", format2);
/**
* yyyy年MM月dd日 HH时mm分ss秒
*/
SimpleDateFormat f3 = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");
String format3 = f3.format(new Date());
Log.i("yyyy年MM月dd日 HH时mm分ss秒", format3);
/**
* yyyy年MM月dd日 HH时mm分ss秒转成Date形式
*/
SimpleDateFormat f4 = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");
try {
Date date = f4.parse(format3);
Log.i("yyyy年MM月dd日 HH时mm分ss秒转成Date", date.toString());
} catch (ParseException e) {
e.printStackTrace();
}

看一下输出吧。