`````````````````yyyy```` [英] `uuuu` versus `yyyy` in `DateTimeFormatter` formatting pattern codes in Java?

问题描述

DateTimeFormatter 类文档说明了其年度的格式代码：

你2004年的一年04

y年份2004年; 04

...

年份：字母数确定最小字段宽度低于该填充使用。如果字母数为2，则使用缩减的两位数形式。对于打印，这将输出最右边的两位数字。对于解析，这将使用基数值2000解析，导致一年在2000到2099之间的范围内。如果字母数小于四（但不是两个），则符号只能按照SignStyle.NORMAL输出为负数。否则，如果超出了垫宽度，则按照SignStyle.EXCEEDS_PAD的方式输出符号。

没有其他提到时代。

那么这两个代码有什么区别， u 对 y ，年与年龄？

应该怎样使用这种模式 uuuu-MM-dd ，而当 yyyy-MM-dd 在Java中使用日期的时候？

看起来像那些知道使用 uuuu 的人编写的示例代码，但为什么？

其他格式化类，如旧版 SimpleDateFormat 只有 yyyy ，所以我很困惑为什么java.time为年代带来这个 uuuu

解决方案

在javadoc部分中格式化模式和解析 DateTimeFormatter 它列出以下3个相关符号：

 符号含义演示文稿示例
 ------ ------- ------------ ------- 
 G时代文字AD;公元; 2004年A 
 04年
年2004年; 04 
  

只是为了比较，这些其他符号很容易理解：

  D每年的日数189 
d每月的日数10 
 E day-本周文字;星期二; T 
  

日期 日期和星期几显然是 给定的范围（年，月，周）。

所以，年龄意味着年份（在时间范围内），右上角时代的示例值为 AD （另一个值当然是 BC ）。

year 是签署的年份，年份 0 是 1 BC 年 -1 是 2 BC 等等。

说明：什么时候凯撒被杀暗杀？

• 公元前44年3月15日（使用模式 MMMM d，y GG ）


• 3月15日-43    （使用模式 MMMM d，u ）

当然，只要年份为零或为负，而且由于这是罕见的，大多数人都不关心，即使他们应该。

结论： strong>如果您使用 y ，您还应该使用 G 。由于 G 很少使用，所以正确的年份符号是 u ，而不是 y ，否则非正年将显示不正确。

这被称为防御性编程：

防御性编程是一种防御性设计，

---

$ b
$ b

请注意， DateTimeFormatter 与 SimpleDateFormat ：

 信函日期或时间组件演示示例
 ------ ---------------- ------------------ -------- 
 G Era指示符文字AD 
 y年份1996; 96 
  

负面的年份一直是一个问题，他们现在通过添加 u 。

The DateTimeFormatter class documentation says about its formatting codes for the year:

u year year 2004; 04

y year-of-era year 2004; 04

…

Year: The count of letters determines the minimum field width below which padding is used. If the count of letters is two, then a reduced two digit form is used. For printing, this outputs the rightmost two digits. For parsing, this will parse using the base value of 2000, resulting in a year within the range 2000 to 2099 inclusive. If the count of letters is less than four (but not two), then the sign is only output for negative years as per SignStyle.NORMAL. Otherwise, the sign is output if the pad width is exceeded, as per SignStyle.EXCEEDS_PAD.

No other mention of "era".

So what is the difference between these two codes, u versus y, year versus year-of-era?

When should I use something like this pattern uuuu-MM-dd and when yyyy-MM-dd when working with dates in Java?

Seems that example code written by those in the know use uuuu, but why?

Other formatting classes such as the legacy SimpleDateFormat have only yyyy, so I am confused why java.time brings this uuuu for "year of era".

解决方案

In the javadoc section Patterns for Formatting and Parsing for DateTimeFormatter it lists the following 3 relevant symbols:

Symbol  Meaning        Presentation  Examples
------  -------        ------------  -------
 G       era            text          AD; Anno Domini; A
 u       year           year          2004; 04
 y       year-of-era    year          2004; 04

Just for comparison, these other symbols are easy enough to understand:

 D       day-of-year    number        189
 d       day-of-month   number        10
 E       day-of-week    text          Tue; Tuesday; T

The day-of-year, day-of-month, and day-of-week are obviously the day within the given scope (year, month, week).

So, year-of-era means the year within the given scope (era), and right above it era is shown with an example value of AD (the other value of course being BC).

year is the signed year, where year 0 is 1 BC, year -1 is 2 BC, and so forth.

To illustrate: When was Julius Caesar assassinated?

• March 15, 44 BC (using pattern MMMM d, y GG)
• March 15, -43      (using pattern MMMM d, u)

The distinction will of course only matter if year is zero or negative, and since that is rare, most people don't care, even though they should.

Conclusion: If you use y you should also use G. Since G is rarely used, the correct year symbol is u, not y, otherwise a non-positive year will show incorrectly.

This is known as defensive programming:

Defensive programming is a form of defensive design intended to ensure the continuing function of a piece of software under unforeseen circumstances.

---

Note that DateTimeFormatter is consistent with SimpleDateFormat:

Letter  Date or Time Component  Presentation  Examples
------  ----------------------  ------------  --------
G       Era designator          Text          AD
y       Year                    Year          1996; 96

Negative years has always been a problem, and they now fixed it by adding u.

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