Java DateTimeFormatter 支持单位数的月份和月份
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DateTimeFormatter Support for Single Digit Day of Month and Month of Year
提问by Mark Maxey
DateTimeFormmaterdoesn't seem to handle single digit day of the month:
DateTimeFormmater似乎不能处理一个月中的个位数：
String format = "MM/dd/yyyy";
String date = "5/3/1969";
System.out.println(new SimpleDateFormat(format).parse(date));
System.out.println(LocalDate.parse(date, DateTimeFormatter.ofPattern(format)));
In this example, SimpleDateFormatcorrectly parses the date, but DateTimeFormatterthrows an exception. If I were to use zero padded dates, e.g., "05/03/1969", both work. However, if either the day of month or the month of year are single digit, then DateTimeFormatterthrows an exception.
在本例中，SimpleDateFormat正确解析日期，但DateTimeFormatter抛出异常。如果我要使用零填充日期，例如“05/03/1969”，两者都有效。但是，如果月份中的某一天或年份中的月份是个位数，则DateTimeFormatter抛出异常。
What is the right DateTimeFormatterformat to parse both one and two digit day of month and month of year?
DateTimeFormatter解析月份和月份的一位数和两位数的日期的正确格式是什么？
回答by Holger
From the documentation:
从文档：
Number: If the count of letters is one, then the value is output using the minimum number of digits and without padding.
Number：如果字母数为 1，则使用最小位数输出该值，并且不进行填充。
So the format specifier you want is M/d/yyyy, using single letter forms. Of course, it will still parse date Strings like "12/30/1969"correctly as for these day/month values, two digits are the “minimum number of digits”.
所以你想要的格式说明符是M/d/yyyy，使用单字母形式。当然，它仍然会String像"12/30/1969"这些日/月值一样正确解析日期，两位数是“最小位数”。
The important difference is that MMand ddrequire zero padding, not that Mand dcan't handle values greater than 9 (that would be a bit… unusual).
最重要的区别是，MM和dd需要补零，而不是M和d不能处理的值大于9（这将是一个有点...不寻常的）。
回答by Joginder Malik
In Java 8 Date Time API, I recently used
在 Java 8 Date Time API 中，我最近使用了
DateTimeFormatter formatter = new DateTimeFormatterBuilder()
.appendOptional(DateTimeFormatter.ofPattern("M/dd/yyyy"))
.appendOptional(DateTimeFormatter.ofPattern(("MM/dd/yyyy")))
.toFormatter();
System.out.println(LocalDate.parse("10/22/2020", formatter));
System.out.println(LocalDate.parse("2/21/2020", formatter));
