SimpleDateFormat problems with 2 year date

问题

I'm trying to understand two things:

Why doesn't the following code throw an exception (since the SimpleDateFormat is not lenient)
It doesn't throw an exception, but why is it parsing the year as 0013 (instead of using the rules here the +80:-20 years from today rule)

Here's the code

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class TestDate {
    public static void main(String[] args) throws Exception {
        SimpleDateFormat format = new SimpleDateFormat("dd/MM/yyyy");
        format.setLenient(false);
        Date date = format.parse("01/01/13"); // Since this only has a 2 digit year, I would expect an exception to be thrown

        System.out.println(date); // Prints Sun Jan 01 00:00:00 GMT 13

        Calendar cal = Calendar.getInstance();
        cal.setTime(date);

        System.out.println(cal.get(Calendar.YEAR)); // Prints 13
    }
}

If it makes a difference, I'm using java 1.6.0_38-b05 on Ubuntu

回答1:

SimpleDateFormat API:

For parsing, if the number of pattern letters is more than 2, the year is interpreted literally, regardless of the number of digits. So using the pattern "MM/dd/yyyy", "01/11/12" parses to Jan 11, 12 A.D.

As for lenient, when it's set to false parse throws exception for invalid dates, eg 01/32/12, while in lenient mode this date is treated as 02/01/12. SimpleDateFormat uses Calendar internally, details about leniency can be found in Calendar API.

回答2:

Question 1. This is a partial duplicate of Why Does Java's SimpleDateFormat parse this. The second answer on the question answers this nicely. The crux of it is this:

Number: For parsing, the number of pattern letters is ignored unless it's needed to separate two adjacent fields. Year: During parsing, only strings consisting of exactly two digits […] will be parsed into the default century. Any other numeric string, such as a one digit string, a three or more digit string, or a two digit string that isn't all digits (for example, "-1"), is interpreted literally. So "01/02/3" or "01/02/003" are parsed, using the same pattern

Question 2. Look carefully: your input format is not the same as the format that you are passing to the function.

new SimpleDateFormat("dd/MM/yyyy");

vs.

format.parse("01/01/13");

Either parse to 01/01/2013 or use the date forat 'dd/MM/yy'.

回答3:

According to the SimpleDateFormat javadoc for JDK 1.6,

For parsing, the number of pattern letters is ignored unless it's needed to separate two adjacent fields.

A look at the source code for the method that does the work, SimpleDateFormat.parse(String, ParsePosition), confirms this. There is a variable obeyCount that is true if the pattern has no delimiters, like "yyyyMMdd", and false otherwise. With your pattern the parser looks for 3 numbers separated by 2 delimiters and does not care about the number of digits in each position.

The answers to your questions:

Lenient is not a factor when delimiters are used.
You did not call SimpleDateFormat.set2DigitYearStart so why should the code do what you didn't tell it to do?

来源：https://stackoverflow.com/questions/18127845/simpledateformat-problems-with-2-year-date

标签

java

simpledateformat