Java8 appendPattern vs pattern defined by appendValue methods produces different result

Question

I have the pattern "ddMMyy" in my code I have specified it using the appendValue methods:

DateTimeFormatter dateTimeFormatter = new DateTimeFormatterBuilder()
                    .appendValue(ChronoField.DAY_OF_MONTH, 2)
                    .appendValue(ChronoField.MONTH_OF_YEAR, 2)
                    .appendValue(ChronoField.YEAR_OF_ERA, 2)
                    .toFormatter();
System.out.println(LocalDate.parse("100199", dateTimeFormatter));

However this produces "0099" for year:

0099-01-10

If I change that to using the appendPattern like that:

DateTimeFormatter dateTimeFormatter = new DateTimeFormatterBuilder()
                    .appendPattern("ddMMyy")
                    .toFormatter();
System.out.println(LocalDate.parse("100199", dateTimeFormatter));

I have the correct result for year "2099" with century in it.

2099-01-10

The code seems equivalent for me why isn't it producing the same result? Why is the century missing in the first case?

Answer 1

Because appendValue takes the year as it is being passed without further manipulation - in your case 99.

If you want to start from a "base year", say 2000, and add the value to that base year (to get 2099) you can use appendValueReduced instead:

DateTimeFormatter dateTimeFormatter = new DateTimeFormatterBuilder()
        .appendValue(ChronoField.DAY_OF_MONTH, 2)
        .appendValue(ChronoField.MONTH_OF_YEAR, 2)
        .appendValueReduced(ChronoField.YEAR_OF_ERA, 2, 2, LocalDate.of(2000, 1, 1))
        .toFormatter();

When you use the yy pattern, you get that behaviour by default as detailed in the javadoc:

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. [...]