DateTimeFormatter can parse, but not format for same input

Question

The DateTimeFormatter for pattern "yyyy_'w'w" is unable to format the value it has parsed.

val df = DateTimeFormatter.ofPattern("yyyy_'w'w")
df: DateTimeFormatter = Value(YearOfEra,4,19,EXCEEDS_PAD)'_''w'Localized(WeekOfWeekBasedYear,1)

val week = df.parse("2017_w19")
week: temporal.TemporalAccessor = {Year=2017, WeekOfWeekBasedYear[WeekFields[SUNDAY,1]]=19},ISO

df.format(week)

The error is:

java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: YearOfEra
  java.time.format.Parsed.getLong(Parsed.java:203)
  java.time.format.DateTimePrintContext.getValue(DateTimePrintContext.java:298)
  java.time.format.DateTimeFormatterBuilder$NumberPrinterParser.format(DateTimeFormatterBuilder.java:2540)
  java.time.format.DateTimeFormatterBuilder$CompositePrinterParser.format(DateTimeFormatterBuilder.java:2179)
  java.time.format.DateTimeFormatter.formatTo(DateTimeFormatter.java:1746)
  java.time.format.DateTimeFormatter.format(DateTimeFormatter.java:1720)

Why is this?

What language is that? Scala? (Not that it really matters for your question, it's just tricky for a Java engineer to read.) If you don't get a useful answer, you might translate it to Java to reach a broader audience. — T.J. Crowder
– T.J. Crowder, Commented Aug 17, 2017 at 7:33

user7605325 · Accepted Answer · 2017-08-17 12:08:05Z

The pattern yyyy represents the year-of-era field. But according to javadoc, there's also the pattern uuuu to represent the year field (read those links to see the small difference between them - although for current dates there's not much difference).

The problem is: when you create a formatter with y, it uses the year-of-era field, as you can see by the value:

Value(YearOfEra,4,19,EXCEEDS_PAD)

But when parsing, the resulting parsed object (in your case, the week variable) is created with the year field - as you can see by the value:

{Year=2017, ...

The formatter is set with the year-of-era field. So when you try to format week, it tries to get this field from the week variable. As this field doesn't exist (week contains only year, but not year-of-era), it throws the UnsupportedTemporalTypeException.

The solution is to use the year field (the u pattern) in the formatter:

val df = DateTimeFormatter.ofPattern("uuuu_'w'w")
println(df)
val week = df.parse("2017_w19")
println(week)
println(df.format(week))

The output will be:

Value(Year,4,19,EXCEEDS_PAD)'_''w'Localized(WeekOfWeekBasedYear,1)
{Year=2017, WeekOfWeekBasedYear[WeekFields[SUNDAY,1]]=19},ISO
2017_w19

Note that now the formatter was created with the year field, and the format method now tries to get this field from the parsed object, and no exception is thrown.

Dynamite · Accepted Answer · 2017-08-17 07:48:30Z

0

For me, you can't use a DateTimeFormatter to format a TemporalAccessor which is a too broad interface for time. Try to cast it to a DateTime object first.

answered Aug 17, 2017 at 7:48

Dynamite

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