Parsing date and AM/PM using java 8 DateTime API

Question

I'm trying to parse a date time string using java 8 DateTime API.

The string to parse contains date, month, year and AM/PM marker such as 17/02/2015 PM. I use the following pattern: dd/MM/yyyy aa and I expect the parsed LocalDateTime time part to be set at 12:00:00 or 00:00:00 depending on the AM/PM marker.

Using the former java.text.SimpleDateFormat API, the parsing works as expected using this pattern.

But when i use java 8 DateTime API and run the following code:

LocalDateTime.parse("17/02/2015 PM", DateTimeFormatter.ofPattern("dd/MM/yyyy aa"));

I got the following exception:

Exception in thread "main" java.lang.IllegalArgumentException: Too many pattern letters: a
at java.time.format.DateTimeFormatterBuilder.parseField(DateTimeFormatterBuilder.java:1765)
at java.time.format.DateTimeFormatterBuilder.parsePattern(DateTimeFormatterBuilder.java:1604)
at java.time.format.DateTimeFormatterBuilder.appendPattern(DateTimeFormatterBuilder.java:1572)
at java.time.format.DateTimeFormatter.ofPattern(DateTimeFormatter.java:534)

If i switch the pattern to dd/MM/yyyy a then i got the folowing exception:

Exception in thread "main" java.time.format.DateTimeParseException: Text '17/02/2015 PM' could not be parsed: Unable to obtain LocalDateTime from TemporalAccessor: {AmPmOfDay=1},ISO resolved to 2015-02-17 of type java.time.format.Parsed
    at java.time.format.DateTimeFormatter.createError(DateTimeFormatter.java:1918)
    at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1853)
    at java.time.LocalDateTime.parse(LocalDateTime.java:492)
Caused by: java.time.DateTimeException: Unable to obtain LocalDateTime from TemporalAccessor: {AmPmOfDay=1},ISO resolved to 2015-02-17 of type java.time.format.Parsed
    at java.time.LocalDateTime.from(LocalDateTime.java:461)
    at java.time.LocalDateTime$$Lambda$7/474675244.queryFrom(Unknown Source)
    at java.time.format.Parsed.query(Parsed.java:226)
    at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1849)
    ... 2 more
Caused by: java.time.DateTimeException: Unable to obtain LocalTime from TemporalAccessor: {AmPmOfDay=1},ISO resolved to 2015-02-17 of type java.time.format.Parsed
    at java.time.LocalTime.from(LocalTime.java:409)
    at java.time.LocalDateTime.from(LocalDateTime.java:457)
    ... 5 more

It's also weird that when i do the reverse operation, format, it works fine:

 System.out.println(LocalDateTime.of(2015, 02, 17, 0, 0, 0).format(DateTimeFormatter.ofPattern("dd/MM/yyyy a")));
 System.out.println(LocalDateTime.of(2015, 02, 17, 12, 0, 0).format(DateTimeFormatter.ofPattern("dd/MM/yyyy a")));

respectively prints 17/02/2015 AM and 17/02/2015 PM.

The javadoc for java.time.DateTimeFormatter says (§Resolving, step #6):

6. A LocalTime is formed if there is at least an hour-of-day available. This involves providing default values for minute, second and fraction of second.

What i understand is that the field hour-of-day is mandatory to parse a LocalDateTime... Isn't there any way to parse the time part only using the AM/PM field only?

Answer 1

You can set the default values for unavailable fields via DateTimeFormatterBuilder.parseDefaulting:

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
    .appendPattern("dd/MM/yyyy a")
    .parseDefaulting(ChronoField.HOUR_OF_AMPM, 0) // this is required
    .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0) // optional, but you can set other value
    .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0) // optional as well
    .toFormatter();
System.out.println(LocalDateTime.parse("17/02/2015 PM", formatter)); // 2015-02-17T12:00
System.out.println(LocalDateTime.parse("17/02/2015 AM", formatter)); // 2015-02-17T00:00