Forcing Java 8 LocalTime toString to report omitted values

Question

I have the following datetime helper method that converts a UTC-zoned Java 8 Date into a datetime string:

public static String dateTimeString(Date date) {
    return date.toInstant().atZone(ZoneId.of("UTC")).toLocalDateTime().toString();
}

The desired result is to always have the resultant String be formatted as:

YYYY-MM-dd'T'HH:mm:ss'Z'

Problem is, Java 8 LocalTime#toString() intentionally strips off time components that are zero. So for instance if I have a Date instance that represents June 8, 2018 at 12:35:00 UTC. Then the output of this method above is: 2018-06-08'T'12:35'Z'. Whereas I want it to contain any zeroed-out second/minute/hour components (e.g. 2018-06-08'T'12:35:00'Z').

Any ideas?

Answer 1

private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ssX");

public static String dateTimeString(Date date) {
    return date.toInstant().atOffset(ZoneOffset.UTC).format(formatter);
}

Just use a fixed format pattern string to get your desired format. Let’s try it:

    System.out.println(dateTimeString(new Date(0)));
    System.out.println(dateTimeString(new Date(1_524_560_255_555L)));

This prints:

1970-01-01T00:00:00Z
2018-04-24T08:57:35Z

• In the first example hours, minutes and seconds are printed even if they are 0.
• In the second example milliseoncds are omitted even when they are non-zero (you see that the milliseconds value I specified ends in 555).

All of this said, the output conforms to the ISO 8601 format no matter if you have 2018-06-08T12:35Z, 2018-06-08T12:35:00Z or even 2018-06-08T12:35:00.000000000Z. So you may want to check once more whether leaving out the second works for your purpose before you take the trouble of defining your own formatter.

Link: Wikipedia article: ISO 8601

Answer 2

I don't see why you portray the default output to have its letters enclosed in single quotes when Java's actual output instead looks like 2018-06-08T12:35Z, but anyway, here's the code that produces it as desired, with no omissions:

final LocalDateTime ldt = LocalDateTime.of(2018, 6, 8, 12, 35, 0);

final ZonedDateTime zdt = ZonedDateTime.of(ldt, ZoneId.of("Z"));

final DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd'''T'''HH:mm:ss''X''", Locale.US);

System.err.println(dtf.format(zdt));

Output:

2018-06-08'T'12:35:00'Z'

Personally, I'd probably prefer a format like this, containing the millis, giving time zone information not requiring additional knowledge from the user, and not having imho superfluous characters:

FORMAT: "yyyy-MM-dd HH:mm:ss'.'SSS Z"

OUTPUT: 2018-06-08 12:35:12.345 +0200

Answer 3

It appears that there is no out of the box solution. The simplest way would be to write your own class that extends DateTimeFormatter and override method public String format(TemporalAccessor temporal) where you would call the original method and then if needed modify the output String. See the flags 'Z' for formatter. If that flag is present then you will need to modify the original output.