Convert date time string like Joda DateTime(String) with Java 8

Question

I have an API that can return date values in JSON in three possible formats:

1. 2017-04-30T00:00+02:00
2. 2016-12-05T04:00
3. 2016-12-05

<

Answer 1

I am presenting two options, each with its pros and cons.

One, build a custom DateTimeFormatter to accept your three possible formats:

public static LocalDateTime parse(String dateFromJson) {
    DateTimeFormatter format = new DateTimeFormatterBuilder().append(DateTimeFormatter.ISO_LOCAL_DATE)
            .optionalStart()
            .appendLiteral('T')
            .append(DateTimeFormatter.ISO_LOCAL_TIME)
            .optionalStart()
            .appendOffsetId()
            .optionalEnd()
            .optionalEnd()
            .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
            .toFormatter();
    return LocalDateTime.parse(dateFromJson, format);
}

On one hand, it’s clean, on the other, someone could easily find it a bit tricky. For the three sample strings in your question it produces:

2017-04-30T00:00
2016-12-05T04:00
2016-12-05T00:00

The other option, try the three different formats in turn and pick the one that works:

public static LocalDateTime parse(String dateFromJson) {
    try {
        return LocalDateTime.parse(dateFromJson);
    } catch (DateTimeParseException e) {
        // ignore, try next format
    }
    try {
        return LocalDateTime.parse(dateFromJson, DateTimeFormatter.ISO_OFFSET_DATE_TIME);
    } catch (DateTimeParseException e) {
        // ignore, try next format
    }
    return LocalDate.parse(dateFromJson).atStartOfDay();
}

I don’t consider this the most beautiful code, still some may think it’s more straightforward than the first option? I think there’s a quality in relying on the built-in ISO formats alone. The results for your three sample strings are the same as above.