Handling multiple formats in DateTimeFormatter

Question

I am developing using Java 8 a function that must handle the conversion from String to LocalDateTime of the following dates:

• 2019-06-20 12:18:07.207 +0000 UTC
• 2019-06-20 12:18:07.20 +0000 UTC
• 2019-06-20 12:18:07.2 +0000 UTC
• 2019-06-20 12:18:07 +0000 UTC

The strings are produced from an external library that I cannot change.

Following the suggestions given in the SO answer Optional parts in SimpleDateFormat, I tried using the optional formatting offered by the type DateTimeFormatter, using the characters [ and ]. I tried the following patterns:

• yyyy-MM-dd HH:mm:ss[.S[S[S]]] Z z
• yyyy-MM-dd HH:mm:ss[.S[S][S]] Z z

However, neither of them works.

Any suggestion?

Answer 1

You can build the pattern using DateTimeFormatterBuilder and reuse ISO_LOCAL_DATE and ISO_LOCAL_TIME constants:

    DateTimeFormatter formatter = new DateTimeFormatterBuilder()
            .append(DateTimeFormatter.ISO_LOCAL_DATE)
            .appendLiteral(" ")
            .append(DateTimeFormatter.ISO_LOCAL_TIME)
            .appendPattern("[ Z z]")
            .toFormatter();

    ZonedDateTime dt = ZonedDateTime.parse(date, formatter);

The trick is that DateTimeFormatter.ISO_LOCAL_TIME handles the different number of digit used to represent milliseconds its own. From DateTimeFormatter.ISO_LOCAL_TIME JavaDoc:

This returns an immutable formatter capable of formatting and parsing the ISO-8601 extended local time format. The format consists of:
[..]
One to nine digits for the nano-of-second. As many digits will be output as required.