Why is Java 8\'s OffsetDateTime class struggling to parse ISO 8601 strings with offsets like \" 0000\"?

Difficulties Parsing ISO 8601 Strings with Java 8 Date API

When attempting to parse an ISO 8601-formatted string ("2018-02-13T10:20:12.120 0000") using Java 8's ZonedDateTime class and a pre-defined format pattern, users may encounter a parsing error due to a missing colon in the offset.

The Culprit: A Java 8 Bug

This parsing issue stems from a bug in Java 8 that prevents the OffsetDateTime class from correctly parsing offsets without a colon between the hours and minutes. This bug affects offsets like " 0000" but not " 00:00".

Workarounds Until the Bug is Fixed:

(a) String Manipulation Hack:

Alter the input string to add the missing colon before parsing:

<code class="java">String input = "2018-02-13T10:20:12.120+0000".replace("+0000", "+00:00");
OffsetDateTime odt = OffsetDateTime.parse(input);</code>

(b) Explicit DateTimeFormatter:

Define a DateTimeFormatter with a specific pattern to guide the parsing:

<code class="java">String input = "2018-02-13T10:20:12.120+0000";
DateTimeFormatter f = DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ss.SSSX");
OffsetDateTime odt = OffsetDateTime.parse(input, f);</code>

Further Enhancements:

For a more adaptable formatting pattern, utilize a DateTimeFormatterBuilder:

<code class="java">DateTimeFormatter f = DateTimeFormatterBuilder.ofPattern("uuuu-MM-dd'T'HH:mm:ss.SSSX")
                                         .appendOffset("+HH:MM", "Z")
                                         .toFormatter();
OffsetDateTime odt = OffsetDateTime.parse(input, f);</code>

Recommendation:

To simplify parsing, ensure that offsets always include a colon, include both hours and minutes (even if zero), and use padding zeros (-05:00 instead of -5).

Additional Notes:

1. For values in UTC, use Instant:

<code class="java">Instant instant = odt.toInstant();</code>

2. To obtain a specific time zone's wall-clock representation:

<code class="java">ZoneId z = ZoneId.of("America/Montreal");
ZonedDateTime zdt = odt.atZoneSameInstant(z);</code>

3. Avoid legacy date-time classes; use java.time instead.

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