<i18n dev> RFR: 8276220: Reduce excessive allocations in DateTimeFormatter [v2]

[Stephen Colebourne]

On Mon, 1 Nov 2021 22:35:58 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> The commentary on this line could probably be improved, but this is in a private printer-parser that will only be used for NANO_OF_SECOND and not any arbitrary `TemporalField` (see line 704), thus I fail to see how this assumption can fail (since NANO_OF_SECOND specifies a value range from 0 to 999,999,999).
>> 
>> I considered writing a more generic integral-fraction printer parser that would optimize for any value-range that fits in an int, but seeing how NANO_OF_SECOND is likely the only one used in practice and with a high demand for better efficiency I opted to specialize for it more directly.
>
> I see what you're saying that an arbitrary `Temporal` could define its own fields with its own ranges, but I would consider it a design bug if such an implementation at a whim redefines the value ranges of well-defined constants such as `ChronoField.NANO_OF_SECOND` or `HOUR_OF_DAY`. I'd expect such a `Temporal` would have to define its own enumeration of allowed `TemporalField`s.

That isn't the design model however. The design model for the formatter is a `Map` like view of field to value. Any value may be associated with any field - that is exactly what `Temporal` offers. [`TempralAccessor.getLong()`](https://download.java.net/java/early_access/loom/docs/api/java.base/java/time/temporal/TemporalAccessor.html#getLong(java.time.temporal.TemporalField)) is very explicit about this.

As indicated above, the positive part is that an hour-of-day of 26 can be printed by a user-written `WrappingLocalTime` class. The downside is the inability to make optimizing assumptions as per this code.

FWIW, I had originally intended to write dedicated private formatters where the pattern and type to be formatted are known, such as `LocalDate` and the ISO pattern.

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PR: https://git.openjdk.java.net/jdk/pull/6188