Default value for date/time

[John Rose]

On May 7, 2018, at 5:06 AM, Stephen Colebourne <scolebourne at joda.org> wrote:
> 
> There is an implicit desire in adding value types that classes like
> `LocalDate` can be treated as value types. But one of the requirements
> of value types is that they have a default value. But what should this
> be?
> 
> For `LocalDate`, possible candidates might be
> 
> - LocalDate.of(1970, 1, 1)
> - LocalDate.of(1, 1, 1)
> - LocalDate.of(2000, 1, 1)
> - LocalDate.MIN
> - LocalDate.MAX
> - LocalDate.now()
> 
> But these are all poor choices IMO. Ultimately, dates are not like
> numbers - there isn't a meaningful default like zero.
> 
> I suppose we'll have to use 1970-01-01 as it is traditional in Java,
> but it doesn't really have *meaning*. I'd be interested in hearing if
> there are any other choices or views?
> 
> (I think this problem of no sensible default value may well occur for
> other value types.)
> 
> Stephen

The VM mandates that the all-zero encoding must be accepted
by all value types.  This is a forced move as you probably know.
We have to define what this expression produces:
  LocalDate dflt = (new LocalDate[1])[0];

And we won't allow the value type itself to define an ad hoc default,
because the implementation costs are high and the programmer
convenience is marginal.

I suggest that, for LocalDateTime, the required default (all-zero)
throw exceptions for all component accessors.  Alternatively, the
zeroes could be taken to encode a "dumb" date like 1970-01-01.

— John