Case default

[Thiago Henrique Hupner]

Hi Brian and Gavin!

Thank you all for the directions,
I had totally forgotten about using Object as a pattern, that makes sense.


Em ter., 15 de jun. de 2021 às 14:25, Gavin Bierman <
gavin.bierman at oracle.com> escreveu:

> Hi
>
> > On 15 Jun 2021, at 15:16, Thiago Henrique Hupner <thihup at gmail.com>
> wrote:
> >
> > Hi.
> >
> > Trying out the JEP 406: Pattern Matching for switch and I've found that
> the
> > compiler
> > is accepting "case default" instead of only "default".
> > However, I couldn't find which JEP added this support.
>
> It’s in that JEP. It’s a little hidden, but it’s in the section "4b. New
> label forms arising from null labels”.
>
>
> > Another thing that came up when migrating some existing code to pattern
> > matching for switch
> > was the ability to "match" a default value. An example of what I tried to
> > do was changing
> > this:
> >
> > private static Long coerceToLong(Object value) {
> >        value = convertToNumber(value);
> >        if (value instanceof Long newValue) {
> >            return newValue;
> >        }
> >        if (value instanceof Number number) {
> >            return number.longValue();
> >        }
> >        if (value instanceof String newValue) {
> >            return Long.valueOf(newValue);
> >        }
> >        throw new Exception(CANNOT_CONVERT_TO.formatted(value, "Long"));
> >    }
> >
> > To this:
> >
> > private static Long coerceToLong(Object value) {
> >        return switch(convertToNumber(value)) {
> >            case Long asLong -> asLong;
> >            case Number asNumber -> asNumber.longValue();
> >            case String asString -> Long.valueOf(asString);
> >            case default asValue -> throw new
> > Exception(CANNOT_CONVERT_TO.formatted(asValue, "Long"));
> >        };
> >    }
> >
> > However, as the default case doesn't allow a pattern variable, I had to
> do
> > the following
> >
> >    private static Long coerceToLong(Object value) {
> >        value = convertToNumber(value);
> >        return switch(value) {
> >            case Long asLong -> asLong;
> >            case Number asNumber -> asNumber.longValue();
> >            case String asString -> Long.valueOf(asString);
> >            case default -> throw new
> > ELException(CANNOT_CONVERT_TO.formatted(value, "Long"));
> >        };
> >    }
> >
> > This is OK, but as long as the switch allows "case default", it gives the
> > impression that
> > the value is available to extract too.
>
> No. The case default possibility is there because you may want to combine
> it with the new case null label, e.g.
>
> case null, default -> // matches null and any other value
>
> A case default by itself is allowed (why would we disallow it?) but a
> default switch label may be more readable.
>
> If you want a pattern variable then you should write a pattern!
>
> Gavin
>
>
>