switch on Class ?

[Tagir Valeev]

Hello!

Here's previous discussion:
http://mail.openjdk.java.net/pipermail/amber-spec-experts/2018-April/000531.html

I still think that switch on class literal is a good idea.

Tagir.


вс, 23 авг. 2020 г., 22:26 Remi Forax <forax at univ-mlv.fr>:

> There is a feature of Pizza (remember Generic Java ++) we have not
> discussed yet,
> being able to do a switch on Class.
>
> public sealed interface Numeric<T extends Numeric<T>>
>   permits Amount, Percentage, Quantity {
>
>   private BigDecimal value() {
>     return switch(this) {
>       case Amount(value) -> value;
>       case Percentage(value) -> value;
>       case Quantity(value) -> value;
>     };
>   }
>
>   private static <T extends Numeric<T>> T fromValue(Class<T> type,
> BigDecimal newValue) {
>     return type.cast(switch(type) {
>       case Amount.class -> new Amount(newValue);
>       case Percentage.class -> new Percentage(newValue);
>       case Quantity.class -> new Quantity(newValue);
>     });
>   }
>
>   default T add(T numeric) { return fromValue(getClass(),
> value().add(numeric.value())); }
> }
>
> with Amount be declared like this
>   record Amount(BigDecimal value) implements Numeric<Amount> { }
>
>
> This kind of switch is interesting because it's also one that can be
> exhaustive, like the switch on type or the switch on Enum.
>
> In the method fromValue, type is typed as a Class<T> so a Class<? extends
> Numeric<...>> and given that Numeric is a sealed class only permitting
> Amount, Percentage and Quantity, the only possible Class for a switch(type)
> are Amount.class, Percentage.class and Quantity.class.
>
> I'm pretty sure the call fromValue(getClass(), ...) doesn't not compile
> because the compiler has no idea that all subtypes of Numeric implements
> Numeric<themselves> but you get the idea.
>
> regards,
> Rémi
>
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