Issues with pattern variables inside Enum

[Gavin Bierman]

Hi Francois,

> On 6 Oct 2021, at 00:04, Francois Green <francois.green at gmail.com> wrote:
> 
> I don't know if it should, but this works:
> 
> public class PatNum {
> 
>    public static void main(String[] args) {
> 
>        enum Suit {
>            Heart("♡"), Club("♣"), Spade("♠"), Diamond("♢");
> 
>            String symbol;
> 
>            Suit(String symbol) {
>                this.symbol = symbol;
>            }
> 
>            public String color() {
>                return switch(this) {
>                    case Club, Spade -> "Black";
>                    case Diamond, Heart -> "Red";
>                };
>            }
>        }
> 
>        for (var suit : Suit.values()) System.out.println(suit.color());
>    }
> }
> 

Yes, that is correct - you are using enum constants as switch labels in a switch expression. All good.

> 
> 
> Trying to use a pattern variable causes an error:
> 
> public class PatNum {
> 
>    public static void main(String[] args) {
> 
>        enum Suit {
>            Heart("♡"), Club("♣"), Spade("♠"), Diamond("♢");
> 
>            String symbol;
> 
>            Suit(String symbol) {
>                this.symbol = symbol;
>            }
> 
>            public String color() {
>                return switch(this) {
>                    case Club c -> "Red";
>                    case Spade s -> "Black";
>                    case Diamond d -> "Red";
>                    case Heart h -> "Red";
>                };
>            }
>        }
> 
>        for (var suit : Suit.values()) System.out.println(suit.color());
>    }
> }
> 
> PatNum.java:16: error: cannot find symbol
>                    case Club c -> "Red";
>                         ^
>  symbol:   class Club
>  location: class Suit
> PatNum.java:17: error: cannot find symbol
>                    case Spade s -> "Black";
>                         ^
>  symbol:   class Spade
>  location: class Suit
> PatNum.java:18: error: cannot find symbol
>                    case Diamond d -> "Red";
>                         ^
>  symbol:   class Diamond
>  location: class Suit
> PatNum.java:19: error: cannot find symbol
>                    case Heart h -> "Red";
>                         ^
>  symbol:   class Heart
>  location: class Suit
> Note: PatNum.java uses preview features of Java SE 17.
> Note: Recompile with -Xlint:preview for details.
> 4 errors
> error: compilation failed

I’m afraid that *is* an error. This preview feature only allows type patterns to appear as switch labels. You have written, e.g. Club c, but Club is not a type, it’s an enum constant; so the compiler correctly complains. 

Gavin