Issues with pattern variables inside Enum

[Remi Forax]

----- Original Message -----
> From: "Gavin Bierman" <gavin.bierman at oracle.com>
> To: "Francois Green" <francois.green at gmail.com>
> Cc: "amber-dev" <amber-dev at openjdk.java.net>
> Sent: Mercredi 13 Octobre 2021 15:09:49
> Subject: Re: Issues with pattern variables inside Enum

> 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.

Hi Francois
A way to see the issue is to declare the enum constants in all caps

enum Suit {
  HEART("♡"), CLUB("♣"), SPADE("♠"), DIAMOND("♢")
}

with this renaming, the switch you are trying to compile becomes

public String color() {
  return switch(this) {
    case CLUB c -> "Red";
    case SPADE s -> "Black";
    case DIAMOND d -> "Red";
    case HEART h -> "Red";
  };
}

As Gavin said CLUB is not a type, it's a constant.


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