RFR: Generate Java enums

[Maurizio Cimadamore]

On Fri, 20 Jun 2025 17:09:19 GMT, Vivek Narang <duke at openjdk.org> wrote:

> We are using jextract in our [cuVS Java](https://github.com/rapidsai/cuvs/tree/branch-25.08/java) project. As of now, jextract generates individual getter functions for the C enums. This results in code where it is difficult to disambiguate which getter functions are logically grouped together for a particular Enum. Additionally, given the lack of this capability, we also have to manually create and maintain Java enums for each enum in the C layer.
> 
> In this PR, I wish to introduce an option to additionally generate Java enums by passing an optional flag `--generate-java-enums`. In case a Java user wishes to use bitwise operations on the enums, they can use the enum's `.ordinal()` method for this. The original static getter methods will still be generated as before. 
> 
> Example C enum:
> 
> enum SIZE {
>    S,
>    M,
>    L
> };
> 
> Produces the following with jextract:
> 
> private static final int S = (int)0L;
> public static int S() {
>      return S;
> }
> 
> private static final int M = (int)1L;
> public static int M() {
>      return M;
> }
> 
> private static final int L = (int)2L;
> public static int L() {
>      return L;
> }
> 
> With this feature enabled using the `--generate-java-enums` flag, the following enum is generated (in addition to the above):
> 
> public enum Size {
>     S(0),
>     M(1),
>     L(2);
> 
>     private final int value;
> 
>     private Size(int value) {;
>         this.value = value;
>     }
> 
>     public int getValue() {
>         return this.value;
>     }
> }
> 
> This PR is created against the `jdk22` branch. I will rebase it to `master` branch if needed.

Another thing that came to mind: if we translate C enums as Java enums -- how is a native function accepting a C enum type translated? If we translate it as an int-accepting function (what we do now) then the Java enum constants cannot be used to call into native functions. If we translate it as a Java-enum-accepting function, then we would make these function stricter than their C counterparts.

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PR Comment: https://git.openjdk.org/jextract/pull/284#issuecomment-3031606147