Hi all, is there a way to write a data class that ...

# getting-started

[Philip Graf]

Hi all, is there a way to write a data class that compiles to the same as the following Java class?

public class Pet {

    private List<String> names;

    public @NonNull List<@NonNull String> getNames() {
        return names;
    }

    // constructor, setter, etc. omitted
}

I tried to write the following data class:

data class Pet(val names: @get:NonNull List<@get:NonNull String>)

For each of the annotations I get two compile errors:
- This annotation is not applicable to target 'undefined target' and use site target '@get'
- This annotation is not applicable to target 'type usage' and use site target '@get'

The NonNull annotation is from the Java library microprofile-graphql-api

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER})
@Documented
public @interface NonNull {
}

I run into the same problem when I try to set the annotations on the field:

data class Pet(val names: @field:NonNull List<@field:NonNull String>)

Is there a way to get annotations on the type and generic type of a getter method or field?

[Sam]

data class Pet(@get:NonNull val names: List<@NonNull String>)

• The first annotation is for the property getter (AnnotationTarget.PROPERTY_GETTER). It goes before the property declaration, but needs the get: use-site target otherwise it would annotate the constructor parameter by default.
• The second annotation is for the String type parameter use. (In Kotlin, AnnotationTarget.TYPE, in Java, ElementType.TYPE_USE). There’s no use-site target for this because it’s the only valid target for an annotation in that position.

[Sam]

Actually looking at your first example, I think both of your annotations are on the type usages. I had thought one of them was on the getter method. So maybe what you want is closer to:

data class Pet(val names: @NonNull List<@NonNull String>)

I’m a bit fuzzy on what the difference would be in the resulting Java code, to be honest. I’ve been spoiled by writing Kotlin for too long 😄

[Sam]

tl;dr: to annotate a type use, you just use the annotation without any prefix (like get:).

[Philip Graf]

Thanks Sam, but I need the annotations on the type (List) and type parameter (String), otherwise the Java library won't do the right things.

data class Pet(val names: @NonNull List<@NonNull String>)

The above snippet will not put any annotation on the method, its type or type parameter. It will just compile them to @kotlin.Metadata where they are not visible to a Java library that is not aware of Kotlin.

The Java code above compiles to

public getNames()Ljava/util/List;
  @Lorg/eclipse/microprofile/graphql/NonNull;()
  @Lorg/eclipse/microprofile/graphql/NonNull;() : METHOD_RETURN, 0;
  @Lorg/eclipse/microprofile/graphql/NonNull;() : METHOD_RETURN, null

and I'm looking for a Kotlin data class that compiles to the same bytecode.

[Philip Graf]

data class Pet(@get:NonNull val names: List<@NonNull String>)

compiles to

public final getNames()Ljava/util/List;
 @Lorg/eclipse/microprofile/graphql/NonNull;()

i.e. it puts the annotation on the method but not the type or type parameter.

[ephemient]

https://youtrack.jetbrains.com/issue/KT-13228/Generate-annotations-on-types-in-JVM-8-target-mode