Interfaces exposing properties and Kotlin data classes

Question

I've got a data class in Kotlin:

data class Image (val name: String, val url: URL) {}

I want to serve a list of these Images via REST (Spring Boot, JPA), but I just want certain properties (in this example just the names, not the URLs) to be served. From what I have read I need to use a projection - define an interface with just a 'getName' function:

interface HasName {
  fun getName () : String
}

..then set the data class to implement this, and get the RestController to serve HasNames rather than Images:

@GetMapping("/images")
fun getImages () : List <HasName> {
  return imageService.getImages()
}

(or that's what I'd do in Java anyway).

However, if I do this in Kotlin:

data class Image (val name: String, val url: URL) : HasName {}

I get an error:

Accidental override: The following declarations have the same JVM signature (getUrl()Ljava/net/URL;):

public final fun <get-url>(): URL defined in com.figmentsignage.server.data.campaign.Image public abstract fun getUrl(): URL defined in com.figmentsignage.server.data.campaign.Image

Is there an easy way around this? I can give the method a different name and redefine it in the Image class, but that's a bit messy. Or I can define the 'name' variable in the interface and override it in the Image, again messy. I ideally want to change the Image class as little as possible.

Joe Joe · Accepted Answer · 2019-08-06T21:51:33

Here's how I'd define these in kotlin, just using a name property (no need for a getName() method) and being sure to use the override keyword:

interface HasName {
   val name: String
}

data class Image(override val name: String, val url: URL): HasName

Interfaces exposing properties and Kotlin data classes

1 Answers