override and implement fn from class in interface - kotlin

Question

I want to override toString() in an interface and have objects that implement that interface to default to using that method (eg: not shadowed)
interface SingletonObjectTrait {
fun toString(): String = this.javaClass.simpleName
}
Is there a straightforward way to define such an interface, preferably with minimal configuration at implementation
object MyEvent: SomeEventLogic(), SomeEventType, SingletonObjectTrait
class SomeEventLogic {}
interface SomeEventType {}

Answer 1

That's not possible, I'm afraid.
Method implementations in interfaces work much like default methods in Java: they're used only if the implementing class doesn't have an implementation already.  But every class already inherits toString() from Any, so the default would never be used.
In fact, the compiler has a specific error for this — if you try to implement toString() in an interface, it says:
An interface may not implement a method of 'Any'
I can't see a good way around this.
As Erik says, one option is to change the interface to an abstract class; but of course that's not viable if any implementations already extend another class.
Another option might be to implement a different method in the interface, and in the comments instruct implementing classes to override toString() and call that method.  Not automatic, but less work for implementers, and less repetition.

Answer 2

There isn't a great way to do this other than using maybe an annotation processor to add the missing override at compile time (by adding an annotation to the interface that you detect and you generate the overrides in the implementation class). This would work, but may be biting off more than you want to, and is likely out of scope of an answer here on Stack Overflow (seek "how to write an annotation processor for Java or Kotlin" and "KAPT" for the one engine that supports Kotlin processors).
Back to your code and why it will not work as-is:
An interface cannot have a method with a signature that matches any of the methods in Any class. And if you try to override one of them you will get a compilation error.
An interface may not implement a method of 'Any'
The minimal code to do something like you want is:
interface SingletonObjectTrait {
fun asString(): String = this.javaClass.simpleName
}
open class SomeEventLogic {}
interface SomeEventType {}
object MyEvent: SomeEventLogic(), SomeEventType, SingletonObjectTrait {
override fun toString(): String = asString() // hope they don't forget to call this!
}
There is no guarantee the implementer will call the trait asString() function but at least you can share the functionality with a one-liner in the implementing class.