How comes kotlin.Char is Serializable? (it is not explicitly defined)

Question

kotlin.Char is defined in the following way:

public class Char private constructor() : Comparable<Char> {
...
}

Where Comparable<T> is defined as:

public interface Comparable<in T> {
    public operator fun compareTo(other: T): Int
}

So how comes a Char is Serializable?

fun main(args: Array<String>) {
  println('A'::class) // class kotlin.Char
  println('A'::class.java) // char
  println('A' is java.io.Serializable) // true
}

• Is it something added on compilation to Byte code?
• Is it documented anywhere?

Answer 1

The basic types such as Int, Double, Char, etc. are represented on the JVM in one of two ways (documentation):

• whenever possible, as primitives (int, double, char),
• when required to be nullable or when used as generic type parameters, as the respective wrapper classes (Integer, Double, Character).

Both of these representations are serializable. Primitive types are serializable by default as they are, and their wrappers all implement Serializable as well, for example, as you can see in the documentation of Character.

This sort of mapping between Kotlin and Java types is also mentioned here in the Java interop documentation.

So the question is, why don't the Kotlin representations have Serializable as a supertype on the source code level? My guess is so that they're kept platform independent, as having them explicitly implement java.io.Serializable would make them depend directly on a JVM type.