How to compare classes and interfaces?

Question

Can anybody explain me how to compare KClasses and interfaces among themselves? I know how to check if classes or interfaces are equal but I don't understand how to check if A class is a superclass of B class, etc.

interface IB {}
interface IC : IB {}

open class A {}
open class B : A() {}
open class C : B(), IC {}

fun main(args: Array<String>) {
  if (B::class == B::class) { println("B class is equal to B class") }
  if (IB::class == IB::class) { println("IB interface is equal to IB interface") }
    
  if (A::class ??? B::class) { println("A class is parent of B class") }
  if (A::class ??? C::class) { println("A class is superclass of C class") }
    
  if (C::class ??? IC) { println("C class implements IC interface") }
  if (IC ??? IB) { println("IC interface implements IB interface") }
}

Answer 1

Kotlin reflection doesn't have an API for retrieving information about KClass hierarchy, so the only way to check if one KClass is superclass or subclass of another KClass is to compare corresponding java classes:

interface IB {}
interface IC : IB {}

open class A {}
open class B : A() {}
open class C : B(), IC {}

fun main(args: Array<String>) {
  if (B::class == B::class) { println("B class is equal to B class") }
  if (IB::class == IB::class) { println("IB interface is equal to IB interface") }

  if (A::class.java.isAssignableFrom(B::class.java)) { println("A class is parent of B class") }
  if (A::class.java.isAssignableFrom(C::class.java)) { println("A class is superclass of C class") }

  if (IC::class.java.isAssignableFrom(C::class.java)) { println("C class implements IC interface") }
  if (IB::class.java.isAssignableFrom(IC::class.java)) { println("IC interface implements IB interface") }
}

UPDATE: You can also define two extension functions that will make this kind of checks a bit nicer:

inline fun <reified L : Any, reified R : Any> isSubClassOf(): Boolean {
  return R::class.java.isAssignableFrom(L::class.java)
}

inline fun <reified L : Any, reified R : Any> isSuperClassOf(): Boolean {
  return L::class.java.isAssignableFrom(R::class.java)
}

fun main(args: Array<String>) {
  if (isSubClassOf<B, B>()) { println("B class is equal to B class") }
  if (isSubClassOf<IB, IB>()) { println("IB interface is equal to IB interface") }

  if (isSuperClassOf<A, B>()) { println("A class is parent of B class") }
  if (isSuperClassOf<A, C>()) { println("A class is superclass of C class") }

  if (isSubClassOf<C, IC>()) { println("C class implements IC interface") }
  if (isSubClassOf<IC, IB>()) { println("IC interface implements IB interface") }
}