Mutable and immutable collection classes are the same kotlin

Question

When using reflection to investigate collection classes, mutable and immutable classes for the same type are referentially equal.

Why is this the case?

    @Test
    fun demonstrate_mutableAndImmutableClassesAreTheSame() {
        println("(MutableIterable::class === Iterable::class) = ${(MutableIterable::class === Iterable::class)}")
        println("(MutableCollection::class === Collection::class) = ${(MutableCollection::class === Collection::class)}")
        println("(MutableList::class === List::class) = ${(MutableList::class === List::class)}")
        println("(MutableSet::class === Set::class) = ${(MutableSet::class === Set::class)}")
        println("(MutableMap::class === Map::class) = ${(MutableMap::class === Map::class)}")
        println("(MutableMap.MutableEntry::class === Map.Entry::class) = ${(MutableMap.MutableEntry::class === Map.Entry::class)}")
    }

prints

(Iterable::class === MutableIterable::class) = true
(Collection::class === MutableCollection::class) = true
(List::class === MutableList::class) = true
(Set::class === MutableSet::class) = true

Answer 1

Collection and MutableCollection are mapped to the same java.util.Collection class (mapped-types), so from POV of JVM (after compilation at runtime) that they are the same.

If you decompile the code you've written, you'll get this (I've decompiled it using Kotlin Bytecode generated by Intellij):

Kotlin:

fun demonstrate_mutableAndImmutableClassesAreTheSame() {
    println("(MutableCollection::class === Collection::class) = ${(MutableCollection::class === Collection::class)}")
}

Java:

import kotlin.jvm.internal.Reflection;

// class declaration
public static final void demonstrate_mutableAndImmutableClassesAreTheSame() {
    String var0 = "(MutableCollection::class === Collection::class) = " + (Reflection.getOrCreateKotlinClass(Collection.class) == Reflection.getOrCreateKotlinClass(Collection.class));
    boolean var1 = false;
    System.out.println(var0);
}

References:

• https://kotlinlang.org/docs/reference/java-interop.html#mapped-types
• (imutable collection is mutable) returns true in Kotlin
• Kotlin make constructor of data class accept both List and MutableList but store a mutable instance of them