Hey, can someone explain me why this compiles? ``` fun foo () { val map: NavigableMap<...

# announcements

[thana]

Hey, can someone explain me why this compiles?

fun foo () {
        val map: NavigableMap<String, String> = TreeMap()
        map["key"] = null
    }

[diesieben07]

My guess is: NavigableMap is a Java class and thus the types become flexible

[Casey Brooks]

My guess is because the set operator is inlined to .put, and NavigableMap is a Java interface so that .put uses platform typing with no null-safety. If you replace NavigableMap with MutableMap, it works as you’d expect

[Michael Böiers]

It’s because NavigableMap is a Java class. Kotlin has relaxed null checks for platform types. https://kotlinlang.org/docs/java-interop.html#null-safety-and-platform-types

[Michael Böiers]

But that means that I have a follow-up question: Why does this code not compile?

import java.util.*

fun foo () {
    val map = TreeMap<String, String>()
    map["key"] = null
}

[thana]

Ha funny didn't realize that. good catch!
So i guess that disproved the explanation around platform types?

[Michael Böiers]

No, but apparently Kotlin has some special rules when it infers the type as opposed to when you set it explicitly.