A modern programming language that makes developers happier.

kotlinlang

Hey, can someone explain me why this compiles?
```    fun foo () {
        val map: NavigableMap&lt;String, String&gt; = TreeMap()
        map["key"] = null
    }```

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

Screen Shot 2021-04-07 at 10.45.40 AM.png

Screen Shot 2021-04-07 at 10.44.09 AM.png

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

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>

But that means that I have a follow-up question: Why does this code *not* compile?
```import java.util.*

fun foo () {
    val map = TreeMap&lt;String, String&gt;()
    map["key"] = null
}```

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

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