kotlin: No nullable safety with Java libraries

The Kotlin compiler protects us against type errors, including nullability, within Kotlin code but there are some edges to look out for.

Java library code

Kotlin will not allow a Kotlin method expecting a reference type Foo to be passed a Foo?.

Kotlin will allow a Java method expecting a reference type Foo to be passed a Foo?! (so long as that's a reference type)

// Given kotlin method:
fun doIt(f: Foo) { }

val a: Foo? = null
doIt(a)  // ❌ Will not compile

// Given java.time.Instant library method:
// bool compareTo(Instant) { }

val a: Instant? = null
Instant.now().compareTo(a)  // 😱 Will compile, but throw NullPointerException at runtime

Direction of comparison operator

Kotlin will not allow an operator comparison between Kotlin types Foo and Foo?.

Kotlin will allow an operator comparison between Java types Foo and Foo? but not between Foo? and Foo!

// Given Kotlin type Foo:
val k1: Foo = Foo()
val k2: Foo? = null

k1 < k2  // ❌ Will not compile
k2 < k1  // ❌ Will not compile

// Given Java type Instant:
val j1: Instant = Instant.now()
val j2: Instant? = null

j1 < j2  // 😱 Will compile, but throw NullPointerException at runtime
j2 < j1  // ❌ Will not compile