Interoperability java-kotlin Nullable string crashing at runtime but why?

Question

I have an alternative to my problem, but I still have 2 questions:

Why it's crashing
Why Intellij/compiler is not complaining

I provided a small example of my issue. Here is an overview (you can find the code just after)

Car.java is the java class with a simple getter annoted as nulllable
MyView.kt is the crashing at runtime example with no warning in Intellij or in gradle. (Crashing when the value is null)
MyView2.kt is not crashing at runtime (even if mType is null)
Library.kt (kotlin stdlib) is simply the expected call for the example (even if it's weird to see string.toString())

Car.java

    @Nullable
    private String mType;

    @Nullable
    public String getCarType() {
        return mType;
    }

MyView.kt (crashing)

    val test: String = myCar.carType.toString()

MyView2.kt (not crashing)

    val carType: String? = myCar.carType
    val test2: String = carType.toString()

Library.kt (kotlin stdlib)

    /**
     * Returns a string representation of the object. Can be called with a null receiver, in which case
     * it returns the string "null".
     */
    public fun Any?.toString(): String

Thanks! I guess this is a corner case of the interop between kotlin and java? Or... someone is aware of a better explanation?

Can you show the stacktrace? Which nullability annotations are you using? Can you show the imports you're using at the top of your Car.java file? My first guess is that you're using a nullability annotation that Kotlin's compiler doesn't recognize. — Tenfour04
– Tenfour04, Commented Sep 23, 2022 at 15:11

AndrewL · Accepted Answer · 2022-09-23 17:49:52Z

1

The clue to this difference is the optional marker ? in this line in MyView2.kt

val carType: String? = myCar.carType

- here you are declaring to the Kotlin compiler that you know that carType is nullable. (In turn the behaviour of .toString() will work even though it is called on a null object, as you showed from the docs.)

However things are different in MyView.kt because the code doesn't get as far as the .toString() call. Here you are directly using a platform type (i.e. Java) where null-safety rules cannot be deduced.

This documentation about Calling Java code from Kotlin explains:

Types of Java declarations are treated in Kotlin in a specific manner and called platform types. Null-checks are relaxed for such types, so that safety guarantees for them are the same as in Java

answered Sep 23, 2022 at 17:49

AndrewL

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