Why is the type of null + null implicitly String in Kotlin?

user_6387618 0
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The following Kotlin code:

val x = null + null

results in x being of type String, which is correct as according to the docs for String.plus:

Concatenates this string with the string representation of the given [other] object. If either the receiver or the [other] object are null, they are represented as the string "null".

However, I don't understand why this happens - is it due to some special feature of the language?

kotlin nullable string
3 Answers

Probably because String?.plus(Any?) is the only plus function which accepts a nullable type as a receiver in Kotlin library. Therefore, when you call null + null, the compiler will treat the first null as String?.

If you define an extension function where the receiver type is Int? and the return type is Int, then x will be inferred as Int.

public operator fun Int?.plus(other: Any?): Int = 1
val x = null + null

If you declare another similar function within the same file (nullable type as the receiver type), when you call null + null, it causes the compile time error: Overload resolution ambiguity. All these functions match..

public operator fun Int?.plus(other: Any?): Int = 1
public operator fun Float?.plus(other: Any?): Float = 1F
val x = null + null    //compile time error
user_83433330

We need to start with the type of Nothing. This type has exactly zero possible values. It's a bottom type, and is a subtype of every other type (not to be confused with Any, which is a supertype of every other type). Nothing can be coerced to any type, so that you can do stuff like:

fun doStuff(a: Int): String =
    TODO("this typechecks")

Moving on to the type of Nothing?, meaning Nothing or null. It has 0 + 1 possible values. So null has a type of Nothing?. Nothing? can be coerced to any nullable type, so that you can do stuff like:

var name: String? = null

Here null : Nothing? is coerced to String?.

For some reason, unfortunately, there's this function defined in stdlib:

operator fun String?.plus(other: Any?): String

that allows null + null leveraging those coercion rules I mentioned above

user_10322860 
val x = null + null

Try to rephrase this as below and you will find you answer:

val x = null.plus(null)

The below is what IntelliJ shows as the signature of the plus method:

public operator fun String?.plus(other: Any?): String

So the first null is treated as String? type and then when you try to plus anything else, the above plus method is the only match you have. Printing out x will result in nullnull

user_47475870
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