Why doesn't toString throw an exception when called on null value in Kotlin?

Question

Given the code

fun main(args: Array<String>) {
    val someText: String? = null
    println(someText.toString())
}

When run, output is

null

Two questions appear:

is that possible to implement custom null-safe method with fallback to some default code (like, I think, toString does)
why no exception is thrown?

Ok, but what about the second question? Is that possible? – Wojciech Wirzbicki Mar 12 '18 at 14:42 — Wojciech Wirzbicki, Mar 12 '18 at 14:42

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

2

From the docs:

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

You can achieve similar behaviour by writing an extension function. For example:

fun Any?.foo() = println(this ?: "Sadly, this is null")

fun main(args: Array<String>) {
    val x: Int? = null
    val y: Int? = 3

    x.foo()       // "Sadly, this is null"
    y.foo()       // "3"
    null.foo()    // "Sadly, this is null"
}

_{Live example.}

edited Jun 20 '20 at 09:12

Community

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answered Mar 12 '18 at 14:43

Oliver Charlesworth

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Interesting extension... calling `foo()` on any nullable object should work without having to first assert not null or `?.foo()`? – Robin Jonsson Mar 14 '18 at 10:20
@RobinJonsson - Indeed. I've edited my answer to include a more complete example. – Oliver Charlesworth Mar 14 '18 at 11:13

Why doesn't toString throw an exception when called on null value in Kotlin?

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