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I'm converting Java to Kotlin with Android Studio. I get double bang after the instance variable. What is the double bang and more importantly where is this documented?

mMap!!.addMarker(MarkerOptions().position(london).title("Marker in London"))
hotkey
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mbr_at_ml
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3 Answers3

267

This is unsafe nullable type (T?) conversion to a non-nullable type (T), !! will throw NullPointerException if the value is null.

It is documented here along with Kotlin means of null-safety.

Simson
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hotkey
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    what does it mean when the `!!` is at the end of a statement? IJ auto-convert to Kotlin did that for me `val price = sale.latest!!` – ycomp Feb 23 '16 at 16:57
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    @ycomp, it means that `sale.latest` can contain `null`; the assignment will succeed only if `sale.latest` is not null and will throw NPE otherwise. This gives null-safety for `val price`: its type will be non-null. See https://kotlinlang.org/docs/reference/null-safety.html – hotkey Feb 23 '16 at 17:46
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    @hotkey: So what is the difference between getting NPE - here OR when latest method is accessed on null object? – Aada Aug 13 '18 at 12:12
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    @Aada, it's common problem to debug an NPE and have a hard time locating the line / execution path that set the value to null. This null assignment could happen on a different context, class or even day! By using `!!` you can fail-fast and locate the root cause of a NPE. I wish Java had a similar feature (that is, w/o ugly `if` statements and/or `assert`ions). – Tasos P. Jan 11 '19 at 13:09
  • As Kotlin is Null safe programming language, to prevent throwing `NullPointerException` , use `let`. `T?.let{ it.addMarker() } `. By this way, you are safe. – Parisa Baastani Dec 08 '20 at 11:53
98

Here is an example to make things clearer. Say you have this function

fun main(args: Array<String>) {
    var email: String
    email = null
    println(email)
}

This will produce the following compilation error.

Null can not be a value of a non-null type String

Now you can prevent that by adding a question mark to the String type to make it nullable.

So we have

fun main(args: Array<String>) {
    var email: String?
    email = null
    println(email)
}

This produces a result of

null

Now if we want the function to throw an exception when the value of email is null, we can add two exclamations at the end of email. Like this

fun main(args: Array<String>) {
    var email: String?
    email = null
    println(email!!)
}

This will throw a KotlinNullPointerException

CoolMind
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Alf Moh
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    So, why would people use '!!' even though it is unsafe because an app will be terminated when that variable has null? – Sam Sep 03 '18 at 14:59
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    @david you can use it only when you are 100% sure that the variable is not null (e.g. you explicitly checked it) and you need non-nullable variable – FMK Sep 13 '18 at 08:53
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    @FMK I get it, thanks! I understand that double bang is used to make it possible that the values of nullable type variable to go into non-nullable types variable, right? – Sam Sep 13 '18 at 09:26
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    @david yes, exactly. – FMK Sep 13 '18 at 10:35
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    I get the reasoning, but if you think about it, if we know that the value of a variable will not be null and we are 100% sure of the fact, then to use a '!!' is in theory 100% of no use as we already are 100% sure that the variable in question will not be null, ever. Surely the usage is when we are not 100% sure that the variable in question may be null and we want an error to be thrown if it is null so we can fix the root cause? – Jeremy Feb 04 '21 at 21:34
16

Double-bang operator is an excellent option for fans of NullPointerException (or NPE for short).

The not-null assertion operator !! converts any value to a non-null type and throws an exception if the value is null. 

val nonNull = a!!.length

So you can write a!!, and this will return a non-null value of a (a String here for example) or throw an NPE if a is null.

If you want an NPE, you can have it, but you have to ask for it explicitly. This operator should be used in cases where the developer is guaranteeing – the value will never be null.

Andy Fedoroff
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    Asking as a beginner: why would I want to convert any value to a non-null type ? – Wolf359 May 13 '19 at 22:11
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    @Wolf359 It allows you to be 100% sure that its value is not null. ("it's common problem to debug an NPE and have a hard time locating the line / execution path that set the value to null. This null assignment could happen on a different context, class or even day!" by Cascader) – reducing activity Jul 12 '19 at 06:16