Kotlin: How do I get characters after "@" in a string?

Question

I have a string that is an email. I want to be able to get the domain part of the email no matter what the string/email is. Essentially I'm wanting to get hold of the characters after the @ part of the string. For example, for testing@kotlin.com, I'm after the kotlin.com part.

val emailString = "hello@world.com"

Answer 1

Note: Ivan Wooll's answer brings up the point of using substringAfterLast, which is a very useful utility, though it is important to keep in mind that it cannot return null and instead defaults to a provided default value (this is the original string, if nothing is specified).

I personally prefer dealing with null in cases where invalid input is a reasonable concern, rather than e.g. an empty string, because it's a much clearer indication that the delimiter was not found, and this special case can be easily handled by chaining ?:, ?., let, etc.

Here's an example of possibly-unwanted behavior:

string           | string.substringAfterLast("@")
-------------------------------------------------
"domain.com"     | "domain.com" !
"@domain.com"    | "domain.com"
"foo@domain.com" | "domain.com"

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Just for the sake of completeness:

val string = "hello@world.com"

val index = string.indexOf('@')

val domain: String? = if (index == -1) null else string.substring(index + 1)

This assigns the part after @ to domain if it exists, otherwise null.

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For learning, IntelliJ's Java -> Kotlin converter may be of use.

By default, this shortcut is usually mapped to Ctrl+Alt+Shift+K.

You could even make this an extension property:

val String.domain: String?
    get() {
        val index = string.indexOf('@')
        return if (index == -1) null else string.substring(index + 1)
    }

and then you would be able to do

println("hello@world.com".domain)

You could shorten this code to one line with let:

string.indexOf('@').let { if (it == -1) null else string.substring(it + 1) }

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Here's a similar question in Java.