icon-cookie
The website uses cookies to optimize your user experience. Using this website grants us the permission to collect certain information essential to the provision of our services to you, but you may change the cookie settings within your browser any time you wish. Learn more
I agree
blank_error__heading
blank_error__body
Text direction?

Type aliases

Type aliases provide alternative names for existing types. If the type name is too long you can introduce a different shorter name and use the new one instead.

It's useful to shorten long generic types. For instance, it's often tempting to shrink collection types:

typealias NodeSet = Set<Network.Node>

typealias FileTable<K> = MutableMap<K, MutableList<File>>

You can provide different aliases for function types:

typealias MyHandler = (Int, String, Any) -> Unit

typealias Predicate<T> = (T) -> Boolean

You can have new names for inner and nested classes:

class A {
    inner class Inner
}
class B {
    inner class Inner
}

typealias AInner = A.Inner
typealias BInner = B.Inner

Type aliases do not introduce new types. They are equivalent to the corresponding underlying types. When you add typealias Predicate<T> and use Predicate<Int> in your code, the Kotlin compiler always expand it to (Int) -> Boolean. Thus you can pass a variable of your type whenever a general function type is required and vice versa:

typealias Predicate<T> = (T) -> Boolean

fun foo(p: Predicate<Int>) = p(42)

fun main(args: Array<String>) {
    val f: (Int) -> Boolean = { it > 0 }
    println(foo(f)) // prints "true"

    val p: Predicate<Int> = { it > 0 }
    println(listOf(1, -2).filter(p)) // prints "[1]"
}
Measure
Measure
Related Notes
Get a free MyMarkup account to save this article and view it later on any device.
Create account

End User License Agreement

Summary | 1 Annotations
It's useful to shorten long generic types
2017/09/21 08:02