下面是一个简单的例子来说明这个问题:
abstract class Base {
abstract fun String.extension(x: Char)
}
class Derived : Base() {
override fun String.extension(x: Char) {
// Calling lots of methods on String, hence extension method
println("${first()} $length ${last()} ${firstOrNull { it == x }} ...")
}
}
Calling the extension method from Java is trivial:
Base o = new Derived();
o.extension("hello world", 'l');
但我不知道如何在纯Kotlin 做到这一点.String和Base似乎都没有extension方法.
首先，请注意，定义为成员的扩展函数需要两个接收器，一个是封闭类的实例(dispatch receiver，通常是封闭类的this)，另一个是函数扩展的类型的实例(extension receiver).这是here条记录.
所以，要从类外调用这样的函数，必须提供两个接收器.Kotlin doesn't have any syntax可以像(x, "abc").stringExtension()那样显式地实现这一点，但您可以使用extension lambda隐式地提供调度接收器:
class C(val name: String) {
fun String.extended() = this + " extended by " + name
}
fun main(args: Array<String>) {
val c = C("c")
with(c) { println("abc".extended()) }
}
In the with(...) { ... } block, c becomes the implicit receiver, thus allowing you to use it as a dispatch receiver in C member extensions. This would work with any other function that uses functional types with receivers: apply, run, use etc.
In your case, it would be with(o) { "hello world".extension('l') }
如@KirillRakhman所述，C的扩展函数的扩展接收器也可以隐式地用作C中定义的扩展的调度接收器:
fun C.someExtension() = "something".extended()