通用类型< T>函数名称前的参数 [英] Generic type <T> parameter BEFORE the function name

问题描述

在Kotlin中，函数名称前<T>类型参数 的用途是什么?

What is the usage of the <T> type parameter before the function name in Kotlin?

示例:

fun <T> MutableList<T>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] 
    this[index1] = this[index2]
    this[index2] = tmp
}

请参阅上面的第一个<T>.

我尝试浏览 Kotlin文档关于泛型以及 Java泛型，但是它们大多只是触及第二个<T>而不是第一个.

I've tried to look through the Kotlin docs regarding generics as well as the Java Generics however they mostly just touch on the 2nd <T> not the first.

推荐答案

它用于指示已使用泛型，但未引用某些类型的T.

It is used to indicate that generics are used and not some type T is referenced.

看看这个完全有效的示例

Have a look at this completly valid example

fun <String> MutableList<String>.swap(index1: Int, index2: Int)

现在可以在任何MutableList<*>上调用它，而不仅仅是MutableList<String>.如果您不愿意在fun关键字后写<String>，那么kotlin怎么会知道您实际上是在引用泛型而不是kotlin.String?

Now this can be called on any MutableList<*> and not only MutableList<String>. If you would not write <String> after the fun keyword, how would kotlin know that in fact you were referencing a generic and not kotlin.String?

您所显示的示例也是如此. fun之后的<>只是引入了一个新的通用参数，否则kotlin会抱怨它不知道类型T

The same goes for the example you've shown. The <> after the fun just introduces a new generic parameter, else kotlin would complain that it wouldn't know the type T

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