Kotlin泛型和可为空的类类型 [英] Kotlin Generics and nullable Class type

问题描述

如何在Kotlin中处理可为空的泛型类类型?

How do I handle a nullable generics Class type in Kotlin?

带有泛型的示例函数:

fun <I> calculateStuff(valueType: Class<I>, defaultValue: I): I {
    // do some work

    return defaultValue;
}

这是一个调用函数(请注意calculateStuff(...)的第二个参数)

Here is a calling function (note the 2nd param for calculateStuff(...))

fun doStuff() {
    // works fine!
    val myVar1 = calculateStuff(String::class.java, "")

    // FAIL (null is not accepted... Error: "Cannot infer type parameter I in....")
    val myVar2 = calculateStuff(String::class.java, null)
}

解决方法(将返回类型更改为I?将defaultValue更改为I?):

Work-around (change return type to I? AND defaultValue to I?):

fun <I> calculateStuff(valueType: Class<I>, defaultValue: I?): I? {
    return defaultValue;
}

首选方法，但Kotlin似乎不支持(请注意"String?:: class.java"):

Preferred method, but does not seemed supported by Kotlin (note "String?::class.java"):

val myVar2 = calculateStuff(String?::class.java, null)

我真的很希望能够将返回类型发送给方法(calculateStuff(...))，如果它可以为null，则将其作为第一个参数...这样，我只需要对null进行检查即可.如果我在第一个参数中传递了一个可为空的Class，则返回值.

I really want to be able to send to the method (calculateStuff(...)) the return type, and if it can be null, as the first parameter... that way I ONLY have to null-check the return value if I pass a nullable Class in the first param.

这在科特林有可能吗?

推荐答案

您需要将Class<I>更改为Class<out I>:

fun <I> calculateStuff(valueType: Class<out I>, defaultValue: I): I {
    return defaultValue;
}

您还可以使用已修改类型参数:

inline fun <reified I> calculateStuff(defaultValue: I): I {
    // do some work

    return defaultValue;
}

用法:

val myVar1 = calculateStuff("")             // myVar1 is String
val myVar2 = calculateStuff<String?>(null)  // myVar2 is String?

这篇关于Kotlin泛型和可为空的类类型的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！