如何检查Kotlin中的泛型类型？

Question

我有课

class Generic<T : SuperType>()

这段代码不对

fun typeCheck(s: SuperType): Unit {
            when(s){
                is T -> //do some thin
            }
        }

但是施放s类型T s as T显示警告 - 不安全的施法.
怎么检查s是什么类型T？

Answer 1

如果您需要检查某些内容是否属于泛型类型T,则需要有一个Class<T>要检查的实例.这是Java中常见的技术,但在Kotlin中我们可以使用内联工厂方法来获取类对象.

class Generic<T : Any>(val klass: Class<T>) {
    companion object {
        inline operator fun <reified T : Any>invoke() = Generic(T::class.java)
    }

    fun checkType(t: Any) {
        when {
            klass.isAssignableFrom(t.javaClass) -> println("Correct type")
            else -> println("Wrong type")
       }

    }
}

fun main(vararg args: String) {
    Generic<String>().checkType("foo")
    Generic<String>().checkType(1)
}

Answer 2

I know that I'm kinda late to this thread, but I just want to recap on the answer provided by Alexander Udalov.

It is, indeed, impossible to determine the type of a generic parameter in Kotlin unless you're using inline functions and declaring the generic type as reified.

Not sure if I'll be able to answer this question entirely and accurately, but I feel like my contribution might still be valuable for someone who is attempting to do just that. So let's say you have a few data classes, and you want to check which type you're dealing with.

You could use a function like that:

inline fun <reified T> checkType() = when (T::class) {
    TypeA::class -> println("TypeA")
    else -> println("Type not recognized")
}

however, functions that call it must also be inline, so you might have to write something like

inline fun <reified T> someOtherFunction(data: T) {
    checkType<T>
}

however, if you cannot allow for an inline function (let's say in an interface!), you can kinda 'cheat' the system by saying, for example

class AmazingTypes {
    inline fun <reified T> checkType(genericParameter: T) = when (T::class) {
        TypeA::class -> println("TypeA")
        else -> println("Type not recognized")
    }
}


fun myAwesomeMethod(someParameter: Any) {
    val amazingClass = AmazingClass()
    amazingClass.checkType(someParameter)
}

Answer 3

通用类型在运行时不会在JVM上实现,因此在Kotlin中无法执行此操作.警告是正确的,因为编译器在完成强制转换时不可能生成任何失败的指令,因此取消选中强制转换,这意味着程序可能会在某个时刻或之后中断.

可能有用的相关特征是内联函数中的具体类型参数.但是,类不能具有已知的类型参数,因此如果您对用例进行详细说明,我可以尝试帮助您实现您似乎需要的内容.