generics - 是否可以在when语句中返回与类型参数相同的类型

Question

例如:

fun <T> f(a: T): T =
    when (a) {
        a is Int -> 0  // if T is Int, then return Int
        a is String -> ""  // if T is String, then return String
        else -> throw RuntimeException()  // Otherwise, throw an exception so that the return type does not matter.
    }

它给出了编译错误:

Error:(3, 20) The integer literal does not conform to the expected type T
Error:(4, 23) Type mismatch: inferred type is String but T was expected

Answer 1

之后您可以将结果转换为 T。你不会得到任何编译器帮助，你会收到警告，但至少它确实编译:

fun <T> f(a: T): T =
    when {
        a is Int -> 0  // if T is Int, then return Int
        a is String -> ""  // if T is String, then return String
        else -> throw RuntimeException()  // Otherwise, throw an exception so that the return type does not matter.
    } as T

注意这里的when (a)是不必要的，只要when {就足够了。