TypeVariable(V) 在 Kotlin 中的可变 Map 分配中需要。 无法将 map 中的值放入自身

Question

The following example is a simplified version of what I am trying to do in my application.

fun main() {
    val test: MutableMap<Int, String> = mutableMapOf(
        1 to "Apple",
    )
    test[2] = test[1]  // test[1] has incorrect type.
}

The code doesn't compile. IntelliJ gives the following hint:

Type mismatch.

    Required:   TypeVariable(V)

    Found:      String?

I don't understand what a TypeVariable is. but when I provide a default value the error disappears

test[2] = test[1] ?: "Grape"

Why the required type is TypeVariable(V) , not String , and what exactly is it? What to do if there's no default value for my application purposes?

Answer 1

... = test[1]

returns a String? as the hint showed you. But test is a MutableMap of <Int, String>, which means you need to assign a String:

... = test[1]!!

Of course this will only work if 1 is a valid key in test . Otherwise your code with the null safety operator is the way to go:

... = test[1] ?: "default value"