nullable - Kotlin mutableMap.put 返回可为空

Question

在 kotlin 标准库中我们有 MutableMap 接口(interface)有这个方法

public abstract fun put(key: K, value: V): V?

如果它接受不可为空的值，为什么返回可空值？是为java互操作做的吗？

Answer 1

看一下定义

/**
 * Associates the specified [value] with the specified [key] in the map.
 *
 * @return the previous value associated with the key, or `null` if the key was not present in the map.
 */
public fun put(key: K, value: V): V?

所以

fun main(args: Array<String>) {
    var m: MutableMap<Int, String> = mutableMapOf(Pair(1, "a"))
    val prev1Value = m.put(1, "b")
    val prev2Value = m.put(2, "c")

    println(m)
    println("Previous value of 1 was: $prev1Value")
    println("Previous value of 2 was: $prev2Value")
}

打印:

{1=b, 2=c}
Previous value of 1 was: a
Previous value of 2 was: null