科特林mutableMap.put返回null的

Question

在科特林標準庫，我們有MutableMap接口 擁有這種方法

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

爲什麼回國空值，如果它接受不可空嗎？它是爲java interop完成的嗎？

Answer 1

看看定義

/** 
* 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