Question

I want to use MutableMap with defaults:

val myMap = mutableMapOf<String, Set<String>>().withDefault { mutableSetOf() }

but I can't use getOrImplicitDefault method because withDefault returns MutableMap<String, Set<String>> type. Moreover, I can't cast to MutableMapWithDefault interface because this is a private interface.

I can't use get method either because it returns a nullable type. It is ok because this is a method on the MutableMap interface (moreover it doesn't call defaultValue callback for take default value).

Seems like this functionality is not correctly implemented in Kotlin, or I am using it wrong. So, how do I use withDefault wrappers correctly?

Answer 1

从Kotlin 1.0开始，withDefault返回的包装器仅在属性委派用例中可用。

val map = mutableMapOf<String, Set<String>>().withDefault { mutableSetOf() }

var property: Set<String> by map // returns empty set by default

Answer 2

在Kotlin 1.1中，如果您使用getValue()函数而不是get()函数，这实际上是有效的。

Answer 3

好吧，getOrImplicitDefault的所有实现都将t重定向到getOrElseNullable。也许试试吧。

Answer 4

我一直在寻找一种从MutableMap返回默认值的方法，但也会同时存储它以供将来检索。 .withDefault仅返回默认值，但不存储它。每次需要检索值时调用.getOrPut都不是一个好主意。我想出了类似的东西：

val myMap = with(mutableMapOf<String, Set<String>>()) {
    withDefault { key -> getOrPut(key, { mutableSetOf<String>() }) }
}

这将在支持MutableMap对象的getOrPut包装器内调用withDefault，该对象将缺少的键值对放入地图并返回它。

Answer 5

使用以下方法：

myMap.getOrPut(someKey,{DEFAULT_VALUE})

文档：https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/get-or-put.html

Answer 6

Map withDefault返回只读映射的包装，具有指定函数defaultValue提供的隐式默认值。当原始地图不包含指定键的值时，将使用此隐式默认值。同时查看getOrElse和getOrDefault的用法，其用法与列表相同-

val emojiMap = mapOf("Grinning Face" to "?", "Winking face" to "?", 
                "Zany face" to "?").withDefault({"?"})

fun main()
{
    println(emojiMap) // {Grinning Face=?, Winking face=?, Zany face=?}
    println(emojiMap.getValue("Grinning Face")) // ?
    println(emojiMap.getValue("Zany face")) // ?
    println(emojiMap.getValue("Smiling face")) // ?

    // getOrElse & getOrDefault
    println(emojiMap.getOrElse("Flushed face"){"?"}) // ?
    println(emojiMap.getOrDefault("Sad face","?")) // ?
}

How to use withDefault wrappers?

6 个答案: