无法将类型强制转换为“类型"，因为“变量"是可变属性，到那时可能已经更改了

问题描述

限时删除！！

Kotlin新手问，为什么下面的代码不能编译?":

And the Kotlin newbie asks, "why won't the following code compile?":

    var left: Node? = null

    fun show() {
         if (left != null) {
             queue.add(left) // ERROR HERE
         }
    }

我知道left是可变变量，但是我明确检查了left != null并且left是Node类型，所以为什么不能将其智能转换为该类型?

I get that left is mutable variable, but I'm explicitly checking left != null and left is of type Node so why can't it be smart-casted to that type?

如何解决这个问题? :)

How can I fix this elegantly? :)

推荐答案

在执行left != null和queue.add(left)之间，另一个线程可能已将left的值更改为null.

Between execution of left != null and queue.add(left) another thread could have changed the value of left to null.

要解决此问题，您有几种选择.这里是一些:

To work around this you have several options. Here are some:

1. 使用局部变量进行智能强制转换:

1. Use a local variable with smart cast:

val node = left
if (node != null) {
    queue.add(node)
}

使用安全呼叫，例如一个以下:

Use a safe call such as one of the following:

left?.let { node -> queue.add(node) }
left?.let { queue.add(it) }
left?.let(queue::add)

将 Elvis运算符与 return 以便从封闭函数中尽早返回:

Use the Elvis operator with return to return early from the enclosing function:

queue.add(left ?: return)

请注意，break和continue可以类似地用于循环内的检查.

Note that break and continue can be used similarly for checks within loops.

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