Smart cast to 'Type' is impossible, because 'variable' is a mutable property that could have been changed by this time

Question

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     } } 

Smart cast to 'Node' is impossible, because 'left' is a mutable property that could have been changed by this time

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?

How can I fix this elegantly?

Answer 1

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

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

1. Use a local variable with smart cast:

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

2. 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) 

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

    queue.add(left ?: return) 

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

Answer 2

1) Also you can use lateinit If you sure do your initialization later on onCreate() or elsewhere.

Use this

lateinit var left: Node 

Instead of this

var left: Node? = null 

2) And there is other way that use !! end of variable when you use it like this

queue.add(left!!) // add !!