为什么在执行nullcheck后Smartcast无法正常工作 [英] Why does Smartcast not work after nullcheck

问题描述

我正在尝试代码的出现，并希望在第10天创建一个类.我知道这些值可以为null，因此我将其声明为可为空.在某个时候，我需要检查是否已分配该值并对其进行处理. 问题来了.我通过high != null进行了事先检查，但是在接下来的一行中，我必须使用!!来使编译器确信它实际上是空的.

I'm attempting advent of code and wanted to create a class for day 10. I know that the values can be null, so I declared them as nullable. At some point, I need to check whether the value is assigned and do something with it. There comes the issue. I check beforehand via high != null, but in the line that follows, I have to use !! to convince the compiler that it actually is null.

尽管先进行空检查，它似乎仍找不到正确的compareTo方法.我想，它没有智能广播我的变量

It seems that it can't find the proper compareTo method, despite nullchecking it first. I guess, it didn't smartcast my variable

private class Bot(val number: Int, var low: Int?, var high: Int?) {

  fun acceptValue(value: Int) {
    if (low == null && high == null) {
      high = value
    } else {
      if (high != null) {
        if (high!! > value) { //it doesn't compile, because appareantly, high is still not considered nonnull at this point
          low = value
        } else {
          low = high
          high = value
        }
      }
    }
  }
}

使用的kotlin版本是1.1.3-2

the kotlin-version is use is 1.1.3-2

那是个错误吗?我在做错什么吗?

Is that a bug? Am I doing something wrong?

推荐答案

在high != null和high > value之间，另一个线程可以执行high = null，从而使空检查无效.所以这是预期的行为.

Between high != null and high > value another thread could do high = null, invalidating the null check. So this is the expected behavior.

解决此问题的方法是使用无法在外部更改的临时变量:

The way to solve this is to use a temporary variable which cannot be externally changed:

val cHigh = high
if (cHigh != null) {
    if (cHigh > value) {
        ....

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