How to set mutable global variable in Kotlin

I’d like to make a global variable mutable, and use it in further methods, try to implement it with companion object, but not sure whether it’s thread safe, here is what I intend to do.

  companion object {
      private var flag = false

      fun modifyFlag(){
        flag = setFlagValue() //this method written in other class to assign value to the flag
      }
      fun getVal(): Boolean = this.flag // this is to get flag value in other methods static way, like GlobalVariable.getVal()
  }
}

can you give me some suggestions, thanks.

In general no, booleans are not guaranteed to be threadsafe.

If you’re on the JVM you can use an AtomicBoolean
https://docs.oracle.com/javase/8/docs/api/index.html?java/util/concurrent/atomic/AtomicBoolean.html

Thanks, what else can I do just with Kotlin to implement this?

The honest answer is that global mutable state is bad, so you should find a different way of handling this.

If you’re on multiplatform you can try to have a different solution for different targets, on JS all is fine, JVM has atomics, on native, I’m not sure really, there could be an answer here: https://github.com/JetBrains/kotlin-native/blob/master/CONCURRENCY.md

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Shared mutable state and concurrency

The companion object you wrote will work fine if you mark the field @Volatile, as long as setFlagValue() doesn’t call getVal()

The modifyFlag() method seems very strange, though, so I don’t know what you intend to do with it. You should be careful, since lots of programmers fail to achieve thread-safety when using thread-safe methods.