CODEWITHSUNDEEP
kotlinmemory-managementglobal-variables
goedi
goedi

Reputation: 2113

Global variables in Kotlin

After using Java for a long time I almost forgot how it is to be allowed to define global variables. I read that it is recommended to use top-level functions in Kotlin instead of wrapping them inside objects like in Scala. How about top-level variables (which I guess we call global variables)? What is the recommendation about using them? What are the pitfalls? How are they deallocated?

Upvotes: 2

Views: 6635

Answers (1)

Silvestru
Silvestru

Reputation: 56

Global variables are supposed to be accessible from anywhere at any point in your program, so it would most likely be a bad idea to deallocate global variables, even if you could because it could lead to instability or crashes in your application.

Now Kotlin is a JVM language, it compiles to Java code and therefore is garbage-collected. As for how the garbage collector works, when a variable is no longer referenced by the program, it gets flagged for deletion and will be removed by the garbage collector later (not necessarily instantly). Global variable's scope is the entire program so they are never garbage-collected. What could you do tho is to set its reference to null (if it is declared nullable) to allow at least the referenced memory to be collected.

The top-level variables that you are referring about aren't exactly free since Kotlin will compile them into a java class named with the file name anyway(that's how you can access kotlin file-specific functions inside java code), so they are just like static fields in Java. For a better explanation you could check this thread: Kotlin: Difference between constant in companion object and top level. Now we know that top-level variables are compiled into static variables inside a Java class. Those don't get flagged for collection unless the program somehow drops the class loader associated to that class itself but that's something I can't speak of more since I never used that.

Now in the end, should you use global variables? They are usually considered a bad practice and this is in big part due to the fact that they can be referenced by any part of the program and it can be hard to remember when and why did they change, especially in large applications. If you keep track of their usages and use them in moderation then they are alright. Personally, if I have to use them, I define them inside a class as a companion object the same way you would define static variables in java, like this:

class GlobalData {
    companion object {
        var varEx= 0
        const val constString = "CONSTANT GLOBAL"
    }
}

Upvotes: 4

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