Kotlin static functions : companion object, @JvmStatic @JvmField

Question

I have just started messing around with the Kotlin programming language, which is pretty much cooler than Java. I have some doubts related to static methods and fields:

Q1: Official document says

Kotlin can also generate static methods for functions defined in named objects or companion objects if you annotate those functions as @JvmStatic.

But if you see below I can access bar() method as a static method, which works without using @JvmStatic annotation. But on official doc its throwing error -> Kotlin static method.

Class C{
    companion object{
        @JvmStatic
        fun foo() { }
        fun bar();
    }
}
fun main(args: Array<String>) {
    C.foo();
    C.bar(); //this line works fine
}

Q2: Do I really need @JvmStatic and @JvmField to make things static? As you can see with companion object, things are working as expected.

Answer 1

You can access members of a companion object as C.bar() in Kotlin, but not in Java. Without @JvmStatic, you would need to use C.Companion.bar() in Java, just as said in the docs.

Note that, without @JvmStatic, the function is compiled to an instance (non-static) method that is called on C.Companion in Java (and Kotlin simply shortens it to a call on C, but it's the same under the hood), so yes, you need either @JvmStatic and @JvmField to make a declaration in a companion object compile into a static member.

Basically, @JvmStatic and @JvmField are tools for Java interoperation that help with creating Java-friendly APIs, and if you don't need to call the Kotlin members from Java (e.g. they are internal to your Kotlin project, or you are developing a library that is unlikely to be used with Java), you can leave them as they are.