Kotlin static functions : companion object, @JvmStatic @JvmField

Yep, you do need @JvmStatic. The problem with your code that you call it in Kotlin while in documentation code was called in Java.

To be more precise, this code won't compile:

public void main(String[] args) {
    C.foo(); // Ok
    C.bar(); // Not ok
}

Kotlin knows about function in companion object so you can call it on class directly, while Java doesn't know anything about it. So you annotate any function with @JvmStatic and it becomes visible from Java code as static method.

And just to clarfiy - purpose of @JvmStatic is interop with Java code. If you write your application in Kotlin only you don't need @JvmStatic at all.

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.