@JvmStatic methods

Similarly to properties, we can apply @JvmStatic to methods defined on companion objects or named objects. Let's examine this by adding a method to our existing companion object:

class Widget {
    companion object {
        ...

        fun goo() { }
    }
}

From Java, we can then use this method by once again using an instance of the companion object:

// Main.java
public class Main {
    public static void main(String[] args) {
        Widget.Companion.goo();
    }
}

In this example, we must reference the companion object instance because the goo() method is a member of the companion object itself, as seen in the following generated code:

public final class Widget {
   ...
   public static final class Companion {
      ...

      public final void goo() {
      }

      ...
   }
}

To move this generated goo() method to the Widget class itself, we can once again apply the @JvmStatic annotation:

class Widget {
    companion object {
        ...

        @JvmStatic
        fun goo() { }
    }
}

Adding the @JvmStatic annotation to the companion object declaration of goo() will indicate to the compiler that it should generate a static method, goo(), on the Widget class itself:

public final class Widget {
   ...

   @JvmStatic
   public static final void goo() {
      Companion.goo();
   }

   public static final class Companion {
      ...
   }
}

Once the compiler has generated the true JVM static method, it can be accessed from Java without needing to reference, or know about, the companion object:

// Main.java
public class Main {
    public static void main(String[] args) {
        Widget.goo();
    }
}

By writing your function with @JvmStatic, the code can be used from Java like any other static method in your Java code base. If you're working in a code base with a lot of Java and Kotlin code, this familiar syntax and usage might be a desirable trait.