How can I refer to companion objects from Java?

The companion object in Kotlin has static fields and support methods for interacting with Java, so you can essentially treat them like a static class if you've annotated them correctly (using @JvmStatic and @JvmField). So:

class C {
    companion object {
        @JvmStatic fun foo() {}
        fun bar() {}
    }
}

      

        
        
        
      

    

foo can be obtained from Java as a static function. The bar can't.

C.foo(); // works fine
C.bar(); // error: not a static method
C.Companion.foo(); // instance method remains
C.Companion.bar(); // the only way it works

      

        
        
        
      

    

You can do the same with fields, except using JvmField

class Key(val value: Int) {
    companion object {
        @JvmField val COMPARATOR: Comparator<Key> = compareBy<Key> { it.value }
    }
}

      

        
        
        
      

    

Then:

// Java
Key.COMPARATOR.compare(key1, key2);
// public static final field in Key class

      

        
        
        
      

    

You can also use const.

// file: Example.kt

object Obj {
    const val CONST = 1
}

class C {
    companion object {
        const val VERSION = 9
    }
}

const val MAX = 239

      

        
        
        
      

    

In Java:

int c = Obj.CONST;
int d = ExampleKt.MAX;
int v = C.VERSION;

      

        
        
        
      

    

See Java's Kotlin interop in the documentation for full details on the details (all examples from copy anyway).

I recommend knowing (and using) the JvmStatic and JvmField annotation well if you want to interact with Java frequently, as they are indeed critical to smoothing Kotlin's interaction with Java.