Api.java

public interface Api {
    public interface Listener {
        void onEvent(String name);
    }
    void listen(Listener listener);
}

class JUser {
    void test() {
        new Impl().listen(name -> System.out.println(name));
    }
}

Impl.kt

class Impl : Api {
    override fun listen(listener: Api.Listener?) {
        // something
    }
}

class UserK {
    fun test() {
        Impl().listen { println(it) }
    }
}