Kotlin Object with inner trait: instancing anonymous trait inside activity

KotlinTrait

// ** Kotlin implementation **
// Singleton with inner trait
object LastFmManager {

    public trait TopArtistsFetchEvent {
        public fun topArtistsFetchSucceeded(artists: List<LastFmArtist>)
        public fun topArtistsFetchFailed(throwable: Throwable)
    }
    
    // Other logic here
}

// Android Activity
public class MainActivity : ActionBarActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        LastFmManager.fetchTopArtists(object : LastFmManager.TopArtistsFetchEvent(){ // Error in TopArtistFetchEvent's parenthesis: "This class does not have a constructor"
            override fun topArtistsFetchSucceeded(artists: List<LastFmArtist>) {
                throw UnsupportedOperationException()
            }

            override fun topArtistsFetchFailed(throwable: Throwable) {
                throw UnsupportedOperationException()
            }
        })
    }
}

// ** Java implementation
public class LastFmManager {

    private static LastFmManager mInstance;
    
    public interface TopArtistsFetchEvent {

        public topArtistsFetchSucceeded(List<LastFmArtist> artists)
        public topArtistsFetchFailed(Throwable throwable)
    }
    
    public static LastFmManager getInstance(){
        if (mInstance == null) {
            mInstance = new LastFmManager();
        }
        return mInstance;
    }

    private LastFmManager(){
    }
}

public class MainActivity extends ActionBarActivity {

    @Override
    public void onCreate( Bundle savedInstanceState) {
        LastFmManager.fetchTopArtists(new LastFmManager.TopArtistsFetchEvent(){
            @Override
            public void topArtistsFetchSucceeded(artists: List<LastFmArtist>) {
                // Do something here
            }

            @Override
            public void topArtistsFetchFailed(throwable: Throwable) {
                // Report error
            }
        });
    }
}