LeaderBoardCompletionHandler.kt

private boolean loading = false;
private class CompletionHandler implements GKLeaderboard.Block_loadLeaderboardsWithCompletionHandler {
    @Override
    public void call_loadLeaderboardsWithCompletionHandler(
            NSArray<? extends GKLeaderboard> leaderboards, NSError nsError) {
        //content below...
    }
}
loading = true;
if (nsError != null) {
    Log.d("Error on loading leaderboards: %s", nsError.toString());
}
if (leaderboards != null && leaderboards.size() > 0) {
    leaderboards.get(0).setPlayerScope(GKLeaderboardPlayerScope.Global);
    leaderboards.get(0).setTimeScope(GKLeaderboardTimeScope.AllTime);
    leaderboards.get(0).loadScoresWithCompletionHandler(new GKLeaderboard.Block_loadScoresWithCompletionHandler() {
        @Override
        public void call_loadScoresWithCompletionHandler(NSArray<? extends GKScore> scores, NSError nsError) {
            ...
        }
    });
}