MapTabCompleter

public class MapTabCompleter implements TabCompleter {

    @Override
    public List<String> onTabComplete(CommandSender sender, Command command, String label, String[] args) {
        if(sender instanceof Player){
            Player player = (Player) sender;
            Game game = GameManager.getInstance().getGame();
            if(command.getLabel().equalsIgnoreCase("map")) {
                if (player.isOp() && args.length > 0) {
                    StringBuilder sb = new StringBuilder();
                    for (String arg : args) {
                        sb.append(arg);
                    }
                    ArrayList<CTFMap> maps = MapManager.getInstance().getMapsByName(sb.toString());
                    ArrayList<String> mapNames = new ArrayList<>();
                    maps.forEach((map) -> mapNames.add(map.getMapname()));
                    mapNames.remove(game.getArena().getMapname());
                    return mapNames;
                }
                return new ArrayList<>();
            }
        }
        return null;
    }

}

@Override
public static void onEnable(){
    this.getCommand("map").setTabCompleter(new MapTabCompleter());
}