Fly

if(cmd.getName().equalsIgnoreCase("fly")) {
    Player player = (Player) sender;
    if(fly.contains(player)) {
        player.setAllowFlight(false);
        player.sendMessage("Fly disabled");
        fly.remove(player);
        return true;
    } else {
        player.setAllowFlight(true);
        player.sendMessage("Fly Enabled");
        fly.add(player);
        return true;
    }
}