FlyCommand.java

@Override
public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
  if(!(sender instanceof Player)) {
    sender.sendMessage("Player only ;s");
    return true;
    }
    
    Player player = (Player) sender;
    
    if(args.length != 1) return false;
    
    player.setAllowFlight(args[0].equalsIgnoreCase("on"));
    player.sendMessage("Set flight status to " args[0].equalsIgnoreCase("on") ? ChatColor.GREEN.toString() + "ON" : ChatColor.RED.toString() + "OFF";
    return true;
}