Help with "Hat" command!

So yes, this code ALMOST works. So look in the code in the comments below and you will see my questions. I am not really sure HOW i should check these things, like if the player is online/existing that i can create my own error Message, PLUS how i check if the material is a material? Because if the player is doing "/hat sdf" i want an own error and how i check if it is not a material.

Thanks.

Code:

    @Override
    public boolean onCommand(CommandSender cs, Command cmd, String label, String[] args) {
        if(!(cs instanceof Player)) {
            cs.sendMessage("Only Players can use Hats");
            return true;
        }
        Player p = (Player)cs;
       
        if(args.length == 0) {
        p.sendMessage(ChatColor.RED + "/hat <material> <player>");
        } else if(args.length == 1) {
           
           
            if(p.hasPermission("hats.self")){
               
               
                /**So how do i check if the material i choose below IS an actual material?**/       
         p.getInventory().setHelmet(new ItemStack(Material.getMaterial(args[0].toUpperCase())));
         p.playEffect(p.getLocation(),Effect.MOBSPAWNER_FLAMES, 5);
         p.sendMessage(ChatColor.GREEN + "You equipped " + args[0] + " as your hat!");
        
       
        }
        } else if(args.length == 2) {
            if(p.hasPermission("hats.otherplayer")){
               
               
               
                Player getter = Bukkit.getPlayer(args[1]);
                /**How do i check if the Player IS a player / online?**/
                /**So how do i check if the material i choose below IS an actual material?**/
                getter.getInventory().setHelmet(new ItemStack(Material.getMaterial(args[0])));
                 getter.playEffect(p.getLocation(),Effect.MOBSPAWNER_FLAMES, 5);
                 getter.sendMessage(ChatColor.GREEN + "You got equipped " + args[0] + " as your hat by "+p.getName()+"!");
               
               
               
               
               
               
            }
        } else {
            p.sendMessage(ChatColor.RED + "/hat <material> <player>");
        }
       
        return true;
}

 

If getMaterial returns null, then it doesn't exist.

 

Okay thanks for that one!

 

If the player is not online, " Bukkit.getPlayer(args[1]);" returns null. Null check to see if the player is online.