getItem() and chenage his value

Hi,
I am working on a plugin, but the plugin needs to check if you have the required items for something and then remove the required amount.
but I get an error while using (i have put in 318 voor a test:

Code:

                                ItemStack item = inv.getItem(318);
                                item.setAmount(item.getAmount()-1);

The error is an arrayouofbounds error and the top "at ..." thing says it it whas this particular code.

I hope someong can help me out,
regards,
Blackshade

Well I got the value changed but now the inventory won't reload it self.. Is that part of the Inventory Instance still missing? I got:

Code:

                                int itemIndex = inv.first(Integer.parseInt(costs[i]));
                                ItemStack item = inv.getItem(itemIndex);
                                item.setAmount(item.getAmount()-1);

But I see the new item amounts when I logout and Log in again. Not that instance that I click something (like I programmed)
am I missing something?

thank you,
Blackshade

EDIT by Moderator: merged posts, please use the edit button instead of double posting.