public class SendMailInventory implements InventoryHolder, Listener {

    private Inventory inv;
    private List<ItemStack> itemsToStore = new ArrayList<ItemStack>();


    public SendMailInventory() {
        inv = Bukkit.createInventory(this, 9, "SEND MAIL");
    }


    @Override
    public Inventory getInventory() {
        return inv;
    }


    public void openInventory(Player p) {
        p.openInventory(inv);
    }



    //STORE ITEMS ON CLOSING INVENTORY

    @EventHandler
    public void onCloseInventory(InventoryCloseEvent e) {

        InventoryHolder holder = e.getInventory().getHolder();

                            //DEBUG MESSAGE
        if(holder != this) {System.out.println("ITS NOT"); return;}

        //remove items from the list if there is any
        if(!itemsToStore.isEmpty()) {

            List<ItemStack> remove = new ArrayList<ItemStack>();

            for(ItemStack i : itemsToStore) {
                remove.add(i);
            }

            itemsToStore.removeAll(remove);

        }

        //store items
        ItemStack[] storeList = e.getInventory().getContents();
        storeItems(storeList);


    }


    private void storeItems(ItemStack[] items) {

        List<ItemStack> listCopy = new ArrayList<ItemStack>();

        for(int i = 0; i<items.length; i++) {
            listCopy.add(items[i]);
        }

        itemsToStore.addAll(listCopy);
        System.out.println(itemsToStore);


    }