sortInventoryItems(inv, Material.COAL, Material.COAL_BLOCK);

public void sortInventoryItems(Inventory inv, Material toSort, Material after) {
		ItemStack[] items = inv.getContents();
		int amount = 9;
        for (int i = 0; i < items.length; i++) {
            ItemStack is = items[i];
            if (is != null && is.getType() == toSort) {
                int newamount = is.getAmount() / amount;
                inv.addItem(new ItemStack(after, newamount));
                if (newamount > 0) {
                    is.setAmount(newamount);
                    break;
                } else {
                    items[i] = new ItemStack(Material.AIR);
                    amount = -newamount;
                }
            }
        }
        inv.setContents(items);
    }