public class BackpackInsideBackpackLimitation implements Listener {

    @EventHandler
    private void onClick(InventoryClickEvent event) {
        Inventory targetInventory;
        ItemStack targetItem;

        if(event.getClickedInventory() == null)
            return;

        if(event.getClick() == ClickType.NUMBER_KEY) {
            targetInventory = event.getClickedInventory();
            targetItem = event.getWhoClicked().getInventory().getItem(event.getHotbarButton());
        }
        else if(event.isShiftClick()) {
            targetInventory = event.getClickedInventory() == event.getView().getBottomInventory() ? event.getView().getTopInventory() : event.getView().getBottomInventory();
            targetItem = event.getCurrentItem();
        }
        else {
            targetInventory = event.getClickedInventory();
            targetItem = event.getCursor();
        }

        if(targetInventory == null)
            return;

        if(targetItem == null)
            return;

        if(!(targetInventory.getHolder() instanceof BackpackInventory))
            return;

        if(!BackpackInventory.isBackpack(targetItem))
            return;

        event.setCancelled(true);
    }
}