BackpackUtils

public class BackpackUtils {

    public static void storeItems(HashMap<Integer, ItemStack> content, ItemStack item){
        ItemMeta data = item.getItemMeta();

        if (content.size() == 0){
            data.getPersistentDataContainer().set(new NamespacedKey(Backpack.getPlugin(), "backpack_id"), PersistentDataType.STRING, "");
            item.setItemMeta(data);
        } else {
            try {
                ByteArrayOutputStream input = new ByteArrayOutputStream();
                BukkitObjectOutputStream output = new BukkitObjectOutputStream(input);

                output.writeObject(content.size());

                content.forEach((Integer, ItemStack) -> {
                    try {
                        output.writeObject(content);
                    } catch (IOException ex) {
                        System.out.println(ex);
                    }
                } );

                output.flush();


            } catch (IOException ex) {
                System.out.println(ex);
            }
        }
    }

    public static HashMap<Integer, ItemStack> getContents(ItemStack item) {
        PersistentDataContainer data = item.getItemMeta().getPersistentDataContainer();

        HashMap<Integer, ItemStack> contents = new HashMap<>();
        String encodedItems = data.get(new NamespacedKey(Backpack.getPlugin(), "Backpack_id"), PersistentDataType.STRING);

        try {
            assert encodedItems != null;
            if (!encodedItems.isEmpty()){
            byte[] rawData = Base64.getDecoder().decode(encodedItems);


                ByteArrayInputStream io = new ByteArrayInputStream(rawData);
                BukkitObjectInputStream in = new BukkitObjectInputStream(io);

                int itemsCount = in.readInt();

                for(int i = 0; i < itemsCount; i++){
                    contents.put((Integer) in.readObject(), (ItemStack) in.readObject());
                    in.readObject();
                }
                in.close();
            }
        } catch (ClassNotFoundException | IOException | NullPointerException ex) {
            System.out.println(ex);
        }
        return contents;
    }

}