So I currently have a backpack working thing is, I want every new backpack to have its own inventory. How would I go about doing this?

Create Item, implement IInventory. Customize your implementation. Since Item is only "description" of what you are holding - it cannot hold data itself (singleton).

 

What holds data  is ItemStack. You can assign NBT with any kind of data to any ItemStack. Save your equipment per-itemStack inside its NBT. For examples (and util methods) lookup vanilla's code (player's inventory for one).

I have the backpack inventory i just want it to be unique for each new player gets new backpack, the new backpack is a different inventory to the last one.

What did I just write? ( ͠° ͟ʖ ͡°)

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You can NOT implement IInventory in an Item class - Items in Minecraft are singletons, so there is only ever one instance of each item, but for an Inventory to work you need the Inventory itself to be a unique instance, otherwise it will be all sorts of messed up.

 

If you want each player to have a unique backpack instance, like a portable Ender Chest, you need to store the inventory in IExtendedEntityProperties and only access the inventory GUI via your item - the item itself will have nothing else to do with the inventory.

 

If you want players to be able to carry as many backpacks as they want, each backpack having storing an independent inventory, you must store the Inventory inside the ItemStack's NBT and basically recreate the inventory from that tag each time you want to interact with it.

 

I have a tutorial that covers both cases.