InventorySystem | a basic drag and drop inventory system | Widget library

Let's image for a game, I have this scheme

Item is what all the game's Item based off

Some Item like Sword implements IEquipable

Some Item like Potion implements IConsumable

When I am giving out a quest reward, I add an Item to the character. When I am listing out the character's inventory, I go through their List of Item. At this time, I only know I have some Item but no longer know that the Item is Equipable or Consumable . Basically, the Interface is no longer known to me. What design can I do in this circumstance?

Let's be more specific.

When an Item is IConsumable, I would like them to "stack" on top of eachother, may be give a different glow to them and a right-click action will result in consuming that item

When an Item is IEquipable, The item will not stack inside the inventory, may be give a different glow to them and a right-click action will result in equipping that item

Should I do type-checking (If item is IEquipable then -> Do IEquipable-specific actions or access specific property). Or maybe there would be a different design altogether for this type of problem?

I am looking for answer that describe if this is not a problem or how I can avoid it if is a design problem (of needing to access to feature of more "refined" class while what we have at hand is the more generic class) - with example if possible

Update: I would argue that it makes sense for inventory to be basing off Item. While Item provides the most basic functionality like Name, GoldValue or ItemIcon.

Though properties like Durability or Enchantments[] can only be applied to Equipables. Let's take a look at Minecraft inventory. I would say every basic interaction are from InventorySystem to Item. Though it is still necessary to know what is the kind of Item the InventorySystem is processing? Like Bread , what I see is that they are consumables and can stack up to 64 in one single slot

I'm quite new to coding, and making a game for fun, it went well until now.

I'm working on the inventorysystem right now and wanted to do an array out of inventoryslots. The problem for me right now is that i get a NullReferenceException: Object reference not set to an instance of an object error, right att the for loop. Sadly I don't see or understand my mistake. I do set det Inventoryslots in Unity and have 3 at the moment.

*Sorry if this is a stupid question i just want to learn

New to coding, let alone java and javaFX. I'm not sure where I'm messing up on moving from the home window to any other one and could use some help figuring it out.

The homewindow opens fine but when trying to move to any other screen, for example AddPart it gives the error log at the bottom of the page. It does this with every single page, so I'm assuming it's something with how I'm trying to load the pages.

HomeWindowController:

I'm trying to launch up a project in JavaFX and am pretty lost at what I'm doing wrong.

The Main

I'm pretty new to programming and have gotten some help with something that I'm working on. That said, now my project isn't recognizing that the main method is there. I've made sure several times that the run's main class is a valid route, but it still doesn't seem to recognize that the file is there. No other files recognize any imports from that package, and the package doesn't recognize any imports from other files.

The error code