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You are comparing objects by reference (==) instead of by contents (ItemStack::areItemStacksEqual).

I would suggest you my own one https://github.com/DarkRoleplay/Dark-Roleplay-Medieval/tree/1.11/src/main/java/net/dark_roleplay/medieval It has a big variety of things and I am trying to do things as simple as possible. And it's nearly cleaned up But in case you read anything about DRPCore that's just an advanced "library" I use. (Well basically it's a mod adding a completely new and universal crafting System and a few more things in close future)

Best "open-source"-ish mod to learn the basics of things? Minecraft. Seriously, a lot of things people ask about can be found here and there in Vanilla. Best open-source mod to learn about conventional coding? Forge. Best open-source mod to learn about how to do some specific things? Your brain and a bit of logic. Best open-source mod to learn something really specific from? Well, depends on the specifics. Most mods nowdays are open-sourced, as there is literally no reason to hide your source code. Every mod does things it's own way though. Want something simple? Pick a simple mod then, Any advanced mod will have several layers of abstractions and by your definition will be 'non learner friendly'. Also I do not think that any mod but the forge example mod is intended to be 'learner-friendly'. Mods are usually designed to work, not to teach people how to code

DrawBlockHighlightEvent would be more appropriate.