Draco18s

Because that's the default Vanilla gui texture size.

Either you can force enchantments onto the item, or you can use Global Loot Modifiers.

See these lines? That's how you add stuff. The items and amounts in question are initialized by values stored in the json:

Your DeerEntity(EntityType<? extends DeerEntity> type, World worldIn) is protected, it should be public. I'm also pretty sure it needs to be DeerEntity(EntityType<T> type, World worldIn) as indicated by the error.

No, attach the capability to the item stack.

LazyOptional is just a lazily-instantiated Optional. It still holds a value, that value is of type T. T as in LazyOptional<T> That value gets cast to T when you call someLazy.cast(), so if the value actually stored in the LazyOptional is not actually of type T, the cast will fail. Optionals are just a box wrapped around a concrete object because you don't know if that object actually exists or not (which is why you have to use ifPresent, orElse, or orElseThrow--depending on circumstances).

There's already a collection that contains all the items in the game, its called the "item registry" and Forge tracks them.

Specifically once on each side for each hand.

Show your capability registration.

Don't register a capability with the event system, make a single class that handles ALL your events and then get the capability from the relevant object given to you by the event data.

...How else would data be transmitted from the client to the server or vice versa?

Complain to Optifine.

No, you're registering a TileEntity with that line. You also, additionally need a TileEntityType.

Your TileEntity's TileEntityType?

^ No parameters specified ^ 1 parameter required

By the way, this is not needed. Code Style #3

You need to have your receiver examine the blocks around it to find your lever, and if it does, and the lever is on, then output a signal. Alternatively if you want a specific lever-receiver pairing system you need a TileEntity to store that information. There is only One Lever instance and One Receiver instance. If you were to have your lever class modify data in the receiver class, then ALL receivers would activate any time you flipped ANY lever.

Yeah, unclear what you're trying to do.

Heck if I know.

All the documentation is found within the source code. There is no externally hosted version because changes are made too rapidly that any such external version would be out of date immediately.

Ore Gen is json based now.

Great, cool, I wonder if there are vanilla blocks that output a redstone signal. So you can go look at it. And figure out how it does it. Then adapt that to your new block.

I wonder if any vanilla blocks have a similar feature. Maybe you could go look at them.

Your error is not "exit code 1." All "exit code 1" means is that the program encountered an error and shut down. Your actual error is this: Learn Java first, mod Minecraft later. Trying to learn how to program by modding is a bad idea.