Draco18s

No, you're creating a new entity, diddling some of its values, and then throwing it in the garbage when the method returns. This is also never false because you set entity = new ItemEntity already.

Nothing about that method registers the biome with the game. https://github.com/Budschie/Deepnether-Mod/blob/1.16.3/src/main/java/de/budschie/deepnether/biomes/biome_data_handler/BiomeDataHandler.java#L13

Of course the cast doesn't work, an ItemStackHandler isn't an IInventory (nor is your TileEntity).

You have two biomes. I only see one being registered: https://github.com/Budschie/Deepnether-Mod/blob/1.16.3/src/main/java/de/budschie/deepnether/biomes/BiomeRegistry.java#L19

You know how when you buy a pair of headphones, they come in a box? You have a box labeled "headphones" right now and you're trying to plug it into your computer, but it doesn't have an audio jack.

No, do not implement ISidedInventory. This is a vanilla system that is replaced by the Forge capabilities system. LazyOptional.empty is...empty

It's called "push keys on keyboard and write code." The "hard" part is translating your thoughts into a sequence of instructions. And of all the methods you have to implement, there's maybe three you actually need to apply your brain to. The rest you can either get by extending an existing implementation (and not overriding--I recommend doing this) or pretty much copy and pasting their contents.

Start by making the stubs not-stubs. You can look at the other recipe implementations for help. Heck, you may even want to extend one of them to make your life easier.

Why are you creating a new ItemEntity every tick? Neither of these will ever be true.

You failed to include any of your actual code.

Well of course. That would only be true if the entity spawned already in the water. (Try throwing your item while under water). If you want to check for it being in water some time later you need to replace the entity with a custom entity and override the relevant method (there should already be one for when an entity is in water, or enters water, but Update would also work).

There's a wonderful website called GitHub BitBucket also exists.

https://lmgtfy.app/?q=java+tutorial+for+beginners

Yes, you do, but the lambda statement you have (which you need) is not inside the StairsBlock constructor. Both the register method and StairsBlocks constructor take a Supplier<Block> argument. (Gasp!) you need two lambdas!

Pretty sure you have: MAGICA_PLANKS.get() And the blocks in the Blocks class have: () -> MAGICA_PLANKS.get()

Blue banner at the top of every page.

complain to the dev.

Depends on your goal, really. Generally speaking you shouldn't create new tags in Forge or Minecraft namespace domains, but sometimes it is appropriate (e.g. if you're adding copper ingots, then forge:ingots might be the right place, because every modder who is adding copper that also do that is going to be interoperable).

Why not?

That's because swimming logic is handled by the LivingEntity class, not the block class. And the check it makes is based on the fluid/water tag.

Maybe don't add it to the water tag?

Have you looked at the vanilla classes involved with Ice and Glass? (Also compare with Leaves) There (was at one time) a method called DoesSideRender which controlled this behavior.

"Finished with non zero value" is not an error. It is the JVM telling you "there was an error, here's the log, its up to you now." Here's the actual error:

This: And this: Should both return the same object. So your code is equivalent to Math.sqrt(x*x).

All this is is the JVM saying "there was an error." It is about as helpful as walking into a doctor's office and saying, "doctor it hurts." You need to actually read the rest of the log to find out what the error was and where it happened.