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More specifically, this post:

Looks like you have your JDK pointing to your JRE. Just to forewarn you, if you do not have previous java experience, modding will be nearly impossible, and only forge is supported on these forums, not basic java.

That would be the client side yes. You need to send a packet to the server (all this packet does is say "this button was clicked"!) and the server does the operations neccessary (including checking to make sure that the player is allowed to click the button). Quick "Am I on the Server?" guide: Is it visual? No, you are on the client Is it input related? No, you are on the client Was a world or player object passed to you? Maybe, check world.isRemote (or player.world.isRemote) isRemote returns true? No, you are on the client Did you get your world by calling Minecraft.getInstance()? No, you are on the client. Also don't do this unless you know what you're doing.

What do you mean "u don't need it"? You need Java to run the installer.

You need a custom slot: https://github.com/Draco18s/ReasonableRealism/blob/1.14.4/src/main/java/com/draco18s/harderores/inventory/SlotDust.java Line 17 is the important one, all you need to do is check for one item. And a sublcass of ItemStackHandler: https://github.com/Draco18s/ReasonableRealism/blob/1.14.4/src/main/java/com/draco18s/harderores/entity/capability/SiftableItemsHandler.java (You won't need onContentsChanged, that's specific to my block, just edit line 15 to be the same logic as the line 17 of your slot class).

This is the wrong bus.