Basically, let's say I wanted to make a mod, and I wanted to addon to Mr. Crayfishes Device Mod. I want this mod to work with, or without the mod. When the mod is not installed, don't register the things for the mod, and if it is, do register them.

 

How would I be able to execute this?

Well, there was a similar question a few days ago, about dependencies, which you state you do not want. (Directly referencing another mods' classes, which happen to not exist, even after checking if it was there or not = Crash when creating runtime, because you imported something that doesn't exist.)

You can see that conversation here.

 

You want to keep your code as subtle as possible. If you simply want "oh, that mod is in here, let me add this" Then simply use the Loader#isModLoaded(id) to flip a boolean, of which your item/block-registration checks before registering the specified object.

If you absolutely need to reference another mod's files, you need to put that code into a separate class, put the relevant calls into a function, and then call that function inside an Loader#isModLoaded(id) check.

 

Ah la

https://github.com/Draco18s/HarderStuff/blob/master/src/main/java/com/draco18s/ores/OresBase.java#L754-L769

Awesome! Thank you both!