So, I understand that an API is just some classes for other mods to interact with mine. But, do I simply create a package for my API classes, compile my mod with it, and have people use the source of my mod for the API? Do I have to compile an API separately? Do I have to define a class or package as an API? I just want to make sure people won't have errors when attempting to interact with my mod.

Yes, you should create a specific package for your API classes, if any, so that you can declare it as an API by including a package-info.java file:

@API(owner = "yourmodid", provides = "UniqueNameForAPI", apiVersion = "1.8-1.0")
package yourmod.api;

import net.minecraftforge.fml.common.API;

This lets FML / Forge know that this is an API package so that it prioritizes making the classes in it available sooner rather than later when loading mods (at least that's how I understand it - may not be 100% correct).

 

Whether you release that API package separately or include it in your mod is up to you; personally I tend to just include them with my mods, but if you plan on having lots of your own mods using the same API, it may be better to release it separately as a dependency, or you could shade it.

Are you sure it's a .info file? Wouldn't you need to do a .java to use the @API annotation?

Are you sure it's a .info file? Wouldn't you need to do a .java to use the @API annotation?

Sorry, it's named "package-info.java"

I tried creating that, but it says you can't use a dash in your class name.

Silly me, just had to create the file outside of eclipse. Thanks!

One more question though, what if someone implements my API classes, and someone using that mod does not have my mod loaded? Wouldn't that crash the game?

If people want to use your API as an optional option, they can use the @Optional annotation so forge knows not to crash when your API is missing. So no need to worry about that.

Alright, would they put that annotation over their Item or Block class? (I created the weight limit mod , so they have to implement my interface in order to provide items with weight).