Detection
I was working on my first mod recently, and needed to detect another one (CC: Tweaked) for the purpose of loading optional integration with it.
I only encountered threads on this forum which were very old and used an API which no longer exists in Forge to accomplish this.
So, for posterity, the correct thing to use to detect another mod is: ModList.get().isLoaded("mod_id_goes_here")
ModList is a class in the net.minecraftforge.fml package, and can be imported like: import net.minecraftforge.fml.ModList;
Integration
To actually integrate with another mod, it seems like you'll want to run some code which uses classes from the other mod.
To do this without introducing a hard dependency on that mod (i.e., to retain the ability to run without it being there), you need to split the code which requires it out into its own class, and then load that class using Class.forName("YourClass") and avoid directly using that class. Instead, dynamically lookup the entry point method for your integration with something like your_class.getDeclaredMethod("yourEntryPoint") and invoke it with invoke.
A complete loading side example might look something like:
import net.minecraftforge.fml.ModList;
import java.lang.reflect.InvocationTargetException;
if (ModList.get().isLoaded("other_mod_id")) {
try {
var yourClass = Class.forName("YourIntegrationClass");
var yourMethod = yourClass.getDeclaredMethod("loadIntegration");
yourMethod.invoke(null, new Object[0]);
} catch (ClassNotFoundException e) {
// fail out in your preferred way, for the case where the integration class somehow isn't in your mod jar
} catch (NoSuchMethodException e) {
// fail out in your preferred way, for the case where the method named here doesn't exist. possibly a typo on your part.
} catch (InvocationTargetException e) {
// fail out in your preferred way, for the case where the method called throws its own exception
}
// okay, I didn't feel like listing all of the failure cases out like that.
// there are additional exceptions to handle here. not all are mandatory, but here's the remaining as an unordered list:
// - LinkageError
// - ExceptionInInitializerError
// - NullPointerException
// - SecurityException
// - IllegalAccessException
// - IllegalArgumentException
}
