When should I use the JAVA generic interface design?

I have some interfaces that I intend third-parties to implement in the future, and I provide a base implementation myself. I'll only be using a couple to show the example.

Currently, they are defined as

Item:

public interface Item {
    String getId();
    String getName();
}
ItemStack:
public interface ItemStackFactory {
    ItemStack createItemStack(Item item, int quantity);
}
ItemStackContainer:
public interface ItemStackContainer {
    default void add(ItemStack stack) {
        add(stack, 1);
    }
    void add(ItemStack stack, int quantity);
}

Now, Item and ItemStackFactory I can absolutely foresee some third-party needing to extend it in the future. ItemStackContainer could also be extended in the future, but not in ways that I can foresee, outside my provided default implementation.

Now, I'm trying to make this library as robust as possible; this is still in the early-stages (pre-pre-alpha) so this may be an act of over engineering (YAGNI). Is this an appropriate place to use generics?