could I get an explanation for what this method signature is doing?
AbstractRoad<T extends AbstractRoad.Builder<T>>
It looks like the type parameter is an extension of the Builder... meaning it is expecting any type that is or extends the Builder Subclass
ArrayList<Inventory<T extends Item>>
This is an array list of inventories containing items or objects that extend item... i.e.
Public class Potion extends Item
would be a valid item in the inventory... T is a generics reference of essentially Object. Please see this link: Generic Types