He asks for a simpler example. How about a collections api? Start with array, then linked lists, then functioms like contains and filter and which need an itera

He asks for a simpler example. How about a collections api? Start with array, then linked lists, then functioms like contains and filter and which need an iterable interface, then different types of linked list, then add hashmap where you iterate keys, then different types of map e.g. linked map. This is all data structures 101 material anyway! After this primer, a simple GUI toolkit isn't as far out of reach.