I think, in most cases, if you have n items in memory, and you want to find m of them by id, or some such thing, your default should be to represent the n items

I think, in most cases, if you have n items in memory, and you want to find m of them by id, or some such thing, your default should be to represent the n items as a hashmap, not a list, and look them up from the hashmap. There will be cases where this isn’t the right choice, but it’s a good default. And it’s almost always virtually no extra complexity to represent them this way, i.e. often something as simple as: myMap = myList.groupBy(_.id) Don’t write complex optimizations until you know you need them, but I think defaulting to code with good O(N) complexity, where it’s simple to do so, is a good default.