Is it safe to assume that collection transformation functions like

map

return Lists whose

get

method is O(1)? If so, is that documented?

well,

map

is defined as an extension method to a number of collection types

Right, that one.

I believe the default implementation is to map the results to an

ArrayList

, which gets mapped to the underlying platform-specific equivalent. I don’t think it’s specifically documented, but it’s easy enough to see by looking at the source

which does return a

List

, but neither the Iterable nor List interfaces make any guarantee as to underlying access complexities

I can read the source and I know what it does. A colleague is wondering if it's ok to rely on this if you need the return value to be O(1)

it really depends on what counts as “good enough” as a guarantee in your situation

For a strict guarantee, you should probably use

mapTo(ArrayList())

instead.

Hmm I guess you can add an extension function like

if (this is Random Access) this else ArrayList(this)

I’m not sure how your extension function would work in a general case - there is no

Random Access

interface or abstract type to match upon, but (edit: apparently there is) sure, you could do something like that, and there isn’t a semantically-comparable function available in the stdlib to my knowledge

There is such an interface... Unless it's internal to the stdlib?

Generally, I wouldn’t rely upon constant-time access for anything in a List, regardless of the type. Usually, you’d use a Map for guarantees of random-access time-complexity.

Would you care to enlighten me @David Glasser? I’m not familiar with the interface in question

oh wow, today I learned

Sure, but arrays are for random access by index if your data is usefully ordered!

cool 👍

Me too!

I suppose

new IndexOutOfBoundsException()

is a

O(1)

operation 🤔