does iterating over `List asReversed ` construct a copy or i kotlinlang #getting-started

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getting-started

Title

y

y

02/08/2023, 3:32 PM

does iterating over

List.asReversed()

construct a copy, or is it an “efficient” operation?

s

Sam

02/08/2023, 3:32 PM

“asReversed” uses the existing list, as a view. It’s different from “reversed”, which makes a copy.

y

y

02/08/2023, 3:33 PM

cool. so is it correct to say that things that return a view don’t generally copy their elements?

s

Sam

02/08/2023, 3:34 PM

Yes, that’s normally what is meant by a “view”. A useful convention is that function names starting with “as…” return a view, whereas function names starting with “to…” return a copy. Mostly.

y

y

02/08/2023, 3:36 PM

thanks. it’s often difficult for me to tell if an operation would create a copy, so this is a good rule of thumb to remember.

e

ephemient

02/08/2023, 4:48 PM

on immutable objects,

to…()

might not create a copy, but the difference should not be observable (unless you're comparing instances)

y

y

02/08/2023, 4:49 PM

what do you mean by observable in this context? for example, I’d expect

Map<K, V>.toSortedMap()

would certainly have (some) runtime cost

e

ephemient

02/08/2023, 4:52 PM

Map

is not immutable, since

MutableMap

is a subtype

but I didn't mean performance anyway

for example,

String.toString()

doesn't return a copy, it returns the

String

instance itself. it could create a copy, but since you can't change any state in

String

, there is no observable behavior difference whether it's a copy or a view (not counting performance and not counting comparing instances with

===

)

the JVM changed

String.substring

from a view to a copy in Java 7, which changed performance and memory usage characteristics, but again, since

String

is immutable, it's not an observable difference as far as we usually care about

y

y

02/08/2023, 5:03 PM

thank you for the explanation. so this is all about the behavior with regards to copying, not whether or not the implementation is expected to be performant.

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