List.copyOf already does what you want. https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/List.java#L1068 https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/ImmutableCollections.java#L168
Paul. > On Aug 23, 2022, at 4:49 PM, Ethan McCue <et...@mccue.dev> wrote: > > Hi all, > > I am running into an issue with the collections framework where I have to > choose between good semantics for users and performance. > > Specifically I am taking a java.util.List from my users and I need to choose > to either > * Not defensively copy and expose a potential footgun when I pass that List > to another thread > * Defensively copy and make my users pay an unnecessary runtime cost. > > What I would really want, in a nutshell, is for List.copyOf to be a no-op > when used on lists made with List.of(). > > Below the line is a pitch I wrote up on reddit 7 months ago for a mechanism I > think could accomplish that. My goal is to share the idea a bit more widely > and to this specific audience to get feedback. > > https://www.reddit.com/r/java/comments/sf8qrv/comment/hv8or92/?utm_source=share&utm_medium=web2x&context=3 > > > Important also for context is Ron Pressler's comment above. > -------------- > > What if the collections api added more marker interfaces like RandomAccess? > > It's already a common thing for codebases to make explicit null checks at > error boundaries because the type system can't encode null | List<String>. > > This feels like a similar problem. > If you have a List<T> in the type system then you don't know for sure you can > call any methods on it until you check that its not null. In the same way, > there is a set of methods that you don't know at the type/interface level if > you are allowed to call. > > If the List is actually a __ > Then you can definitely call > And you know other reference holders might call > And you can confirm its this case by > null > no methods > no methods > list == null > List.of(...) > get, size > get, size > ??? > Collections.unmodifiableList(...) > get, size > get, size, add, set > ??? > Arrays.asList(...) > get, size, set > get, size, set > ??? > new ArrayList<>() > get, size, add, set > get, size, add, set > ??? > While yes, there is no feasible way to encode these things in the type > system. Its not impossible to encode it at runtime though. > interface FullyImmutable { > // So you know the existence of this implies the absence > // of the others > default Void cantIntersect() { return null; } > } > > interace MutationCapability { > default String cantIntersect() { return ""; } > } > > interface Addable extends MutationCapability {} > interface Settable extends MutationCapability {} > > If the List is actually a __ > Then you can definitely call > And you know other reference holders might call > And you can confirm its this case by > null > no methods > no methods > list == null > List.of(...) > get, size > get, size > instanceof FullyImmutable > Collections.unmodifiableList(...) > get, size > get, size, add, set > !(instanceof Addable) && !(instanceof Settable) > Arrays.asList(...) > get, size, set > get, size, set > instanceof Settable > new ArrayList<>() > get, size, add, set > get, size, add, set > instanceof Settable && instanceof Addable > In the same way a RandomAccess check let's implementations decide whether > they want to try an alternative algorithm or crash, some marker "capability" > interfaces would let users of a collection decide if they want to clone what > they are given before working on it. > > > -------------- > > So the applicability of this would be that the list returned by List.of could > implement FullyImmutable, signifying that there is no caller which might have > a mutable handle on the collection. Then List.of could check for this > interface and skip a copy. > >
