A use case that doesn't cover is adding to a collection. Say as part of a method's contract you state that you take ownership of a List. You aren't going to copy even if the list is mutable.
Later on, you may want to add to the list. Add is supported on ArrayList so you don't need to copy and replace your reference, but you would if the list you were given was made with List.of or Arrays.asList On Wed, Aug 24, 2022, 8:13 AM John Hendrikx <john.hendr...@gmail.com> wrote: > Would it be an option to not make the receiver responsible for the > decision whether to make a copy or not? Instead put this burden (using > default methods) on the various collections? > > If List/Set/Map had a method like this: > > List<T> immutableCopy(); // returns a (shallow) immutable copy if > list is mutable (basically always copies, unless proven otherwise) > > Paired with methods on Collections to prevent collections from being > modified: > > Collections.immutableList(List<T>) > > This wrapper is similar to `unmodifiableList` except it implements > `immutableCopy` as `return this`. > > Then for the various scenario's, where `x` is an untrusted source of List > with unknown status: > > // Create a defensive copy; result is a private list that cannot be > modified: > > List<T> y = x.immutableCopy(); > > // Create a defensive copy for sharing, promising it won't ever > change: > > List<T> y = Collections.immutableList(x.immutableCopy()); > > // Create a defensive copy for mutating: > > List<T> y = new ArrayList<>(x); // same as always > > // Create a mutable copy, modify it, then expose as immutable: > > List<T> y = new ArrayList<>(x); // same as always > > y.add( <some element> ); > > List<T> z = Collections.immutableList(y); > > y = null; // we promise `z` won't change again by clearing the only > path to mutating it! > > The advantage would be that this information isn't part of the type system > where it can easily get lost. The actual implementation knows best whether > a copy must be made or not. > > Of course, the immutableList wrapper can be used incorrectly and the > promise here can be broken by keeping a reference to the original (mutable) > list, but I think that's an acceptable trade-off. > > --John > > PS. Chosen names are just for illustration; there is some discussion as > what "unmodifiable" vs "immutable" means in the context of collections that > may contain elements that are mutable. In this post, immutable refers to > shallow immutability . > On 24/08/2022 03:24, Ethan McCue wrote: > > Ah, I'm an idiot. > > There is still a proposal here somewhere...maybe. right now non jdk lists > can't participate in the special casing? > > On Tue, Aug 23, 2022, 9:00 PM Paul Sandoz <paul.san...@oracle.com> wrote: > >> 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. >> > >> > >> >>
