RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v6]

[Per Minborg]

On Mon, 17 Mar 2025 01:58:55 GMT, Luca Kellermann <duke at openjdk.org> wrote:

> > I see that, probably due to prior `java.util` contracts, a stable list or map cannot present a `toString` with unset component values.  A stable list or map uses a “canned” `toString` method that calls `get`, which must force all component values to be evaluated before the `toString` can be printed.
> 
> I also noticed this issue of `toString` eagerly setting all elements of stable collections and agree that it probably shouldn't do this. Note that all views of these collections (obtained via `List.subList`, `List.reversed`, `Map.entrySet`, `Map.values`, etc.) would also need their own `toString` implementation.
> 
> > Just as `WeakHashMap` bends the `Map` API (regarding `equals`), I think `StableValue` composites should bend the `List` and `Map` APIs, regarding `toString`.  Sometimes the contracts have to be bent for the whole design to fit together.
> 
> Neither `List`, `Set`, nor `Map` mention any requirements for `toString` in their interface specification. Only `AbstractCollection` and `AbstractMap` have a default implementation of `toString`. So I don't think any contract would be bent here.

I think requiring the `toString()` method of `StableList::subList` and `StableList::reversed` is a bit too much, at least in the first incarnation. However, I have noted this as a candidate in future implementations.

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PR Comment: https://git.openjdk.org/jdk/pull/23972#issuecomment-2765490546