RFR: 8266571: Sequenced Collections

[Stuart Marks]

On Wed, 5 Oct 2022 13:30:08 GMT, Viktor Klang <duke at openjdk.org> wrote:

>> PR for Sequenced Collections implementation.
>
> src/java.base/share/classes/java/util/LinkedHashMap.java line 384:
> 
>> 382:             return this.put(k, v);
>> 383:         } finally {
>> 384:             putMode = PUT_NORM;
> 
> @stuart-marks Would it be an alternative to have an  `internalPut(mode, k, v)` so there is no need to have an internal variable which needs to be read/written multiple time per operation? 🤔

Yeah, the coupling here is rather distasteful. (Otherwise known as a quick and dirty hack.) Unfortunately the coupling between HashMap and LinkedHashMap is pretty special-purposed for exactly the intended usage modes (insertion-order and access-order). It could be improved, but it would probably require some refactoring in HashMap, which I didn't want to do right now, in order to keep the sequenced stuff separate.

> src/java.base/share/classes/java/util/SequencedCollection.java line 155:
> 
>> 153:      */
>> 154:     default E getLast() {
>> 155:         return this.reversed().iterator().next();
> 
> @stuart-marks Are these default implementation expected to be used (actually) in the JDK? From a performance PoV, it might make sense to not have default implementations unless strictly needed, and instead keep the code in the JavaDoc as a guideline for "worst-case" performance profile. 🤔

Good question. They might not actually be used in the JDK... but it isn't obvious! In fact some of these methods are used in the `sequencedValues()` view of `SequencedMap`. They could be used if there is a `SortedMap` implementation that doesn't implement `NavigableMap`. I don't think there are any of these in the JDK, but there are "in the wild." Yes, there are potentially performance issues with these, but I think it's valuable to have a complete set of default implementations available.

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PR Review Comment: https://git.openjdk.org/jdk/pull/7387#discussion_r1012436476
PR Review Comment: https://git.openjdk.org/jdk/pull/7387#discussion_r1012418175