RFR: JDK-8170945: Collectors$Partition should override more Map methods

[Viktor Klang]

On Wed, 5 Apr 2023 12:49:31 GMT, Tagir F. Valeev <tvaleev at openjdk.org> wrote:

>> Adds overrides for common Map operations to avoid having to allocate the entrySet for Collectors$Partition maps.
>
> Another thing I noticed is that `keySet()` of this map is a constant. If we want to preserve the order, we can use something like this:
> 
> 
>     @Override
>     public Set<Boolean> keySet() {
>       return KeySet.INSTANCE;
>     }
> 
>     private static class KeySet extends AbstractSet<Boolean> {
>       private static final KeySet INSTANCE = new KeySet();
>       
>       public Iterator<Boolean> iterator() {
>         return List.of(false, true).iterator();
>       }
> 
>       public int size() {
>         return 2;
>       }
> 
>       @Override
>       public int hashCode() {
>         // Boolean.hashCode(true) + Boolean.hashCode(false)
>         return 1237 + 1231;
>       }
> 
>       @Override
>       public boolean equals(Object o) {
>         return o instanceof Set<?> s &&
>                 s.size() == 2 &&
>                 s.contains(false) && s.contains(true);
>       }
> 
>       public boolean isEmpty() {
>         return false;
>       }
> 
>       public boolean contains(Object k) {
>         return k instanceof Boolean;
>       }
> 
>       @Override
>       public void forEach(Consumer<? super Boolean> action) {
>         Objects.requireNonNull(action);
>         action.accept(false);
>         action.accept(true);
>       }
> 
>       @Override
>       public String toString() {
>         return "[false, true]";
>       }
>     }
> 
> 
> Overengineering, probably? On the other hand, it might be useful to expose this set somewhere, like `Collections.booleans()`? This would allow using `for(boolean b: Collections.booleans()) {...}`. In our codebase, I see quite many loops like `for (boolean b : new boolean[]{false, true})`. Declaring it as a `SequencedSet` would be even more useful, as one may control the order via `Collections.booleans().reversed()`.

@amaembo I think ideally the entire Partition class could be replaced in favor of some `OrderedMap.of(Boolean.TRUE, forTrue, Boolean.FALSE, forFalse)` version. I'll let @stuart-marks chime in if there's other options on the table here. :)

-------------

PR Comment: https://git.openjdk.org/jdk/pull/13347#issuecomment-1497483295