RFR: 8293630: Simplify TreeMap.get() with Comparator.naturalOrder()

[ExE Boss]

On Sat, 20 Aug 2022 19:17:18 GMT, Сергей Цыпанов <duke at openjdk.org> wrote:

>> src/java.base/share/classes/java/util/TreeMap.java line 3318:
>> 
>>> 3316:             // Adapt or create a key-based comparator
>>> 3317:             Comparator<? super K> treeComparator = tree.comparator;
>>> 3318:             return Map.Entry.comparingByKey(treeComparator == null ? natural() : treeComparator);
>> 
>> You can probably have:
>> 
>> return treeComparator == nul ?
>>         Map.Entry.comparingByKey() :
>>         Map.Entry.comparingByKey(treeComparator);
>> 
>> instead.
>
> Nope, there'd be a compilation error

It'll work fine if an unchecked cast is used:
Suggestion:

            @SuppressWarnings("unchecked")
            Comparator<Map.Entry<K, V>> entryComparator = treeComparator == null ?
                    (Comparator) Map.Entry.comparingByKey()
                    Map.Entry.comparingByKey(treeComparator);
            return entryComparator;


This also ensures that when `treeComparator` is `null`, this method keeps returning a constant lambda.

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PR: https://git.openjdk.org/jdk/pull/9901