RFR: 6356745: Introduce constructor for PriorityQueue with existing collection and custom comparator [v2]

[Alan Bateman]

On Thu, 14 Dec 2023 20:36:50 GMT, Valeh Hajiyev <duke at openjdk.org> wrote:

>> This commit addresses the current limitation in the `PriorityQueue` implementation, which lacks a constructor to efficiently create a priority queue with a custom comparator and an existing collection. In order to create such a queue, we currently need to initialize a new queue with custom comparator, and after that populate the queue using `addAll()` method, which in the background calls `add()` method (which takes `O(logn)` time) for each element of the collection (`n` times).  This is resulting in an overall time complexity of `O(nlogn)`. 
>> 
>> 
>> PriorityQueue<String> pq = new PriorityQueue<>(customComparator);
>> pq.addAll(existingCollection);
>> 
>> 
>> The pull request introduces a new constructor to streamline this process and reduce the time complexity to `O(n)`.  If you create the queue above using the new constructor, the contents of the collection will be copied (which takes `O(n)` time) and then later  `heapify()` operation (Floyd's algorithm) will be called once (another `O(n)` time). Overall the operation will be reduced from `O(nlogn)` to `O(2n)` -> `O(n)` time.
>> 
>> 
>> PriorityQueue<String> pq = new PriorityQueue<>(existingCollection, customComparator);
>
> Valeh Hajiyev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   updated the javadoc

src/java.base/share/classes/java/util/PriorityQueue.java line 215:

> 213:      * Creates a {@code PriorityQueue} containing the elements in the
> 214:      * specified collection. The {@code PriorityQueue} will order its
> 215:      * elements according to the specified comparator.

The wording "will order its elements" is consistent with the other constructors but I wonder if "its" might be mis-read to mean the collection rather than the queue. If you change it to "will order the priority queue's elements" will avoid that.

You'll need to add `@since 23`. Also new tests will need to be added for this constructor.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17045#discussion_r1429103333