CopyOnWriteArrayList Collection.shuffle

[Jason Mehrens]

CopyOnWriteArrayList implements an efficient List::replaceAll so you could do something like:

====
public static void main(String[] args) {
	List<String> cowl = new CopyOnWriteArrayList<>(new String[]{"1","2","3"});
	List<String> copy = Arrays.asList(cowl.toArray(new String[0]));
	Collections.shuffle(copy);
	Iterator<String> it = copy.iterator();	
	try {
		cowl.replaceAll(e -> it.next());
	} catch (NoSuchElementException nsee) {
		throw new ConcurrentModificationException();
	}
		
	if (it.hasNext()) {
		throw new ConcurrentModificationException();
	}

	System.out.println(cowl);
}
====

Some of the non-random access branches in Collections.java could be updated to use List::replaceAll over ListIterator next/set.  Once Collections.java was using List::replaceAll you could then just wrap CopyOnWriteArrayList in a non-random access list.

Jason

________________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Zelva Lia <jolyjdia at gmail.com>
Sent: Friday, August 19, 2022 5:49 AM
To: core-libs-dev at openjdk.org
Subject: CopyOnWriteArrayList Collection.shuffle

Hello, when shuffling the CopyOnWrite list with the standard Collections.shuffle method, performance anomalies occur, due to the fact that it calls the set method, which copies the array each time, a possible solution (crutch) is a random comparator for sorting, so sorting in COW is redefined to its own sub - blocking implementation

Another problem with Collections.shuffle is that it's not exactly thread safe because it calls the size() method; and then iterates from it, also COW does not support modification inside the iterator (works on snapshots)


COWCollectionsShuffle      0,008 ops/ms
COWListRandomSort         1,089 ops/ms
syncListCollectionsShuffle  0,950 ops/ms