CODEWITHSUNDEEP
javacollectionsshuffle
Martin Hansen
Martin Hansen

Reputation: 2101

Collections.shuffle multiple times in a row

dI got a little question about the Collections.shuffle() method.

Case:

I have 2 lists I need to shuffle, then union / merge them into one list, then shuffle the new complete list. I have used the shuffle method with the Random class - using system.nanoTime() as seed.

Code looks as below:

public List<PresentationArticle> shuffleUnionShuffleLists(List<PresentationArticle> list1, List<PresentationArticle> list2) {
    shuffleList(list1);
    shuffleList(list2);

    List<PresentationArticle> resultList = //merge/union the two lists

    shuffleList(resultList);

    return resultList;
}

public void shuffleList(List<PresentationArticle> articleList) {
    long seed = System.nanoTime();
    Collections.shuffle(articleList, new Random(seed));
}

My question is: Will this be a proper random shuffling of the lists, when the methods runs right after eachother, with a new Random object and a new (but almost identical) seed?

The method shuffleUnionShuffleLists() will be run approximately every 3 minutes.

Upvotes: 0

Views: 1307

Answers (3)

Kayaman
Kayaman

Reputation: 73528

First of all, you don't generally create a new Random object for every invocation. It doesn't make it "more random". Secondly, shuffling several times doesn't increase entropy either, so a single shuffle is all that is needed.

Upvotes: 0

Peter Lawrey
Peter Lawrey

Reputation: 533472

The default seed is based on nanoTime and a counter, so it is slightly more random than what you have. Also there is no need to shuffle more than once. Once it is randomized doing it more than once just takes longer.

So in your case, all you need to do is add the two list together and shuffle that using a new Random(), or reusing an old one.

public List<PresentationArticle> shuffleUnionShuffleLists(List<PresentationArticle> list1, List<PresentationArticle> list2) {
    List<PresentationArticle> list = new ArrayList<>(list1.size()+list2.size());
    list.addAll(list1);
    list.addAll(list2);
    Collections.shuffle(list);
    return list;
}

Also it is usually best not to modify arguments passed to you.

Upvotes: 2

OldCurmudgeon
OldCurmudgeon

Reputation: 65793

There's no reason I can think of to shuffle the lists before merging. Randomising/shuffling something twice does not make them any more random.

Will this be a proper random shuffling of the lists

Yes but you are doing too much.

Upvotes: 0

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