What is the fastest way to change the order of elements in an ArrayList?

Question

I have a first ArrayList:

ArrayList moves = new ArrayList<>();

and want to reorder the elements randomly into a second one:

ArrayList randomMoves = new ArrayList<>();

Normally I do:

    while (randomMoves.size() < moves.size()) {
    
        int index = (int) (Math.random() * moves.size());

        while (randomMoves.contains(moves.get(index))) {

            index = (int) (Math.random() * moves.size());
        }
        
        randomMoves.add(moves.get(index));
    }

but it goes without saying that this is a performance disaster (the last elements will take time to get chosen randomly)...

normally this is not so much of an issue, but this code is going to be executed in a time-critical part of the application and I don't have a lot experience with time-critical code...

enlighten me :)

S.

user_10819573 · Answer

You can use Collections.shuffle(List list) to shuffle the elements in the list.

ArrayList randomMoves = new ArrayList<>(moves);
Collections.shuffle(randomMoves);

The first line of the code creates an ArrayList randomMoves containing the elements of moves and the second line of the code shuffles the elements in randomMoves.

What is the fastest way to change the order of elements in an ArrayList?

1 Answers