Converting List<List<Integer>> to ArrayList<int[]>

Question

So i'm creating a method that looks like this:

public static ArrayList<int[]> permuteArray(int[] array)

I have a helper method that looks like this:

  public static List<List<Integer>> permuteArray(List<List<Integer>> list, List<Integer> result, int [] arr) {
    if(result.size() == arr.length){
        list.add(new ArrayList<>(result));
    }
    else{
        for(int i = 0; i < arr.length; i++){
            if(result.contains(arr[i]))
            {
                continue;
            }
            result.add(arr[i]);
            permuteArray(list, result, arr);
            result.remove(result.size() - 1);
        }

    }
    return list;
}

I have this line : List<List<Integer>> permute = permuteArray(list, new ArrayList<>(), array); But i want to convert the List<List<Integer>> to ArrayList<int[]>. Is this possible for the helper method to return ArrayList<int[]> or if the original method can?

Are you asking if the helper method can directly return an ArrayList<int[]> instead of List<List<Integer>>? It probably can. It would be easier to answer if you included the code of that method. — Eran
– Eran, Commented Jul 12, 2020 at 5:57
practicing with array-list and the prompt wants me to return an ` ArrayList<int[]>` — Fire assassin
– Fire assassin, Commented Jul 12, 2020 at 6:04

deadshot · Accepted Answer · 2020-07-12 06:13:46Z

4

Try this:

List<List<Integer>> lst = new ArrayList<>();
lst.add(List.of(1, 2, 3));
lst.add(List.of(4, 5, 6));
lst.add(List.of(7,8,9));

ArrayList<int[]> newList = lst.stream()
        .map(x -> x.stream().mapToInt(k -> k).toArray())
        .collect(Collectors.toCollection(ArrayList::new));

answered Jul 12, 2020 at 6:13

deadshot

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Rubydesic · Accepted Answer · 2020-07-12 06:14:00Z

2

While modifying the original method might be the better way to go, this is the answer to the title, which is 'Converting List<List<Integer>> to ArrayList<int[]>'

List<List<Integer>> original = ...;
ArrayList<int[]> arraylist = original.stream()
    .map(sub -> sub.stream().mapToInt(i -> i).toArray())
    .collect(Collectors.toCollection(ArrayList::new));

answered Jul 12, 2020 at 6:14

Rubydesic

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Vignesh T I · Accepted Answer · 2020-07-12 13:54:35Z

0

You can also try using the below to convert List<List> to ArrayList<int[]>,

list.stream().map(x -> x.stream().mapToInt(Integer::intValue).toArray())
                .collect(Collectors.toCollection(ArrayList::new));

answered Jul 12, 2020 at 13:54

Vignesh T I

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