如何在Java的ArrayList中存储很多数组？

Question

I have a function that through recursion prints every combination of numbers of an input array.

public class comb {
    public static void main(String[] args)

        List<int[]> list = new ArrayList<int[]>();


        int[] test = {1,2,3,4};
        combinations(test,2,0,new int[2]);

        listToString(list);
    }


    static void combinations(int[] arr, int n, int pos, int[] result){
        if(n == 0){
            System.out.println(Arrays.toString(result));
            return;
        }
        for(int i = pos; i <= arr.length - n; i++){
            result[result.length - n] = arr[i];
            combinations(arr,n-1,pos+1,result);
        }
    }

Outputs

[1, 2]
[1, 3]
[1, 4]
[2, 2]
[2, 3]
[2, 4]
[3, 2]
[3, 3]
[3, 4]

What should I do if instead of having each array printed, I want it added to a list? I can't just add result to the list because it is constantly changing.

Answer 1

There's multiple ways you might achieve it...

You could...

Make List<int[]> list = new ArrayList<int[]>(); a static global

public class comb {
    static List<int[]> list = new ArrayList<int[]>();
    public static void main(String[] args) {

Then you could just add the values to the ArrayList ...

static void combinations(int[] arr, int n, int pos, int[] result) {
    if (n == 0) {
        list.add(result);
        return;
    }

But this is generally a bad idea, is sloppy programming and leads to bad habits

You could...

Pass a reference of the List to the combinations method...

static void combinations(int[] arr, int n, int pos, int[] result, List<int[]> list) {
    if (n == 0) {
        list.add(Arrays.copyOf(result, result.length));
        return;
    }

This is better, as it makes the API re-usable and capable of been called multiple times within the same instance of the program without otherwise destroying what was in the List , like the previous example

You could...

Have the combinations method return a List ...

static List<int[]> combinations(int[] arr, int n, int pos, int[] result) {
    List<int[]> list = new ArrayList<>(25);
    if (n == 0) {
        list.add(Arrays.copyOf(result, result.length));
    } else {
        for (int i = pos; i <= arr.length - n; i++) {
            result[result.length - n] = arr[i];
            list.addAll(combinations(arr, n - 1, pos + 1, result));
        }
    }
    return list;
}

This is kind of cool in the fact that you don't need to care about how the List was created, only that it will return all combinations of the arr to your specifications. It's probably not as efficient as the previous example, as you are creating a new List on each recursion, but does hide the implementation details

Answer 2

You can copy the array, and add the copy to the list.

static void combinations(int[] arr, int n, int pos, int[] result, List<int[]> allResults){
    if(n == 0){
        allResults.add(Arrays.copyOf(result));
        return;
    }
    // etc...