java add new element to an arraylist

Question

I have the following code for the problem "Given a set of distinct integers, return all possible subsets. If nums = [1,2,3] , a solution is [ [3], [1], [2], [1,2,3], [1,3], [2,3], [1,2], [] ] ."

public class Solution {
public ArrayList<ArrayList<Integer>> subsets(int[] num) {
    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
    if(num == null || num.length == 0) {
        return result;
    }
    ArrayList<Integer> list = new ArrayList<Integer>();
    Arrays.sort(num);  
    subsetsHelper(result, list, num, 0);

    return result;
}


private void subsetsHelper(ArrayList<ArrayList<Integer>> result,
ArrayList<Integer> list, int[] num, int pos) {

    result.add(new ArrayList<Integer>(list));

    for (int i = pos; i < num.length; i++) {
        list.add(num[i]);
        subsetsHelper(result, list, num, i + 1);
        list.remove(list.size() - 1);
    }
}

In result.add(new ArrayList<Integer>(list)) why do I need to use new ArrayList<Integer>(list)) ? I know simply using result.add(list) would make the items in the result all the same, but I want to know the reason.

Answer 1

 new ArrayList<Integer>(list) 

This is called a copy constructor , " It is used to create a copy of an existing object of the same class. "

Without the copy constructor each call to result.add(list) would add a reference to the same list repeatedly to result . With the copy constructor a new list is created with the current values of list . That way subsetHelper() can repeatedly modify list without also changing the lists already stored in result .

Make sure you understand how Java passes around method parameters ; it's not complicated, but it is critical to being an effective Java programmer.