Arraylist constructor not creating new object reference

Question

So when I insert an Arraylist A into an Arraylist constructor I expect a new Arraylist object B created with the same set of objects as A. In my case I have

Arraylist<Arraylist<Integer>> powerSet
Arraylist<Arraylist<Integer>> OG

Both powerSet and OG seem to share the same reference despite OG being constructed from powerSet like this:

ArrayList<ArrayList<Integer>> OG  = new ArrayList<>(powerSet);

Here is the full code:

public static ArrayList<ArrayList<Integer>> generatePower (ArrayList<Integer> s){
    ArrayList<ArrayList<Integer>> powerSet = new ArrayList<>();
    generatePower(s,powerSet);
    return powerSet;
}
public static void generatePower(ArrayList<Integer> s,ArrayList<ArrayList<Integer>> powerSet ){
    if(s.size()==0){
        powerSet.add(s);
        return;
    }
    else{
        int temp = s.remove(0);
        generatePower(s,powerSet);
        ArrayList<ArrayList<Integer>> OG  = new ArrayList<>(powerSet);
        for(ArrayList<Integer> el: OG){
            el.add(temp); //for some reason any changes I make to OG here is
                         //reflected in the powerSet
        }
        powerSet.addAll(OG);

    }
}

Why do OG and powerSet have the same reference and how do I make OG be a new arraylist containing all of powerSet's elements without have OG share powerSet's reference

shmosel · Accepted Answer · 2017-08-11 22:21:23Z

1

The ArrayList constructor doesn't clone elements; it just copies references over. Here's one way to make a deep copy using streams

List<List<Integer>> copy = powerSet.stream()
        .map(ArrayList::new)
        .collect(Collectors.toList());

answered Aug 11, 2017 at 22:21

shmosel

51.1k7 gold badges81 silver badges148 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John Tribe · Accepted Answer · 2017-08-11 22:21:02Z

0

ArrayList<ArrayList<Integer>> OG = new ArrayList<>(powerSet);

this line pass all elements (copy references) stored in powerSet to OG.

It's work in the same way like example:

List<User> usersList_1 = Arrays.asList(user1, user2, user3);
List<User> usersList_2 = new ArrayList(userList_1);

user1.setName("John");

userList_2.get(0).getName();

output: John

answered Aug 11, 2017 at 22:21

John Tribe

1,68219 silver badges28 bronze badges

Collectives™ on Stack Overflow

Arraylist constructor not creating new object reference

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related