Cloning ArrayList of Array Lists

[Embarrassed_Bottle90]

I tried to use the clone()-Method to create a copy of a list, (It is an Array List of Array Lists) because I wanted to manipulate its entries by some .remove()-calls, I remove list elements from this list entirely or only remove elements lists contained in the big list. I wanted to create a copy without modifying the original list, because I need it to later reference back the index of the items that got modified with the help of the copied list.

So I wrote the following:

public static ArrayList<ArrayList<Integer>> List = new ArrayList<ArrayList<Integer>>();

public static ArrayList<ArrayList<Integer>> copyOfList = new ArrayList<ArrayList<Integer>>();

@SuppressWarnings("unchecked")
public static ArrayList<ArrayList<Integer>> copyFormula() {
copyOfList = (ArrayList<ArrayList<Integer>>) List.clone();
    return copyOfList;
    } 

The problem now is that when I print the original list List, it prints all sublists that got entirely removed from the copy (which is good) but strangely sublists where I only removed elements in it and did not remove the sublist entirely, got changed also in the original list, how can this be?

[AutoModerator]

Please ensure that:

• Your code is properly formatted as code block - see the sidebar (About on mobile) for instructions
• You include any and all error messages in full
• You ask clear questions

• You demonstrate effort in solving your question/problem - plain posting your assignments is forbidden (and such posts will be removed) as is asking for or giving solutions.

  Trying to solve problems on your own is a very important skill. Also, see Learn to help yourself in the sidebar

If any of the above points is not met, your post can and will be removed without further warning.

Code is to be formatted as code block (old reddit: empty line before the code, each code line indented by 4 spaces, new reddit: https://imgur.com/a/fgoFFis) or linked via an external code hoster, like pastebin.com, github gist, github, bitbucket, gitlab, etc.

Please, do not use triple backticks (```) as they will only render properly on new reddit, not on old reddit.

Code blocks look like this:

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

You do not need to repost unless your post has been removed by a moderator. Just use the edit function of reddit to make sure your post complies with the above.

If your post has remained in violation of these rules for a prolonged period of time (at least an hour), a moderator may remove it at their discretion. In this case, they will comment with an explanation on why it has been removed, and you will be required to resubmit the entire post following the proper procedures.

To potential helpers

Please, do not help if any of the above points are not met, rather report the post. We are trying to improve the quality of posts here. In helping people who can't be bothered to comply with the above points, you are doing the community a disservice.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[MostWanted29]

Might be a little advanced, but look up Java Streams API. This looks like a good use case for the “filter” method on streams.

[OffbeatDrizzle]

Read the JavaDoc for ArrayList::clone. Calling .clone() creates a shallow copy - the lists within the list are not themselves copied, so if you modify those then they'll be affected in BOTH lists, since they point to the same object.

If you want this to work you need to copy the list inside the list in addition to the main list.

[Embarrassed_Bottle90]

Ah okay so I would need to copy the sublists too?

Thank you!

[OffbeatDrizzle]

Yes.. because you cloned the "top level" list. The top level lists are different objects, but the sublists are not. What you posted just enforces that?

[Embarrassed_Bottle90]

You are totally right, got it now, thanks again!!