Deep copy of an ArrayList in JAVA

Question

I checked other answers but I could not find a proper answer to my question. I want to create a copy of my ArrayList<ArrayList>, since I need the original one somewhere else. I used the .clone() method in different ways:

public class WordChecker {

    private ArrayList<ArrayList> copyOfList = new ArrayList<ArrayList>();

    public WordChecker(ArrayList<ArrayList> list) {
        for (int i = 0; i < list.size(); i++)
            for (int j = 0; j < 7; j++)
                copyOfList = (ArrayList<ArrayList>) list.clone(); // without error
                // copyOfList = list.clone();cannot convert from Object to
                // ArrayList<ArrayList>
                // copyOfList = list.get(i).clone();cannot convert from Object to
                // ArrayList<ArrayList>
                // copyOfList = list.get(i).get(j).clone();
    }

but still my main ArrayList changes as I work on its copy. Could anybody tell me how I should get a deep copy in this case?

Answer: I put the copying mechanism in my class constructor:

private ArrayList<List> checkedTags = new ArrayList<List>();
public WordChecker(ArrayList<ArrayList> list)
  {
     for (ArrayList word: list) copyOfList.add((ArrayList) word.clone());

}

the only problem is that this is not applicable to copy from ArrayList which made me to go through a for loop use .get() method.I feel they are basically the same at the end.

Don't use clone(). Use the ArrayList constructor that takes another collection as argument. And you'll need to loop if you want a deep copy. — JB Nizet, Mar 27 '14 at 10:32
Take a look here: http://stackoverflow.com/questions/715650/how-to-clone-arraylist-and-also-clone-its-contents — Robert Niestroj, Mar 27 '14 at 10:33
I made a nested loop to reach to every item in the inner ArrayList. it throws the error mentioned in front of the code. I used this code: >> for (ArrayList word: list) copyOfList.add(word.clone())... too. but it cannot accept .add . — msc87, Mar 27 '14 at 11:02

sanbhat · Accepted Answer · 2014-03-27T10:41:01.917

-1

You can simply use ArrayList(Collection c) constructor

public <T> ArrayList<ArrayList<T>> deepCopy(ArrayList<ArrayList<T>> source) {
    ArrayList<ArrayList<T>> dest = new ArrayList<ArrayList<T>>();
    for(ArrayList<T> innerList : source) {
        dest.add(new ArrayList<T>(innerList));
    }
    return dest;
}

Caution: As mentioned by @Tim B this does not deep copy the elements within ArrayList

edited Mar 27 '14 at 10:41

answered Mar 27 '14 at 10:33

sanbhat

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Be aware that while this will deep copy the lists it will not deep copy the items within the lists. – Tim B Mar 27 '14 at 10:34
I did as I edited my post. I am not really good at using generic methods...:D – msc87 Mar 28 '14 at 15:54
If it does not deep copy the elements of list then its not deep copy. – DoOrDoNot Jul 18 '16 at 23:28
@Vatsal I have already mentioned this in my answer.. am I missing anything else ? – sanbhat Jul 19 '16 at 11:52

Deep copy of an ArrayList in JAVA

1 Answers1