"Copy" nested array list

Question

I have a nested ArrayList of the form

ArrayList<ArrayList<PointF>> nestedArraylist

I want to create a "copy" nestedArraylistCopy of nestedArraylist in the following sense:

The elements of nestedArraylistCopyshould be independent copies of the elements in nestedArraylist, i.e. should be ArrayLists holding the references to the same PointF objects in the original nestedArraylist.

• Can I somehow use Collections.copy(dest, src) to do what I want? The documentation is not exactly detailed unfortunately...

• Does the following code do what I want?

  for(int i = 0; i < nestedArraylist.size(); i++) 
      nestedArraylistCopy.add(new ArrayList<PointF>(nestedArraylist.get(i)));
  

• Is there a more efficient and or elegant solution?

Answer 1

Q1: after you Collections.copy, your new List object will contain the same elements as src (assuming the size is the same) which means, it holds same ArrayList<PointF> objects, hence the PointF objects are the same too. If you cannot get the info you want from the api java doc, read the source codes.

Q2: What you did is different from Collections.copy, since your copied arrayList has new Arraylist<PointF> as elements, but they contain the same elements (PointF) as the source list.

Q3: I don't know. because I don't know what do you want to have eventually. All new objects? Only ArrayList should be new objects, or all references?

Answer 2

According to my knowledge your solutions will update only references, as does Collection.copy(). You can use the below method, which I prefer:

 List<ArrayList<PointF>> newList = new ArrayList<ArrayList<PointF>>(oldList);

A change of the old list would not affected to new list.

I tested your second option and it also has the property that changes to the old one will not affect the new List.

Note - These will also update only the references. If you change elements in your new list it will update old list too. I think Your second array will create brand new objects, but I am not 100% sure about that. I am adding my testing code below for you reference.

package test;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by Lasitha Benaragama on 4/28/14.
 */
public class StckTest {

public static void main(String[] args) {
    List<String> oldList = new ArrayList<String>();
    oldList.add("AAAAA");
    oldList.add("BBBBB");
    oldList.add("CCCCC");
    oldList.add("DDDDDD");
    oldList.add("EEEEEE");

    StckTest test = new StckTest();

    List<String> newListCopy = new ArrayList<String>(oldList);
    List<String> newListClone = new ArrayList<String>();

    for(int i = 0; i < oldList.size(); i++)
        newListClone.add(oldList.get(i));

    test.printArray(newListCopy);

    test.changeList(oldList);
    test.printArray(oldList);

    test.printArray(newListCopy);
    test.printArray(newListClone);
}

public void changeList(List<String> oldList) {
    oldList.remove(2);
    oldList.add("FFFFF");
}

public void printArray(List<String> oldList){
    for(int i = 0; i < oldList.size(); i++){
        System.out.print(oldList.get(i)+",");
    }
    System.out.println();
}

}

Answer 3

You can use the clone() method to make a shallow copy of your object.

        ArrayList<ArrayList<String>> nestedArraylist=new ArrayList<ArrayList<String>>();
        ArrayList<String> x=new ArrayList<String>();
        x.add("Deepak");
        nestedArraylist.add(x);
        System.out.println(nestedArraylist);
        ArrayList<ArrayList<String>> nestedArraylistcopy=(ArrayList<ArrayList<String>>)nestedArraylist.clone();
        System.out.println(nestedArraylistcopy);