How to copy elements from an ArrayList to another one NOT by reference?

Question

I'm trying to copy each element from one ArrayList (av) to another one (copia). The thing is that they're copied by reference, so whenever I make any changes in the original one, the copy is modified as well. Of course, this behavior is not wanted. How should I write this method?

public void copiarArrayList(ArrayList<Articulo_Venta> copia, ArrayList<Articulo_Venta> av){
    copia.clear();
    for (int i = 0; i < av.size(); i++) {
        copia.add(av.get(i));
    }
}

Articulo_Venta has these fields:

int codigo;
String nombre;
float cantidad;

PS: I also tried the next:

copia = new ArrayList<Articulo_Venta>(av);

but it still has its elements pointing to the original ArrayList.

score 11 · Accepted Answer · edited May 23 '17 at 12:08

11

What you want is the deep copy. If your object contains only primitive you could use clone(), otherwise best way is to do manually:-

Make a constructor in your Articulo_Venta class which takes another Articulo_Venta object and initializes member variables.

Then change the code as:-

public void copiarArrayList(ArrayList<Articulo_Venta> copia, ArrayList<Articulo_Venta> av){
        copia.clear();
        for (int i = 0; i < av.size(); i++) {
            copia.add(new Articulo_Venta(av.get(i)));
        }

Also read here - how-do-you-make-a-deep-copy-of-an-object-in-java

edited May 23 '17 at 12:08

Community

1
1

answered Dec 30 '16 at 06:33

Mangat Rai Modi

5,397
8
45
75

That was it! It's quite annoying having to do this, but the solution is to copy each field of the object individually into a new object, and to add it to the new ArrayList. Thanks and happy New Year! – Ezequiel Berto Dec 30 '16 at 17:46
1

you save my life :) – Masoud Mokhtari Jun 15 '19 at 05:40

Faraz · Answer 2 · 2016-12-30T06:46:10.190

1

Cloning the objects before adding them. For example, instead of newList.addAll(oldList);

for(Articulo_Venta av : oldList) {
    newList.add(av.clone());
}

clone should be correctly overriden in Articulo_Venta.

This is how you do it.

public class Articulo_Venta {

String a;  //assuming you have these fields, then
Date d;
...

public Articulo_Venta clone(){
    Articulo_Venta  av = new Articulo_Venta();
    av.a = this.a.clone();
    av.d = this.d.clone();
    ...
    return av;
}
}

edited Dec 30 '16 at 06:46

answered Dec 30 '16 at 06:32

Faraz

6,025
5
31
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But what if av contains more references to other objects, won't clone copy them all? – Mangat Rai Modi Dec 30 '16 at 06:35
Try not to override clone in Java:-http://stackoverflow.com/questions/270369/what-have-you-used-object-clone-for – Mangat Rai Modi Dec 30 '16 at 06:42
@MangatRaiModi yeah I know you cannot clone primitives. Thats why I didn't include any primitive in my example :P – Faraz Dec 30 '16 at 06:43

score 1 · Answer 3 · answered Dec 30 '16 at 07:29

1

Create a new constructor in your class Articulo_Venta.

public Articulo_Venta(int codigo, String number, float candidad)
{
  this.codigo = codigo;
  this.number = number;
  this.candidad = candidad;
}

public void copiarArrayList(List<Articulo_Venta> copia, List<Articulo_Venta> av)
{
   av.stream().forEach(t -> {
     Articulo_Venta newObj = new Articulo_Venta(t.getCodigo(), t.getNumber(), t.getCandidad());
     copia.add(newObj);
   });
}

answered Dec 30 '16 at 07:29

Nikita Mantri

142
6

This would work too, but it's a little simpler to make a constructor with Articulo_Venta as argument. – Ezequiel Berto Dec 30 '16 at 17:50

How to copy elements from an ArrayList to another one NOT by reference?

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