Unable to change elements in a nested ArrayList

Question

I have an ArrayList

in my App, the Menu object looks like:

public class Menu {
    public int id;
    public String desc;
    public boolean opzionale;
    public int limite;
    public List prodotti;
}

On the user selection i show in my RecyclerView prodotti. each Prodotti on user action can be modified but have to be modified only at the specific position of ArrayList

.

So initially i just show items at the first position of the Array like:

List listProdotti = listMenu.get(fase).prodotti;

fase is a dynamic value which specify which items of which Menu has to be shown.

Then in another function i change the value of the Prodotto at position like:

    Menu.Prodotti prodotto = listProdotti.get(position);
    prodotto.setQta(qtaProdotto + qta);

The issue is that after doing so all prodotti of ArrayList

at position will update it's quantity.

So if in my array i have data like:

ArrayList prodotti = new ArrayList<>();
prodotti.add(new Menu.Prodotti("TEST1", "1", 0, 0, 1, 0));
prodotti.add(new Menu.Prodotti("TEST2", "2", 0, 0, 1, 0));
prodotti.add(new Menu.Prodotti("TEST3", "3", 0, 0, 1, 0));
prodotti.add(new Menu.Prodotti("TEST4", "4", 0, 0, 1, 0));
prodotti.add(new Menu.Prodotti("TEST5", "5", 0, 0, 1, 0));

ArrayList listMenu = new ArrayList<>();
listMenu.add(new Menu(1, "Fase 1", false, 1, prodotti));
listMenu.add(new Menu(2, "Fase 2", false, 1, prodotti));
listMenu.add(new Menu(3, "Fase 3", false, 1, prodotti));
listMenu.add(new Menu(4, "Fase 4", true, 2, prodotti));

And i change the listMenu.get(0).prodotti.get(1).setQta(2) ALL the products in the listMenu at position 1 will be set with Qta 2.

All my logics are in RecyclerView.Adapter.

user_9901515 · Answer

Here is a simple sketch what your test-case looks like.

(The outer boxes are the ArrayList and the inner boxes are the objects of Menu and Prodotti)

So if you change Prodotti 1 from Menu 1, you are also changing Prodotti 1 from Menu 2, because they are referring to the same List and therefore also to the same Prodotti-Object.

Try it with a testcase like this:

  ArrayList prodottiList1 = new ArrayList<>();
  prodotti.add(new Menu.Prodotti("TEST1", "1", 0, 0, 1, 0));
  prodotti.add(new Menu.Prodotti("TEST2", "2", 0, 0, 1, 0));
  prodotti.add(new Menu.Prodotti("TEST3", "3", 0, 0, 1, 0));

  ArrayList prodottiList2 = new ArrayList<>();
  prodotti.add(new Menu.Prodotti("TEST4", "4", 0, 0, 1, 0));
  prodotti.add(new Menu.Prodotti("TEST5", "5", 0, 0, 1, 0));
  prodotti.add(new Menu.Prodotti("TEST6", "6", 0, 0, 1, 0));


  ArrayList listMenu = new ArrayList<>();
  listMenu.add(new Menu(1, "Fase 1", false, 1, prodottiList1));
  listMenu.add(new Menu(2, "Fase 2", false, 1, prodottiList2));

Here both Menu objects are referring to two different Lists, which are containing different objects.

Here another sketch what the objects in the code above look like :

Unable to change elements in a nested ArrayList

1 Answers