How to copy an ArrayList that cannot be influenced by the original ArrayList changes? [duplicate]

Question

I've been using ArrayLists on a project of mine, and I need to create a default ArrayList so I can reset the original one whenever I want. So, I copy the original ArrayList to create the default one. However, whenever I modify something on the original, it also changes the default one. How can I make the copy "static" and unchangeable?

Here is my code: (It's in portuguese)

private ArrayList<Compartimento> listaCompartimentos;
private ArrayList<Compartimento> listaCompartimentosDEFAULT;

public Simulador() {
        this.listaCompartimentos = new ArrayList<>();
        this.listaCompartimentosDEFAULT=new ArrayList<>();
    }

//Copy of the array
public void gravarListaDefault(){
        this.listaCompartimentosDEFAULT=(ArrayList<Compartimento>)listaCompartimentos.clone();
    }

Note: I don't know if it can be the reason behind it, but the ArrayList listaCompartimentos has a listaEquipamentos. For each "Compartimento" there is an ArrayList "listaEquipamentos".

I don't think you should use the clone there. Not a Java expert, but i think a simple assignment would work. — Gjordis
– Gjordis, Commented May 15, 2013 at 10:42

Arvind Kumar Avinash · Accepted Answer · 2023-01-04 09:04:36Z

6

clone() for ArrayLists should be avoided because even if it creates a new List instance, it holds references to the same elements. So an element changed on the first List will also be changed on the second one. Two different objects holding the same references.

The code below will create a new instance with new elements.

ArrayList<Object> realClone = new ArrayList<Object>();
for(Object o : originalList)
   realClone.add(o.clone());

edited Jan 4, 2023 at 9:04

Arvind Kumar Avinash

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answered May 15, 2013 at 10:42

aran

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4 Comments

NilsH Over a year ago

@AsierAranbarri Cloning does create a shallow copy. It says in the docs

Carlito Over a year ago

The thing is: In my main class, using the respective object that contains these two ArrayLists, I fill the original one, only THEN I clone it. These two ArrayList both belong to the class "Simulador" so, you are telling me to create two "Simulador" if I have to use a copy-constructor?

aran Over a year ago

You have to. When you clone a list, you are creating a new list but NOT new elements.

NilsH Over a year ago

Seems like new ArrayList(originalList) also creates a shallow copy, with references identical to originalList.

johnchen902 · Accepted Answer · 2013-05-15 10:42:50Z

2

this.listaCompartimentosDEFAULT = new ArrayList<Compartimento>(
            listaCompartimentos);

answered May 15, 2013 at 10:42

johnchen902

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Comments

AllTooSir · Accepted Answer · 2013-05-15 11:04:27Z

2

I would suggest to clone each object . Make your Compartimentoclass implements Cloneable. And clone each object in the List and add to the other List.

for(Compartimento c : this.listaCompartimentos) {
    this.listaCompartimentosDEFAULT.add(c.clone());
}

answered May 15, 2013 at 11:04

AllTooSir

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1 Comment

aran Over a year ago

that's in my answer as well ; ). +1

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