Making a new ArrayList containing objects the same as the first list?

Question

I have an ArrayList containing custom objects.

What is the best way to make a separate ArrayList that has the exact same content, but isn't using the same references? As in, if I edit the first object in list1, it doesn't touch the first object in list2, but otherwise they look the same through and through.

Is it considered correct / good practice to do the following, or is there a built-in way?

List<MyObject> firstList = getArrayListFromSQLiteDb(criteria);
List<MyObject> secondList = new ArrayList<>();

for (MyObject object : firstList) {
    MyObject newObject = new MyObject();
    newObject.setField1(object.getField1());
    newObject.setField2(object.getField2());
    newObject.setField3(object.getField3());
    secondList.add(newObject);
}

It looks like what you're doing is just fine. Be careful though; if any of the fields are references, they're going to be copied over and it sounds like that isn't what you want. — Trent Small
– Trent Small, Commented May 18, 2016 at 15:31
List<MyObject> firstList = getArrayListFromSQLiteDb(criteria); and List<MyObject> secondList = new ArrayList<MyObject>(firstList); — Manu AG
– Manu AG, Commented May 18, 2016 at 15:32
@Manu & DjMethaneMan: And if you would read the question, you would understand that the asker doesn't just want a copy of the list, he wants a deep copy that also copies all contained objects (So that both lists have different objects in them). Neither the copy constructor nor the clone() function of ArrayList do that. — user6073886
– user6073886, Commented May 18, 2016 at 15:37

Daniel Almeida · Accepted Answer · 2016-05-18 15:42:50Z

3

A simple way of doing this would be to clone the original ArrayList, thus not sharing references and having the other list remain untouched when you alter the original one. As @911DidBush mentioned, this will only work if the lists contents are cloneable and implement the clone() method correctly.

List<MyObject> firstList = getArrayListFromSQLiteDb(criteria);
List<MyObject> secondList = new ArrayList<>();

for(MyObject obj : firstList) {
    secondList.add(obj.clone());
}

edited May 18, 2016 at 15:42

answered May 18, 2016 at 15:34

Daniel Almeida

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1 Comment

user6073886 Over a year ago

You should probably also mention that this will only work if the lists contents are cloneable and implement the clone() method correctly. Otherwise you will get a CloneNotSupportedException

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