How to Copy the Contents of One Arraylist into Another

How do I copy the contents of one ArrayList into another?

You can use such trick:

myObject = new ArrayList<Object>(myTempObject);

or use

myObject = (ArrayList<Object>)myTempObject.clone();

You can get some information about clone() method here

But you should remember, that all these ways will give you a copy of your List, not all of its elements. So if you change one of the elements in your copied List, it will also be changed in your original List.

how to copy one arrayList to another without refrencing first?

mListPreviousData = new ArrayList<>(); 
mListPreviousData.addAll(mAddedList);

Java ArrayList copy

Yes, assignment will just copy the value of l1 (which is a reference) to l2. They will both refer to the same object.

Creating a shallow copy is pretty easy though:

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

(Just as one example.)

how to deep copy one arraylist into another

As I mentioned in my comments to your question, the clone() method returns an Object, and since the compiler can see and know only this, you must (dangerously) cast the object returned to be ArrayList for the method to work:

arrli.add((ArrayList<Integer>) arrli1.clone());

Having said this, I implore you not to use clone() in this situation and that you read Why people are so afraid of using clone() (on collection and JDK classes)? to see why.

If this were my code, I'd make new ArrayLists objects within the for loop rather than cloning existing ones. I'm not 100% sure of what you're trying to achieve here, but perhaps something similar to:

// list to hold other lists
List<List<Integer>> listOfLists = new ArrayList<>();

// inner list to be added to above list
List<Integer> innerList = new ArrayList<>();
for (int i = 0; i < arr.length; i++) {  

    // if we need to create a new inner list, add old to 
    // list of lists and create new one          
    if (i != 0 && arr[i] < arr[i - 1]) {
        listOfLists.add(innerList);
        innerList = new ArrayList<>();
    }

    // always add to the current inner list
    innerList.add(arr[i]);
}

// when all is done, add last inner list to list of lists
listOfLists.add(innerList);

This above code looks to be cleaner and safer. Also, if you're wondering why I'm using:

List<Integer> foo = new ArrayList<>();

rather than

ArrayList<Integer> foo = new ArrayList<>();

Please check out:

Why do some people use the List base class to instantiate a new ArrayList? as well as
What does it mean to “program to an interface”?

How to copy only distinct elements from one arrayList to another ArrayList

Note: This piece of code only works, because the questioner uses this if statement in his constructor:

if(n1 < n2 ){
    this.num1 = n1;
    this.num2 = n2;
} else {
    this.num2 = n1;
    this.num1 = n2;
}

I dont know if I understood your problem correctly, but try this method:

public List<TwoNumbers> doStuff(List<TwoNumbers> list) {
    List<TwoNumbers> returnList = new ArrayList<>();
    for (int i = 0; i < list.size(); i++) {
        boolean flag = true;
        for (int j = i + 1; j < list.size(); j++) {
            if (list.get(i).getNum1() == list.get(j).getNum1() && list.get(i).getNum2() == list.get(j).getNum2()) {
                flag = false;
            }
        }

        if (flag) {
            returnList.add(list.get(i));
        }
    }
    return returnList;
}

This method will loop through the list and if the flag variable is set to false we know that the object is a duplicate, so we dont have to add it to returnList.

If you have further questions or if this didnt answer your question feel free to hit me up.

Copy one item from an ArrayList to another ArrayList Java 8

You can use get() method to get a item at index from ArrayList

Producto product = listaProductos.get(index);

Where index is a integer

Use add() method to add it to second list , Since the second list is of type Pedido you can't add it unless it is of compatible type. You need to convert your Producto object to Pedido to do that you can create a new `Pedido' object and set all the attributes from the 'Producto' , if you have getters and setters you can do like this

Pedido pedido = new Pedido();
pedido.set...(product.get...()); //for all other fields 
listaPedido.add(pedido);

If getters and setters is not written and attributes are public you can directly assign like

pedido.attributeName = producto.attribute;

Take a look into documentation:ArrayList JavaDoc

Copy some values from one ArrayList to another kotlin

var skills: List<SkillModel> = skillsList.map { SkillModel(it.name,it.title) }