How should I go about cloning a nest ArrayList?

Question

I need to write out a method to clone a nested ArrayList .

The nested ArrayList looks like this:

ArrayList<ArrayList<Ship>> theSea = new ArrayList<ArrayList<Ship>>();

I want to copy it to a clone:

ArrayList<ArrayList<Ship>> seaClone = new ArrayList<ArrayList<Ship>>();

I've tried iterating it and copying over the lists:

for(int i = 0; i < theSea.size(); i++){
    seaClone.add(theSea.get(i));
}

However, this doesn't clone the elements of the nested ArrayList<Ship> and instead just copies over a reference to it.

How should I go about cloning the elements of the nested ArrayList<Ship> ?

Answer 1

However, this doesn't clone the elements of the nested ArrayList and instead just copies over a reference to it.

Because it is actually what you are doing : you add the reference of the existing objects in the new List.
So instead of, create a new nested ArrayList for each cloned sublist and create new Ship objects as you add elements in the new List s.
You could define a copy constructor in Ship for example :

public Ship(Ship model){     
  foo = model.foo;
  bar = model.bar;    
}

You can so write :

for(List<Ship> list : theSea){
    List<Ship> currentList = new ArrayList<>();
     for(Ship ship : list){
        currentList.add(new Ship(ship));
     }
    clone.add(currentList);
  }

Answer 2

Assuming Ship objects are cloneable:

List<List<Ship>> clone = theSea.stream()
    .map(l -> l.stream().map(s -> (Ship)s.clone()).collect(toList()))
    .collect(toList());

Answer 3

Something like:

ArrayList<ArrayList<Ship>> listCopy = new ArrayList<>();
for (int i=0; i<list.size(); i++) {
    ArrayList<Ship> newList = new ArrayList<>();
    for (int j=0; j<list.get(i).size(); j++) {
        newList.add(new Ship(list.get(i).get(j)));
    }
    listCopy.add(newList);
}

Observe this line: newList.add(new Ship(list.get(i).get(j)));

Where you need to pass the object to the constructor and from there duplicate all the attributes, otherwise you will just create a reference to the same class.

If your ship class is to complex with many other objects inside this task may be difficult and you can use a tool like serializing to json and then revert back to object or using a java deep object cloner.