Question

I am trying to implement a clone() method on a DoubleLinkedList. Now, the problem is that implementing it by “the convention” is a lot more troublesome than just creating a new DoubleLinkedList and filling it with all the elements of my current DoubleLinkedList.

Is there any inconvenient I am not seeing when doing that?

Here is my current approach:

@Override
public DoubleLinkedList<T> clone() {
    DoubleLinkedList<T> dll = new DoubleLinkedList<T>();

    for (T element : dll) {
        dll.add(element);
    }

    return dll;
}

Here is what it would be by the convention:

@Override
public DoubleLinkedList<T> clone() {
    try {
        DoubleLinkedList<T> dll = (DoubleLinkedList<T>)super.clone();
        //kinda complex code to copy elements
        return dll;
    } catch (CloneNotSupportedException e) {
        throw new InternalError(e.toString());
    }
}

Answer 1

As you correctly point out, the convention is to always call super.clone() in the beginning of an implementation of clone(). From the API docs on Object#clone():

By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == x.getClass().

Your first attempt (without using super.clone()) has the following problem:

Suppose I have

class IntDoubleLinkedList extends DoubleLinkedList<Integer> implements Cloneable

(and that IntDoubleLinkedList does not bother to override clone()) and I run the following code:

IntDoubleLinkedList idll = new IntDoubleLinkedList();
IntDoubleLinkedList idll2 = (IntDoubleLinkedList) idll.clone();

What will happen? The clone method of your DoubleLinkedList will be executed, which, if it doesn’t go through super.clone(), returns an instance of DoubleLinkedList which in turn can not be casted to an IntDoubleLinkedList. A ClassCastException will be thrown!

So how does super.clone() solve this issue? Well, if everybody stick to the convention of calling super.clone() in an overriden clone method, Object.clone() will eventually be called, and this implementation will create an instance of a proper type (IntDoubleLinkedList in this case)!