Add cast to same type

Question

I've got an outer class DoublyLinkedList<T> and two inner classes DoublyLinkedNode<T> as well as DoublyLinkedListIterator<T>. They are arranged as follows:

public class DoublyLinkedList<T> extends Collection<T> {

    private DoublyLinkedNode<T> head;
    private DoublyLinkedNode<T> tail;

    public class DoublyLinkedNode<T> {

        private DoublyLinkedNode<T> prev;
        private DoublyLinkedNode<T> next;
        T element;

    }

    public class DoublyLinkedListIterator<T> implements ListIterator<T> {

        private DoublyLinkedNode<T> current = head;

    }

}

Now since I heard / read on multiple pages in stackoverflow and other sources that an Iterator should be implemented in the corresponding iterable class like DoublyLinkedList<T>, I thought it would be easier since you have access to the outer classes fields. Now I want to initialize an instance of DoublyLinkedListIterator<T>, but Eclipse says at this point:

private DoublyLinkedNode<T> current = head;

inside the DoublyLinkedListIterator<T> that:

Type mismatch: cannot convert from DoublyLinkedList.DoublyLinkedNode< T > to DoublyLinkedList.DoublyLinkedNode< T >

And the only way to resolve this is by casting it with the same type again:

private DoublyLinkedNode<T> current = (DoublyLinkedNode<T>) head;

Why is that so?

Answer 1

You've declared inner classes DoublyLinkedNode and DoublyLinkdListIterator. Inner classes have in scope all generic type parameters of the enclosing class, so T is already available to them. When you declare them again, you are hiding the T on the enclosing class. They are different Ts.

Remove the re-declarations of T that are hiding the outer T (and your class should implement Collection, which is an interface), and let your inner classes use the outer T:

public class DoublyLinkedList<T> implements Collection<T> {

    private DoublyLinkedNode head;
    private DoublyLinkedNode tail;

    public class DoublyLinkedNode {
        private DoublyLinkedNode prev;
        private DoublyLinkedNode next;
        T element;
    }

    public class DoublyLinkedListIterator implements ListIterator<T> {
        private DoublyLinkedNode current = head;
    }
}