Pass inner class object as function parameter

Question

I am writing a linked list in Java. Below is the code:

public class FunctionalList<T> {

    class Node  {
        private T data;
        private Node next;

        //constructor
        public Node(T data, Node next)
        {
            this.data = data;
            this.next = next;
        }
    }
    private Node head;
    /**
     * default constructor, create an empty list
     */
    public FunctionalList() {
        head = null;
    }

    public FunctionalList<T> add( T element ) {
        FunctionalList<T> newList = new FunctionalList<T>();
        add_aux(newList.head, element);

        return newList;
    }

    private void add_aux(Node node, T element)
    {
        if (node == null)
        {
            Node newNode = new Node(element, null);
            node = newNode;
        }
        else if (node.next != null)     // go to the end of the list
        {
            add_aux(node.next, element);
        }
        else
        {
            Node newNode = new Node(element, null);   // create new node
            node.next = newNode;    //add the element to the list
        }
    }
}

I implemented the add method in a recursive way. When I try to add an element to the list, I failed. I tracked the problem which is after the add_aux(newList.head, element) -- the newList.head is still null.

Answer 1

    Node newNode = new Node(element, null);
    node = newNode;

This is because you are assigning a reference to a node variable which is local to the method and you are assuming that it will be reflected into the newList.head .

One way is you always return the node and assigning it to newList.head . That way, it will have the start of the list. So your method definition will be like:

private Node add_aux(Node node, T element) {
.... // all the code is same.
return node; // add this in the end.
}