Issue creating my own LinkedList class in Java

Question

Okay, so I'm trying to create my own LinkedList class (with generic typing) in Java but am running into problems. I have created a LinkedListNode class which basically sets and gets the next and right pointers and also the node's key. My LinkedList class code is posted below, if you compile and run you'll see it's not setting the list up as it should. At the moment, I am trying to insert node y after node x, but my printout looks like:

Key: 5 - prev key: 6 - next key: 6 Key: 6 - prev key: null - next key: null

My code:

public class LinkedList<T> {

    private LinkedListNode<T> m_head = null;
    private LinkedListNode<T> m_tail = null;

    public LinkedList() {
        m_head = new LinkedListNode<T>();
        m_tail = new LinkedListNode<T>();
        m_head.setNext(m_tail);
        m_tail.setPrev(m_head);
    }

    public LinkedListNode<T> getHead() {
        return m_head;
    }

    public LinkedListNode<T> getTail() {
        return m_tail;
    }

    public void insert(LinkedListNode<T> node, LinkedListNode<T> prev_node) {
        prev_node.getNext().setPrev(node);
        node.setPrev(prev_node);
        node.setNext(prev_node.getNext());
        prev_node.setNext(node);
    }

    public void delete(LinkedListNode<T> node) {

    }

    @Override
    public String toString() {
        LinkedListNode<T> pointer = m_head.getNext();
        String result = "";
        while(pointer.getKey() != null) {
            T prev_key = pointer.getPrev() == null ? null : pointer.getPrev().getKey();
            T next_key = pointer.getNext() == null ? null : pointer.getNext().getKey();
            result += "\nKey: " + pointer.getKey() + " - prev key: " + prev_key + " - next key: " + next_key;
            pointer = pointer.getNext();
        }
        return result;
    }

    public static void main(String[] args) {
        LinkedListNode<Integer> x = new LinkedListNode<Integer>(5);
        LinkedListNode<Integer> y = new LinkedListNode<Integer>(6);
        LinkedList<Integer> list = new LinkedList<Integer>();
        list.insert(x, list.getHead());
        list.insert(y, x);
        System.out.println(list.toString());
    }

}

LinkedListNode.java:

public class LinkedListNode<T> {

    private T m_key;
    private LinkedListNode<T> m_next = null;
    private LinkedListNode<T> m_prev = null;

    public LinkedListNode() {

    }

    public LinkedListNode(T key) {
        m_key = key;
    }

    public T getKey() {
        return m_key;
    }

    public void setKey(T key) {
        m_key = key;
    }

    public LinkedListNode<T> getNext() {
        return m_next;
    }

    public LinkedListNode<T> getPrev() {
        return m_next;
    }

    public void setNext(LinkedListNode<T> node) {
        m_next = node;
    }

    public void setPrev(LinkedListNode<T> node) {
        m_prev = node;
    }

}

Answer 1

What you try to implement is Double Linked List (double as you have next and previous).

And you have a problem in your structure design, maintain the head and tail can be issue prone.

You should try to implement it using single element that refer to other.

LinkedListNode<T> header = new LinkedListNode<T>(); 

header.getNext(); // head
header.getPrevious()//tail 

The example of insert

 public void insert(T element) { //We add to the begin

     LinkedListNode<T> newElement = LinkedListNode<T>(element);

     newElement.setNext(header); //previous begin we set to be next
     newElement.setPrevious(header.getPrevious()); //previous end we set to be end. 

     newElement.getPrevious().setNext(newElement); //to previous end we set new begin.
     newElement.getNext().setPrevious(newElement); //to previous begin we set new end. 

 }