Problem with output from DoubleLinkedList data structure

[smoothedatol412]

I have been working on a two reference Linked List data structure that has references to both ends of a the data structures. The [DoubleNode] class that is the reference to each end of the nodes is an subclass of normal [Node] class, I thought having a subclass of the [Node] class was a good next step because this two referenced linked list should in the be a subclass of a normal Linked List data structure. The only part of the subclassing I have done is just the nodes, after I got this part to work I will begin to implement the subclass of the Linked List data structure inheritance.

The problem is not a syntax, but yet a run-time error.
The output is as follows:
List (first->last)
0 0 0 0 0 0
List (last->first)
0 0 0 0 0 0
List (first->last)
0 0 0 0
List (first->last)
0 0 0 0

And The complete source code is below.

// DoubleLinkedList class
public class DoubleLinkedList{
	private DoubleNode first, last;

	// DoubleLinkedList() method
	public DoubleLinkedList(){
		first=null;
		last=null;
	} // end constructor

	// isEmpty() method
	public boolean isEmpty(){
		return first==null;
	} // end method

	// insertFirst(int) method
	public void insertFirst(int value){
		DoubleNode temp=new DoubleNode(value);
		if(isEmpty()){
			last=temp;
		}else{
			first.setPrevious(temp);
		} // end condition
		temp.setNext(first);
		first=temp;
	} // end method

	// insertLast(int) method
	public void insertLast(int value){
		DoubleNode temp=new DoubleNode(value);
		if(isEmpty()){
			first=temp;
		}else{
			last.setNext(temp);
			temp.setPrevious(last);
		} // end condition
		last=temp;
	} // end method

	// deleteFirst() method
	public DoubleNode deleteFirst(){
		DoubleNode temp=first;
		if(first.getNext()==null){
			last=null;
		}else{
			first.next.previous=null;
		} // end condition
		first=first.getNext();
		return temp;
	} // end method

	// deleteLast() method
	public DoubleNode deleteLast(){
		DoubleNode temp=last;
		if(first.getNext()==null){
			first=null;
		}else{
			last.previous.next=null;
		} // end condition
		last=last.getPrevious();
		return temp;
	} // end method

	// insertAfter(int, int) method
	public boolean insertAfter(int key, int value){
		DoubleNode current=first;
		while(current.getData()!=key){
			current=current.getNext();
			if(current==null){
				return false;
			} // end condition
		} // end loop
		DoubleNode temp=new DoubleNode(value);
		if(current==last){
			temp.setNext(null);
			last=temp;
		}else{
			temp.setNext(current.getNext());
			current.next.previous=temp;
		} // end condition
		temp.setPrevious(current);
		current.setNext(temp);
		return true;
	} // end method

	// deleteKey(int) method
	public DoubleNode deleteKey(int key){
		DoubleNode current=first;
		while(current.getData()!=key){
			current=current.getNext();
			if(current==null){
				return null;
			} // end condition
		} // end condition
		if(current==first){
			first=current.getNext();
		}else{
			current.previous.next=current.getNext();
		} // end condtion
		current.next.previous=current.getPrevious();
		return current;
	} // end method

	// displayForward() method
	public void displayForward(){
		System.out.println("List (first->last)");
		DoubleNode current=first;
		while(current!=null){
			current.display();
			current=current.getNext();
		} // end loop
		System.out.println();
	} // end method

	// displayBackward() method
	public void displayBackward(){
		System.out.println("List (last->first)");
		DoubleNode current=last;
		while(current!=null){
			current.display();
			current=current.getPrevious();
		} // end loop
		System.out.println();
	} // end method

	// main(String[]) method
	public static void main(String[] args){
		DoubleLinkedList list=new DoubleLinkedList();
		list.insertFirst(2);
		list.insertFirst(4);
		list.insertFirst(6);
		list.insertLast(1);
		list.insertLast(3);
		list.insertLast(5);
		list.displayForward();
		list.displayBackward();
		list.deleteFirst();
		list.deleteLast();
		list.deleteKey(1);
		list.displayForward();
		list.insertAfter(2, 7);
		list.insertAfter(3, 8);
		list.displayForward();
	} // end method
} // end class

// DoubleNode class
public class DoubleNode extends Node{
	public int data;
	public DoubleNode next, previous;

	// DoubleNode(int) constructor
	public DoubleNode(int data){
		super(data);
	} // end constructor
	
	// DoubleNode(int, DoubleNode) constructor
	public DoubleNode(int data, DoubleNode next, DoubleNode previous){
		super(data, next);
		this.previous=previous;
	} // end constructor
	
		// getPrevious()
		public DoubleNode getPrevious(){
			return previous;
		} // end method
		
		// setPrevious(DoubleNode) method
		public void setPrevious(DoubleNode previous){
			this.previous=previous;
		} // end method
	
		// getNext() method
		public DoubleNode getNext(){
			return next;
		} // end method
		
		// setNext(Node) method
		public void setNext(DoubleNode next){
			this.next=next;
		} // end method
		
		// getData() method
		public int getData(){
			return data;
		} // end method
		
		// setData(int) method
		public void setData(int data){
			this.data=data;
		} // end method

	// display() method
	public void display(){
		System.out.print(data+" ");
	} // end method)
} // end class

// Node class
public class Node{
	public int data;
	public Node next;

	// Node() constructor
	public Node(){
		next=null;
	} // end constructor
	
	// Node(int)
	public Node(int data){
		this.data=data;
		next=null;
	} // end constructor
	
	// Node(int, Node) constructor
	public Node(int data, Node next){
		this.data=data;
		this.next=next;
	} // end constructor
	
	// getNext() method
	public Node getNext(){
		return next;
	} // end method
	
	// setNext(Node) method
	public void setNext(Node next){
		this.next=next;
	} // end method
	
	// getData() method
	public int getData(){
		return data;
	} // end method
	
	// setData(int) method
	public void setData(int data){
		this.data=data;
	} // end method

	// display() method
	public void display(){
		System.out.print(getData()+" ");
	} // end method)
} // end class

[Norm]

The problem is not a syntax, but yet a run-time error.

Can you explain what the problem is and post what the output should look like.

For debugging, add lots of print statements to show the values of variables as the are changed and used.
Add a toString() method to the Node class that returns the data and next values. That makes it easy to print out a node for debugging.

[smoothedatol412]

List (first->last)
2 4 6 1 3 5
List (last->first)
2 4 6 1 3 5
List (first->last)
5 3 1 6 4 2

The thing the the program worked fine until I subclasses [DoubleNode] from [Node].

[Norm]

what did the new class do to change how the code works?
Did you try adding some printlns to show the values of the variables as they are set and used?
For example why are all the values that print out 0? Where is a non-zero value stored in a variable? Where is it retrieved from that variable?