Java Doesn't Recognize Something That Exists?

Question

I'm writing an XML checker for a homework assignment. I'm using a stack which uses a double linked list which uses nodes with a generic data type.
I ran into a problem after testing the program for the first time. I got a NullPointerException error at the instance where I tried to add a new node to the head. After a little digging I found out that apparently Java doesn't remember that I set the successor of "head" to be "tail". I've already spent 20 minutes going over the code that sets the tail as head's successor but I just can't see what I did wrong.
Exception in thread "main" java.lang.NullPointerException
     at DLLNode.setNext(DLLNode.java:59)
     at DLLNode.<init>(DLLNode.java:17)
     at DLL.<init>(DLL.java:20)
     at Stack.<init>(Stack.java:11)
     at ParseTokens.<init>(ParseTokens.java:9)
     at Driver.main(Driver.java:16)The program is already too long to post here and broken up into multiple classes, so I will only post the important stuff. Note: Token is just a Record class that keeps two Strings, type and tagName for the xml tag to store.
DLLNode.java //node with generic datatype T
     private T data;
     private DLLNode<T> successor;
     private DLLNode<T> predecessor;
     public DLLNode(T d, DLLNode<T> next, DLLNode<T> prev) {
          this.setNodeData(d);
          this.setNext(next); //line 17
          this.setPrev(prev);
       public T getNodeData() {
            return this.data;
       public void setNodeData(T newData) {
            this.data = newData;
       public DLLNode<T> getNext() {
            return this.successor;
       public void setNext(DLLNode<T> newNext) {
            this.successor = newNext;
            System.out.println(newNext.toString()); //zeroed in on the problem being here; throws NullPointerException; line 59
       public DLLNode<T> getPrev() {
            return this.predecessor;
       public void setPrev(DLLNode<T> newPrev) {
            this.predecessor = newPrev;
       }DLL.java //manages the DLLNode objects
     private DLLNode<T> head;
     private DLLNode<T> tail;
//other vars
     public DLL() {
          this.setHead(new DLLNode<T>(null, tail, null)); //problem is probably here; after this, java doesn't see tail as head's successor; //line 20
          this.setTail(new DLLNode<T>(null, null, head));
          this.setSize(0);
       public void setHead(DLLNode<T> value) {
            this.head = value;
       public void setTail(DLLNode<T> value) {
            this.tail = value;
     public boolean addAtHead(T dllData) {
          DLLNode<T> newNode = new DLLNode<T>(dllData, this.head.getNext(), this.head);          
          this.getHead().getNext().setPrev(newNode); //original NullPointerException thrown here at the first instance of this method being used
          this.getHead().setNext(newNode);
          this.setSize(this.getSize() + 1);
          return ((newNode != null) && (newNode.getNext() != null) &&
                    (newNode.getPrev() != null));
     }Stack.java //manages a DLL object as a stack
     private DLL<T> dll;
     public Stack() {
          dll = new DLL<T>(); //line 11
          this.setSize(dll.getSize());
       public void push(T data) {
            dll.addAtHead(data); //original NullPointerException at first instance of push() being used
       }ParseTokens.java //class to actually go through the xml and list what it finds
     private Stack<Token> stack;
     public ParseTokens() {
          stack = new Stack<Token>(); //original error; line 9
     }Driver.java //main
ParseTokens parse = new ParseTokens();//line 16Thank you for any help.
Edited by: WhoCares357 on Feb 16, 2010 5:10 PM
Edited by: WhoCares357 on Feb 16, 2010 5:17 PM

Answer 1

A user on another forum h(http://preview.tinyurl.com/yjqx4a9) helped me find the problem. I had to create the head and tail first and then point them at each other. Thanks for all the help here.
@flounder Even though I've already solved my problem I am still confused by what you're saying. The double linked list I created has two placeholders so that I don't have to worry about how many nodes are included. To connect my new node I start at the head and point at whatever is connected to the head (whether it is the tail or another node) to set the successor and predecessor for the new node. I then break the old connections (from the head to the old node after it) and connect them to the new node. I don't see a problem in this, but I am most likely not understanding your concern.
I don't really want to let this go, because there might be a problem in my code that I don't see right now.
@AndrewThompson64 I'll use that the next time I have a problem. Thanks.