Stack copy constructor

Question

I'm having a lot of difficulty with my stack copy constructor. DSStack extends a Stack abstract class and uses a doubly-linked-list to store data (tokens). It keeps failing (NullPointerException) at the line this.push(oldListNode.getToken()); in the constructor. I've included the methods/classes I think will help with solving my problem. It needs to be a deep copy. Any help or guidance would be awesome.

public class DSStack extends Stack {

    // The list containing the data
    private DSList theStack;

    // Constuct an empty stack using a LinkedList as the container
    public DSStack() {
        theStack = new DSList();
    }

    public DSStack(DSStack other) {
    // Create a reference for the stack to be copied
    Node oldListNode = other.theStack.head;

    while (oldListNode != null) {
        this.push(oldListNode.getToken());
        oldListNode = oldListNode.next;
        }
    }

    /**
     * Adds the given object to the top of the Stack. 
     * @return The given object. 
     */
    public Token push(Token obj) {      
        theStack.add(obj);
        return obj;
    }
}

public class DSList implements List {

    /**
     * Appends the specified element to the end of this list.
     * @param obj The object to add. 
     * @return True if the object has been added to the list. 
     * 
     * @throws NullPointerException if the specified object is null.
     */
    public boolean add(Token obj) {
        if (obj == null) {
            throw new NullPointerException();
        }
        // If list is empty, add new node to front.
        if(isEmpty()) {  
            // Create a new Node. Add Token obj as data
            Node newNode = new Node(null, null, obj);  
            // point head to new Node, newNode
            head = newNode;
            return true;
        } else { 
            // create a reference of the start position in the list
            Node current = head;
            // While there are nodes remaining in the list, advance through to reach the end.
            while (current.next != null) 
                current = current.next;
            // Create a new node and append it to the end of the list, with 'prev' pointing to current (2nd to last node)
            Node newNode = new Node(null, current, obj);
            // Point 2nd to last element to the newest , last node in the list (in next variable)
            current.next = newNode;
            // Return true if successful addition of node.
            return true;
        }
    }
 }

public class Node {

    public Node next;
    public Node prev;

    // data being stored in each node
    private Token t;

    // Node constructor
    public Node(Node next, Node prev, Token token) {
        this.next = next;
        this.prev = prev;
        this.t = token;
    }

    public Token getToken() {
        return t;
    }
}

public class Token {

    public enum Type { OPERATOR, OPERAND, PAREN };
    public Type type;

    private String operator;
    private double operand;

    public Token(double result) {
        this.operand = result;
        this.type = Type.OPERAND;
    }

    public Token(String op) {
        this.operator = op;
        this.type = Type.OPERATOR;

        if ( this.operator.equals("(") || this.operator.equals(")") ) {
            this.type = Type.PAREN;
        }
    }

    public Token(Token other) {
        this.operator = other.operator;
        this.operand = other.operand;
        this.type = other.type;
    }
}

Answer 1

You have to initialize theStack inside your paramterized constructor public DSStack(DSStack other). I didn't see that happened, hence it still contains null value for any object constructed using your parameterized constructor. Notice that I am talking about the theStack reference in your push method

Answer 2

Writing Constructors could be work of art sometimes. All you have to do

public DSStack()
{
    theStack = new DSList();
}

public DSStack( DSStack other )
{
    this();
    // Rest of the code goes on 
    // Create a reference for the stack to be copied
    Node oldListNode = other.theStack.head;

    while (oldListNode != null) 
    {
        this.push(oldListNode.getToken());
        oldListNode = oldListNode.next;
    }

}

This will make sure that you always initialize theStack variable no matter which constructor you initialized to allocate memory for DSStack object. this() statement actually calls the no-arg constructor where you initialized theStack variable