list ADTs

[tom eustace]

hi all,
been having a problem which for the life of me i can't figure out. The assignment requires the use of a doubly linked list ADT, but i keep getting a NullPointerException which i don't understand, chances are its something silly which i'm just not seeing well at least i hope it is. Heres what i've got, i've tracked the bug to the insertAfterCurrent() method, there are other methods that might have problems aswell and some that are downright silly, but one step at a time.

/**
 * User: tom
 * Date: Dec 18, 2003
 * Time: 3:12:05 PM
 */
public class List {
    int size;
    Object list_element = new Object();
    DLNode current;
    DLNode header = new DLNode();
    DLNode trailer = new DLNode();
    List() {
        setHeader_Trailer();
        current = header;
    }
    List(int data) {//creates list with one item
        Integer num = new Integer(data);
        if(size == 0) {
            setHeader_Trailer();
        }
        current = header;
        insertBeforeFirst(num);//create new node at beginning of list
    }
    void setHeader_Trailer() {
            header.setNext(trailer);
            trailer.setPrev(header);
        }
    void incrementSize() {
            size++;
        }
        void decrementSize() {
            size--;
        }
    boolean isEmpty() {
        if(size == 0) {
            return true;
        }
        else {
           return false;
        }
    }
    boolean atFirst() throws NullPointerException{//returns true if first element is current
        boolean flag = false;
        try {
            if(size > 0) {
                if(header.getNext() == current) {
                    flag = true;
                }
            }
            else {
                flag = false;
            }
        }
        catch(NullPointerException nul) {
           System.out.println("list is empty");
        }
        return flag;
    }
    boolean atLast() {
        if(trailer.getPrev().equals(current)) {//null means it has nothing after it
            return true;
        }
        else {
            return false;
        }
    }
    boolean offEnd() {//returns true if no element is current
        if(current.equals(null)) {
            return true;
        }
        else {
            return false;
        }
    }
    Object getFirst() {//return first element in list
        DLNode first = header.getNext();
        list_element = first.getElement();
        return list_element;
    }
    Object getLast() {//return last element in list
        DLNode last = trailer.getPrev();
        list_element = last.getElement();
        return list_element;
    }
    Object getCurrent() {//return current element
        if(!offEnd()) {
            list_element = current.getElement();
            return list_element;
        }
        else {
            return null;
        }
    }
    int getLength() {
        return size;
    }
    boolean equals(List list) {
        if(list == this) {//todo change
           return true;
        }
        else {
            return false;
        }
    }
    //************************************************************************//
    void moveFirst() {//makes the first element current
        current = header.getNext();
    }
    void moveLast() {//makes the last element current
        current = trailer.getPrev();
    }
    void moveNext() {//steps the current marker towards end of list
        if(!offEnd()) {
            //current = current.getNext();//todo changed this to .
            current = current.next;
        }
    }
    void movePrev() {//steps the current marker towards beginning of list
        current = current.getPrev();
    }
    void insertBeforeFirst(Object data) {//adds new element at beginning of list
        DLNode second = header.getNext();//returns next node
        DLNode first = new DLNode(data,header,second);//initialises new node pointing to header and header.getNext, ie what is after it
        second.setPrev(first);//set second to point to new node
        header.setNext(first);//set header to point to new node
        current = header.getNext();
        incrementSize();//increment counter so we know how many nodes we have
    }
    void insertAfterLast(Object data) {//adds new element to end of list
        DLNode second = trailer.getPrev();
        DLNode first = new DLNode(data,second,trailer);
        second.setNext(first);
        trailer.setPrev(first);
        incrementSize();
    }
    void insertBeforeCurrent(Object data) {//inserts new element just before current
        DLNode before_current = new DLNode();
        DLNode second = current.getPrev();//get previous
        before_current.setNext(current);//set new node pointers
        current.setPrev(before_current);//reset current pointer
        before_current.setPrev(second);
        second.setNext(before_current);//reset second pointer
        before_current.setElement(data);//set new node element
        current = before_current;
        incrementSize();
    }
    void insertAfterCurrent(Object data) {//inserts new element just after current
        DLNode second = current.getNext();//get next
        DLNode after_current = new DLNode();
        after_current.setElement(data);
        after_current.setNext(current.getNext());
        after_current.setPrev(current);//set new node pointers
        second.setPrev(after_current);//reset current pointer
        current.setNext(after_current);
        current = after_current;
        incrementSize();
    }
    void deleteFirst() {//deletes first element
        DLNode delete = header.getNext();
        DLNode aft_delete = delete.getNext();
        aft_delete.setPrev(header);
        header.setNext(aft_delete);
        delete = null;
        current = header.getNext();
        decrementSize();
    }
    void deleteLast() {//remove pointers to deleted node, and set to null
        DLNode delete = trailer.getPrev();//get last node
        DLNode prev_delete = delete.getPrev();//get node before last node
        prev_delete.setNext(trailer);//set this node to point to trailer
        trailer.setPrev(prev_delete);//set trailer to point to new last node
        delete = null;//garbage collection
        decrementSize();//decrement list size
    }
    void deleteCurrent() throws NullPointerException{//not sure whats going on here
        try {
        if(!current.equals(null)) {
            DLNode first = current.getPrev();
            DLNode second = current.getNext();
            second.setPrev(current.getPrev());
            if(!current.getNext().equals(null)) {
                first.setNext(current.getNext());//todo problem here
                }
            current = first;
            decrementSize();
        }
        }
        catch (NullPointerException n) {
            System.out.println("current is null, exception: "+n);
        }
    }
    List copy() {
        return this;
    }
    public String toString() {
        return "" + list_element;
    }
    class DLNode implements Position{
        Object element;//make private again
        //Object obj = new Object();
        private DLNode next,prev;//make private again
        DLNode () {
            this(null,null,null);
        }
        DLNode(Object e,DLNode p,DLNode n) {
            element = e;
            next = n;
            prev = p;
        }
        void setElement(Object newElem) {
            element = newElem;
        }
        void setNext(DLNode newNext) {
            next =  newNext;
        }
        void setPrev(DLNode newPrev) {
            prev = newPrev;
        }
        Object getElement() {
            return element;
        }
        DLNode getNext() {
            return next;
        }
        DLNode getPrev() {
            return prev;
        }
        public Object element() throws InvalidPositionException {
            if((prev == null) && (next == null)) {
                throw new InvalidPositionException("position is not in list");
            }
            return element;
        }
    }
}

thanks for any help
later
y'all.

[Jeff Langr]

You should post a short piece of client (testing) code that demonstrates the problem.
-Jeff-

[tom eustace]

ok, i should have done so before. so here is some test code that throws the NullPointerException.

import java.io.IOException;
/**
 * User: tom
 * Date: Dec 20, 2003
 * Time: 12:04:59 PM
 */
public class Test {
     public static void main(String args[])throws IOException {
        testinsertaftercurrent();
    }
    static void testinsertaftercurrent() {
        List li = new List();//default constructor seems fine
        initList(2, li);
    }
    static void initList(int size, List li) {
        List l = new List();
        l = li;
        for(int i = 0; i < size; i++) {//
           li.insertAfterCurrent(new Integer(i));//todo changed to afterCurrent  from before
           System.out.println(li.getCurrent());
           if(!li.offEnd()) {
              li.moveNext();
           }
           l.insertAfterCurrent(li.current);
           System.out.println(l.getCurrent());
        }
    }
}

For my assignment i need to be able to create possible many lists and manipulate them. Hence this line which i think is where the problem starts

   l.insertAfterCurrent(li.current);
    

but the debugger points to the insertAfterCurrent() method and the second.setPrev(after_current); line specifically.
once again thanks for any halp
t

[Jeff Langr]

I think in your test you don't want to advance after an insertion:

import java.io.IOException;
public class Test {
   public static void main(String args[]) throws IOException {
      testinsertaftercurrent();
   }
   static void testinsertaftercurrent() {
      List list = new List();
      final int size = 2;
      for (int i = 0; i < size; i++) {
         list.insertAfterCurrent(new Integer(i));
         list.insertAfterCurrent(list.getCurrent());
      }
      list.moveFirst();
      while (!list.offEnd()) {
         System.out.println(list.getCurrent());
         list.moveNext();
      }
   }
}

And offEnd seems like it should be:

boolean offEnd() {  //returns true if no element is current
   return current == trailer;
}

-Jeff-