Java Linked List Sequence insertFirst method inserts object twice

Question

I am trying to implement a Linked List Sequence in Java however my method for inserting an object at the start of the sequence inserts the object twice and I don't understand why.

If anyone can help it would be really appreciated.

The current output of my test program is

30 30

but I want it to be 30

Thanks in advance

Below is the code for the sequence and the method for insertFirst

public class Sequence implements SequenceInterface{

private Node listHead; 
private Node listTail; 

protected class Node{

      protected Object datum; 
      protected Node next; 

      public Node(Object o, Node n) { 
           datum = o; 
           next = n; 
      }

}


//Constructor 
public Sequence(){ 
   listHead = null; 
   listTail = null;
}


public void insertFirst(Object o) {

    if(listHead == null){

         listHead = new Node(o, listTail); 
         listTail = listHead; 

    }else{
        Node oldLH = listHead;
        listHead = new Node(o, oldLH);
    }       
}
}

Here is my test program

public class SequenceTest {
/**
 * @param args
 * @throws Exception 
 */
public static void main(String[] args) throws Exception {
    Sequence s = new Sequence();    
    s.insertFirst(new Integer(30));

    for(int i=0; i<2; i++){
        System.out.println(s.element(i));
    }
}
}

Answer 1

This is the final version tested that works perfectly! Hope this helps:

class SequenceDLListException extends Exception {
  SequenceDLListException() {
    super();
  }
  SequenceDLListException(String s) {
    super(s);
  }
}

/**
 * <dl>
 * <dt>Purpose: Implementation of Sequence ADT.
 * <dd>
 *
 * <dt>Description:
 * <dd>This class is an implementation of the Sequence using an linked list as
 * the underlying data structure. The capacity is therefore unlimited and
 * overflow does not need to be checked.
 * </dl>
 *
 *
 * @version $Date: 2015/01/30
 */

public class SequenceDLList {
  /**
   * Member class Node encapsulates the nodes of the linked list in
   * which the stack is stored. Each node contains a data item and a
   * reference to another node - the next in the linked list.
   * 
   * 
   */


  public Node lastNode() {
            return listTail;
        }

  public Node firstNode() {
            return listHead;
        }
  protected class Node {

    public Node(Object o) {
      this(o, null, null);
    }

      /**
      * I added another null because it has to point at the beginning and at the end 
      * to some kind of terminator to facilitate the traversal of the list
      */

    public Node(Object o, Node p, Node n) {
      datum = o;
      prev =p;
      next = n;
    }
      /**
      * I added another Node because every node has two links now with the Doubly Linked List
      */



    //The Node data structure consists of two object references.
    //One for the datum contained in the node and the other for
    //the next node in the list.

    protected Object datum;
    protected Node next;
    protected Node prev;
  }

  //We use object references to the head and tail of the list (the head
  //and tail of the sequence, respectively).
  private Node listHead;
  private Node listTail;

  //Only require a single constructor, which sets both object
  //references to null.
  /**
   * Constructs an empty sequence object.
   */
  public SequenceDLList() {
    listHead = null;
    listTail = null;
  }

  /**
   * Adds a new item at the start of the sequence.
   */
  public void insertFirst(Object o) {
    //There is a special case when the sequence is empty.
    //Then the both the head and tail pointers needs to be
    //initialized to reference the new node.
    if (listHead == null) {
      listHead = new Node(o, listHead, null);
      listTail = listHead;
    }

    //In the general case, we simply add a new node at the start
    //of the list via the head pointer.
    else {
      listHead = new Node(o, listHead, null);
    }
  }

  /**
   * Adds a new item at the end of the sequence.
   */
  public void insertLast(Object o) {
    //There is a special case when the sequence is empty.
    //Then the both the head and tail pointers needs to be
    //initialized to reference the new node.
    if (listHead == null) {
      listHead = new Node(o, listHead, null);
      listTail = listHead;
    }

    //In the general case, we simply add a new node to the end
    //of the list via the tail pointer.
    else {
      listTail.next = new Node(o, listTail, null);
      listTail = listTail.next;
    }
  }

  /**
   * Adds a new item at a specified position in the sequence.
   */
  public void insert(Object o, int index) throws SequenceDLListException {
    //Check the index is positive.
    if (index < 0) {
      throw new SequenceDLListException("Indexed Element out of Range");
    }

    //There is a special case when the sequence is empty.
    //Then the both the head and tail pointers needs to be
    //initialized to reference the new node.
    if (listHead == null) {
      if (index == 0) {
        listHead = new Node(o, listHead, null);
        listTail = listHead;
      } else {
        throw new SequenceDLListException("Indexed element is out of range");
      }
    }

    //There is another special case for insertion at the head of
    //the sequence.
    else if (index == 0) {
      listHead = new Node(o, listHead, null);
    }

    //In the general case, we need to chain down the linked list
    //from the head until we find the location for the new
    //list node. If we reach the end of the list before finding
    //the specified location, we know that the given index was out
    //of range and throw an exception.
    else {
      Node nodePointer = listHead;
      int i = 1;
      while (i < index) {
        nodePointer = nodePointer.next;
        i += 1;
        if (nodePointer == null) {
          throw new SequenceDLListException("Indexed Element out of Range");
        }
      }
       Node newNode = new Node(o, nodePointer, nodePointer.next);
        if (nodePointer.next != null) {
            nodePointer.next.prev = newNode;
        }
      //Now we've found the node before the position of the
      //new one, so we 'hook in' the new Node.

      nodePointer.next = newNode;

      //Finally we need to check that the tail pointer is
      //correct. Another special case occurs if the new
      //node was inserted at the end, in which case, we need
      //to update the tail pointer.
      if (nodePointer == listTail) {
        listTail = newNode;
      }
    }
  }

  /**
   * Removes the item at the start of the sequence.
   */
  public void deleteFirst() throws SequenceDLListException {
    //Check there is something in the sequence to delete.
    if (listHead == null) {
      throw new SequenceDLListException("Sequence Underflow");
    }

    //There is a special case when there is just one item in the
    //sequence. Both pointers then need to be reset to null.
    if (listHead.next == null) {
      listHead = null;
      listTail = null;
    }

    //In the general case, we just unlink the first node of the
    //list.
    else {
      listHead = listHead.next;
      listHead.prev = null;
    }
  }

  /**
   * Removes the item at the end of the sequence.
   */
  public void deleteLast() throws SequenceDLListException {
    //Check there is something in the sequence to delete.
    if (listHead == null) {
      throw new SequenceDLListException("Sequence Underflow");
    }

    //There is a special case when there is just one item in the
    //sequence. Both pointers then need to be reset to null.
    if (listHead.next == null) {
      listHead = null;
      listTail = null;
    }

    //In the general case, we need to chain all the way down the
    //list in order to reset the link of the second to last
    //element to null.
    else {
      Node nodePointer = listHead;
      while (nodePointer.next != listTail) {
        nodePointer = nodePointer.next;
      }

      //Unlink the last node and reset the tail pointer.
      nodePointer.next = null;
      listTail = nodePointer;
    }
  }

  /**
   * Removes the item at the specified position in the sequence.
   */
  public void delete(int index) throws SequenceDLListException {
    //Check there is something in the sequence to delete.
    if (listHead == null) {
      throw new SequenceDLListException("Sequence Underflow");
    }

    //Check the index is positive.
    if (index < 0) {
      throw new SequenceDLListException("Indexed Element out of Range");
    }

    //There is a special case when there is just one item in the
    //sequence. Both pointers then need to be reset to null.
    if (listHead.next == null) {
      if (index == 0) {
        listHead = null;
        listTail = null;
      } else {
        throw new SequenceDLListException("Indexed element is out of range.");
      }
    }

    //There is also a special case when the first element has to
    //be removed.

    else if (index == 0) {
      deleteFirst();
    }

    //In the general case, we need to chain down the list to find
    //the node in the indexed position.
    else {
      Node nodePointer = listHead;
      int i = 1;
      while (i < index) {
        nodePointer = nodePointer.next;
        i += 1;
        if (nodePointer.next == null) {
          throw new SequenceDLListException("Indexed Element out of Range");
        }

      }

      //Unlink the node and reset the tail pointer if that
      //node was the last one.
      if (nodePointer.next == listTail) {
        listTail = nodePointer;
      }

      //When we remove the node we have to change the reference as well!
        if (nodePointer.next.next != null) {
            nodePointer.next.next.prev = nodePointer;
        }
      nodePointer.next = nodePointer.next.next;
    }
  }

  /**
   * Returns the item at the start of the sequence.
   */
  public Object first() throws SequenceDLListException {
    if (listHead != null) {
      return listHead.datum;
    } else {
      throw new SequenceDLListException("Indexed Element out of Range");
    }
  }

  /**
   * Returns the item at the end of the sequence.
   */
  public Object last() throws SequenceDLListException {
    if (listTail != null) {
      return listTail.datum;
    } else {
      throw new SequenceDLListException("Indexed Element out of Range");
    }
  }

  /**
   * Returns the item at the specified position in the sequence.
   */
  public Object element(int index) throws SequenceDLListException {
    //Check the index is positive.
    if (index < 0) {
      throw new SequenceDLListException("Indexed Element out of Range");
    }

    //We need to chain down the list until we reach the indexed
    //position

    Node nodePointer = listHead;
    int i = 0;
    while (i < index) {
      if (nodePointer.next == null) {
        throw new SequenceDLListException("Indexed Element out of Range");
      } else {
        nodePointer = nodePointer.next;
        i += 1;
      }
    }

    return nodePointer.datum;
  }

  /**
   * Tests whether there are any items in the sequence.
   */
  public boolean empty() {
    return (listHead == null);
  }

  /**
   * Returns the number of items in the sequence.
   */
  public int size() {
    //Chain down the list counting the elements

    Node nodePointer = listHead;
    int size = 0;
    while (nodePointer != null) {
      size += 1;
      nodePointer = nodePointer.next;
    }
    return size;
  }

  /**
   * Empties the sequence.
   */
  public void clear() {
    listHead = null;
    listTail = null;
  }
}

Answer 2

Assuming you are trying to create a singly linked list, you've got this backwards:

     listHead = new Node(o, listTail); 
     listTail = listHead;

When you execute that, you are assigning the new node to both listHead and listTail . You should have

     listTail = listHead;
     listHead = new Node(o, listTail); 

Answer 3

It looks like it is only added once, but is being printed twice.

If you unroll the loop:

for(int i=0; i<2; i++){
    System.out.println(s.element(i));
}

you essentially get:

System.out.println(s.element(0));
System.out.println(s.element(1));

so if this is unexpected behaviour, what we really need to look at to diagnose it is the element() method.

What do you want to happen when s.element(2) is called and 's' only contains one element? Should it throw an index out of range exception? Should it wrap or crop to within range when it is indexed past the lists range (presumably the current behaviour)?

Answer 4

public void insertFirst(Object o) {

    Node newNode = new Node(o, listHead);
    if (listHead == null) {
        listHead = newNode; 
        listTail = listHead;
    } else {
        listHead = newNode; 
    }       
}