Is this implementation of doubly linked list correct?

I am trying to follow the template provided here to create a doubly linked list in java. This homework is based on this lecture. Can someone please review my implementation?

No change to this part of the code

/* DListNode.java */



/**
 *  A DListNode is a node in a DList (doubly-linked list).
 */

public class DListNode {

  /**
   *  item references the item stored in the current node.
   *  prev references the previous node in the DList.
   *  next references the next node in the DList.
   *
   *  DO NOT CHANGE THE FOLLOWING FIELD DECLARATIONS.
   */

  public Object item;
  protected DListNode prev;
  protected DListNode next;

  /**
   *  DListNode() constructor.
   *  @param i the item to store in the node.
   *  @param p the node previous to this node.
   *  @param n the node following this node.
   */
  DListNode(Object i, DListNode p, DListNode n) {
    item = i;
    prev = p;
    next = n;
  }
}

This is the code that I updated

/* DList.java */



/**
 *  A DList is a mutable doubly-linked list ADT.  Its implementation is
 *  circularly-linked and employs a sentinel (dummy) node at the head
 *  of the list.
 *
 *  DO NOT CHANGE ANY METHOD PROTOTYPES IN THIS FILE.
 */

public class DList {

  /**
   *  head references the sentinel node.
   *  size is the number of items in the list.  (The sentinel node does not
   *       store an item.)
   *
   *  DO NOT CHANGE THE FOLLOWING FIELD DECLARATIONS.
   */

  protected DListNode head;
  protected int size;
  private DListNode sentinel;

  /* DList invariants:
   *  1)  head != null.
   *  2)  For any DListNode x in a DList, x.next != null.
   *  3)  For any DListNode x in a DList, x.prev != null.
   *  4)  For any DListNode x in a DList, if x.next == y, then y.prev == x.
   *  5)  For any DListNode x in a DList, if x.prev == y, then y.next == x.
   *  6)  size is the number of DListNodes, NOT COUNTING the sentinel,
   *      that can be accessed from the sentinel (head) by a sequence of
   *      "next" references.
   */

  /**
   *  newNode() calls the DListNode constructor.  Use this class to allocate
   *  new DListNodes rather than calling the DListNode constructor directly.
   *  That way, only this method needs to be overridden if a subclass of DList
   *  wants to use a different kind of node.
   *  @param item the item to store in the node.
   *  @param prev the node previous to this node.
   *  @param next the node following this node.
   */
  protected DListNode newNode(Object item, DListNode prev, DListNode next) {
    return new DListNode(item, prev, next);
  }

  /**
   *  DList() constructor for an empty DList.
   */
  public DList() {
    //  Your solution here.
      sentinel=newNode(null,null,null);
      sentinel.prev=sentinel;
      sentinel.next=sentinel;
      head=sentinel;

  }

  /**
   *  isEmpty() returns true if this DList is empty, false otherwise.
   *  @return true if this DList is empty, false otherwise. 
   *  Performance:  runs in O(1) time.
   */
  public boolean isEmpty() {
    return size == 0;
  }

  /** 
   *  length() returns the length of this DList. 
   *  @return the length of this DList.
   *  Performance:  runs in O(1) time.
   */
  public int length() {
    return size;
  }

  /**
   *  insertFront() inserts an item at the front of this DList.
   *  @param item is the item to be inserted.
   *  Performance:  runs in O(1) time.
   */
  public void insertFront(Object item) {
    // Your solution here.
      sentinel.next=newNode(item,sentinel,sentinel.next);
      if(sentinel.prev==sentinel)
      {
          sentinel.prev=sentinel.next;
      }
      size++;

  }

  /**
   *  insertBack() inserts an item at the back of this DList.
   *  @param item is the item to be inserted.
   *  Performance:  runs in O(1) time.
   */
  public void insertBack(Object item) {
    // Your solution here.
      sentinel.prev=newNode(item,sentinel.prev,sentinel);
      if(sentinel.next==sentinel)
      {
          sentinel.next=sentinel.prev;
      }
      size++;
  }

  /**
   *  front() returns the node at the front of this DList.  If the DList is
   *  empty, return null.
   *
   *  Do NOT return the sentinel under any circumstances!
   *
   *  @return the node at the front of this DList.
   *  Performance:  runs in O(1) time.
   */
  public DListNode front() {
    // Your solution here.
      if(size>0)
      {
          return sentinel.next;
      }
      else
      {
          return null;
      }
  }

  /**
   *  back() returns the node at the back of this DList.  If the DList is
   *  empty, return null.
   *
   *  Do NOT return the sentinel under any circumstances!
   *
   *  @return the node at the back of this DList.
   *  Performance:  runs in O(1) time.
   */
  public DListNode back() {
    // Your solution here.
      if(size>0)
      {
          return sentinel.prev;
      }
      else
      {
          return null;
      }
  }

  /**
   *  next() returns the node following "node" in this DList.  If "node" is
   *  null, or "node" is the last node in this DList, return null.
   *
   *  Do NOT return the sentinel under any circumstances!
   *
   *  @param node the node whose successor is sought.
   *  @return the node following "node".
   *  Performance:  runs in O(1) time.
   */
  public DListNode next(DListNode node) {
    if(node==null || node.next==sentinel)
    {
        return null;
    }
    else
    {
        return node.next;
    }
  }

  /**
   *  prev() returns the node prior to "node" in this DList.  If "node" is
   *  null, or "node" is the first node in this DList, return null.
   *
   *  Do NOT return the sentinel under any circumstances!
   *
   *  @param node the node whose predecessor is sought.
   *  @return the node prior to "node".
   *  Performance:  runs in O(1) time.
   */
  public DListNode prev(DListNode node) {
      if(node==null || node.prev==sentinel)
        {
            return null;
        }
        else
        {
            return node.prev;
        }
  }

  /**
   *  insertAfter() inserts an item in this DList immediately following "node".
   *  If "node" is null, do nothing.
   *  @param item the item to be inserted.
   *  @param node the node to insert the item after.
   *  Performance:  runs in O(1) time.
   */
  public void insertAfter(Object item, DListNode node) {
    // Your solution here.
      if(node!=null);
        {
          node.next=newNode(item,node,node.next);
          size++;
        }
  }

  /**
   *  insertBefore() inserts an item in this DList immediately before "node".
   *  If "node" is null, do nothing.
   *  @param item the item to be inserted.
   *  @param node the node to insert the item before.
   *  Performance:  runs in O(1) time.
   */
  public void insertBefore(Object item, DListNode node) {
    // Your solution here.
      if(node!=null);
        {
          node.prev=newNode(item,node.prev,node);
          size++;
        }
  }

  /**
   *  remove() removes "node" from this DList.  If "node" is null, do nothing.
   *  Performance:  runs in O(1) time.
   */
  public void remove(DListNode node) {
    // Your solution here.
      if(node!=null);
        {

                node.prev.next=node.next;
                node.next.prev=node.prev; 
                size--;
        }
  }

  /**
   *  toString() returns a String representation of this DList.
   *
   *  DO NOT CHANGE THIS METHOD.
   *
   *  @return a String representation of this DList.
   *  Performance:  runs in O(n) time, where n is the length of the list.
   */
  public String toString() {
    String result = "[  ";
    DListNode current = head.next;
    while (current != head) {
      result = result + current.item + "  ";
      current = current.next;
    }
    return result + "]";
  }
}