Doubly Linked List Deque implementation

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Java

I am trying to do a Doubly linked list implementation of the Deque and I keep getting this error:

15 120
Exception in thread "main" EmptyDequeException: Deque is empty.
    at ListDeque.getFirst(ListDeque.java:83)
    at ListMain.main(ListMain.java:14)

I went over my code 100 times and can't figure out what's wrong..can someone please help.. my code is copied below...thanks in advance

import java.lang.*;
public class ListDeque
{
   protected DNode header, trailer;  // dummy nodes
   protected int size;    // number of elements
   public ListDeque()     // constructor: initialize an empty deque
   {
      header = new DNode( 0, null, null );
      trailer = new DNode( 0, null, null );
      header.setNext(trailer);  // make header point to trailer
      trailer.setPrev(header);  // make trailer point to header
      size = 0;
   }

    /**
     * Display the content of the deque
     *
     */
    public void printDeque()
    {
    for ( DNode p = header.getNext(); p != trailer; p = p.getNext() )
        System.out.print( p.getElement() + " " );
    System.out.println();
    }

   // ***************************************
   // DO NOT MODIFY THE CODE ABOVE THIS LINE.
   // ADD YOUR CODE BELOW THIS LINE.
   //
   // ***************************************
   /**
     * Returns the number of items in this collection.
     * @return the number of items in this collection.
     */
    public int size()
    {
      return size; // replace this line with your code
    }

    /**
     * Returns true if this collection is empty.
     * @return true if this collection is empty.
     */
    public boolean isEmpty()
    {
        if(size==0)
        {
             return true;
        }
        return false;
        //return header.getNaxt()== trailer;

    }

    /**
     * Returns the first element of the deque
     *
     */
    public int getFirst() throws EmptyDequeException
    {
        if(isEmpty())
        {
            throw new EmptyDequeException("Deque is empty.");
        }
        return header.getNext().getElement();
    }

    /**
     * Returns the last element of the deque
     *
     */
    public int getLast() throws EmptyDequeException
    {
        // COMPLETE THIS METHOD
        if(isEmpty())
        {
            throw new EmptyDequeException("Deque is empty.");
        }
        return trailer.getPrev().getElement(); // replace this line with your code
    }

    /**
     * Inserts e at the beginning (as the first element) of the deque
     *
     */
    public void insertFirst(int e)
    {
        DNode newNode = new DNode(e, header, header.getNext());
        header.getNext().setPrev(newNode);
        header.setNext(newNode);
        //addAfter(header, e);
    }

    /**
     * Inserts e at the end (as the last element) of the deque
     *
     */
    public void insertLast( int e )
    {
        DNode newNode = new DNode(e, trailer, trailer.getPrev());
        trailer.getPrev().setNext(newNode);
        trailer.setPrev(newNode);
        //addBefore(trailer, e);
    }

    /**
     * Removes and returns the first element of the deque
     *
     */
    public int removeFirst() throws EmptyDequeException
    {
      if(isEmpty())
      {
        throw new EmptyDequeException("Deque is empty.");
      }
      DNode first= header.getNext();
      int o = first.getElement();
      DNode second = first.getNext();
      header.setNext(second);
      second.setPrev(header);
      size--;
      return o;   // replace this line with your code
    }

    /**
     * Removes and returns the last element of the deque
     *
     */
    public int removeLast() throws EmptyDequeException
    {
      if(isEmpty())
      {
        throw new EmptyDequeException("Deque is empty.");
      }
      DNode last = trailer.getPrev();
      int o = last.getElement();
      DNode secondtolast = last.getPrev();
      trailer.setPrev(secondtolast);
      secondtolast.setNext(trailer);
      size--;
      return o;
    }

} // end class