/*
     *     Creates a ListNOde containing value
     *
     *     insert the newly created ListNOde at Front of List
     */
    public void addFirst(Object value)
    {
        add(0,value);
    }

    /*
     *     return the Object stored in the first ListNOde
     */
    public Object getFirst()
    {
        return get(0);
    }

    /*
     *     Creates a ListNOde containing value
     *
     *     insert the newly created ListNOde at End of List
     */
    public void addLast(Object value)
    {
        add(size(),value);
    }

    /*
     *     return the Object stored in the last ListNOde
     */
    public Object getLast()
    {
        return get(size()-1);
    }

    //  return the object previously at index ind.
    //  replace that value with obj!
    //  preCondition: 0 < ind < size()
    //                size() > 0
    public Object set(int ind, Object obj)
    {
        Object o = get(ind);
        getNodeAtIndex(ind).setValue(obj);
        return o;
    }

    /*
     *     return a reference to the middle ListNode
     *
     *     That is, the Listnode at index = size() / 2   (remember to use integer Math - truncate/round down
     *
     *     if size() == 0 return null
     */
    public ListNode getMiddleNode()
    {
        if(size()==0)
        {
            return null;
        }
        int ind = (size()/2);
        return getNodeAtIndex(ind);
    }