LinkedList构造函数有（两种）：

1. public LinkedList()

2. public LinkedList(Collection
          c)

/**     * Constructs an empty list.     */    public LinkedList() {    }    /**     * Constructs a list containing the elements of the specified     * collection, in the order they are returned by the collection's     * iterator.     *     * @param  c the collection whose elements are to be placed into this list     * @throws NullPointerException if the specified collection is null     */    public LinkedList(Collection
      c) {        this();        addAll(c);    }