addpublic void add(Object o)
Inserts the specified element into the list (optional operation). The element is inserted immediately before the next element that would be returned by next, if any, and after the next element that would be returned by previous, if any. (If the list contains no elements, the new element becomes the sole element on the list.) The new element is inserted before the implicit cursor: a subsequent call to next would be unaffected, and a subsequent call to previous would return the new element. (This call increases by one the value that would be returned by a call to nextIndex or previousIndex.)
Parameters:
o - the element to insert.
（在极品老狼的指导下）我是这么理解的：
将指定的元素插入到list中（这是个可有可无的操作）。这个元素被插入到 被next和被previous返回的元素之间（如果这个list是空的，这就是唯一的元素。）这个新元素会在默认位置以前被插入：next( )不会受到影响，previous( )将会返回一个新元素。（这句不好说了！！！）