泛型的通配符

http://stackoverflow.com/questions/14346622/list-vs-list-extends-object

http://stackoverflow.com/questions/8055389/whats-the-difference-between-and-extends-object-in-java-generics

http://stackoverflow.com/questions/12340808/whats-the-difference-between-listobject-and-list

这仨帖子讲泛型的通配符讲的太好了