java泛型中

（1）<? super String> is any class which is a superclass of String (including String itself). (In this case, the only other suitable class is Object.)

即包括String的父類和它本身的類。

（2） <? extends String> (which in this specific case wouldn't be very useful as String is final, so it can have no subclasses).

即包括String的子類，而String沒有子類，爲null。