泛型

（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的父类和它本身的类。

（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的子类，而String没有子类，为null。