（1）<? super String> is any class which is asuperclass of String (including String itself). (In this case, the only other suitable class isObject.)
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 asString is final, so it can have no subclasses).
即包括String的子类，而String没有子类，为null。