java泛型中<? super String>和<? extends String> 的区别

（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。