（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。
QUESTION 217 Given:
3. import java.util.*;
4. public class Hancock {
5. // insert code here
6. list.add("foo");
7. }
8. }
Which two code fragments, inserted independently at line 5, will compile without warnings? (Choose two.)
A. public void addStrings(List list) {
B. public void addStrings(List<String> list) {
C. public void addStrings(List<? super String> list) {
D. public void addStrings(List<? extends String> list) {
Answer: BC