???????flist??add()???????????????????????????add()???????????“? extends Fruit”???????????????????????????
????//: generics/Holder.java
????public class Holder<T> {
????private T value;
????public Holder() {}
????public Holder(T val) { value = val; }
????public void set(T val) { value = val; }
????public T get() { return value; }
????public boolean equals(Object obj) {
????return value.equals(obj);
????}
????public static void main(String[] args) {
????Holder<Apple> Apple = new Holder<Apple>(new Apple());
????Apple d = Apple.get();
????Apple.set(d);
????// Holder<Fruit> Fruit = Apple; // Cannot upcast
????Holder<? extends Fruit> fruit = Apple; // OK
????Fruit p = fruit.get();
????d = (Apple)fruit.get(); // Returns ‘Object’
????try {
????Orange c = (Orange)fruit.get(); // No warning
????} catch(Exception e) {
????System.out.println(e);
????}
????// fruit.set(new Apple()); // Cannot call set()
????// fruit.set(new Fruit()); // Cannot call set()
????System.out.println(fruit.equals(d)); // OK
????}
????} /* Output: (Sample)
????java.lang.ClassCastException: Apple cannot be cast to Orange
????true
????*///:~
??????????????????????е?fruit???????set()??????????????“? extends Fruit”??????ζ????????????????? ????????????????????Fruit ???????????????????“????????”???????????
?????????????????????Ч????
????//: generics/SuperTypeWildcards.java
????import java.util.*;
????class Jonathan extends Apple {}
????public class SuperTypeWildcards {
????static void writeTo(List<? super Apple> apples) {
????apples.add(new Apple());
????apples.add(new Jonathan());
????// apples.add(new Fruit()); // Error
????}
????} ///:~
?????????????д???????ú?????????Apple??????????????????д???????а??????????Jonathan??Apple????????????????????????????????Apple?????Fruit?????????????????????????
?????????????????????????
???????????List<int>??????????List<Integer>??????????????????????????д??????????????????????????
???????????????
???????????????????????????????壬?????????????????????????
????//: generics/MultipleInterfaceVariants.java
????// {CompileTimeError} (Won’t compile)
????interface Payable<T> {}
????class Employee implements Payable<Employee> {}
????class Hourly extends Employee
????implements Payable<Hourly> {} ///:~
????Hourly?????????????????Payable???????÷?????????????????????????????????????????????δ??????????
????????
???????′???????????????????????????????????????????
????//: generics/UseList.java
????// {CompileTimeError} (Won’t compile)
????import java.util.*;
????public class UseList<W??T> {
????void f(List<T> v) {}
????void f(List<W> v) {}
????} ///:~
?????????????
????class SelfBounded<T extends SelfBounded<T>> { // ...