- add()接收一个具有泛型参数类型的参数,contains()和indexOf()将接收Object类型的参数;
- 当指定了一个ArrayList<? extends Fruit>时,add()的参数就变成了“? extends Fruit”;
- 编译期将直接拒绝对参数列表中涉及通配符的方法(例如add())的调用;
package chapter15._10._1;
import java.util.Arrays;
import java.util.List;
class Fruit{}
class Apple extends Fruit {}
class Jonathan extends Apple {}
class Orange extends Fruit {}
public class CompilerIntelligence {
public static void main(String[] args) {
List<? extends Fruit> flist = Arrays.asList(new Apple());
Apple a = (Apple) flist.get(0);
flist.contains(new Apple());
flist.indexOf(new Apple());
}
}
- Holder<Apple>不能向上造型成Holder<Fruit>,但可以向上造型成Holder<? extends Fruit>;
- fruitHolder的set()方法将不能被调用,因为其参数也是“? extends Fruit”;
package chapter15._10._1;
public class Holder<T> {
private T value;
public Holder() {}
public Holder(T value) {
this.value = value;
}
public void set(T value) {
this.value = value;
}
public T get() {
return this.value;
}
public boolean equals(Object object) {
return this.value.equals(object);
}
public static void main(String[] args) {
Holder<Apple> appleHolder = new Holder<Apple>(new Apple());
Apple d = appleHolder.get();
appleHolder.set(d);
// Holder<Fruit> fruitHolder = appleHolder;
Holder<? extends Fruit> fruitHolder = appleHolder;
Fruit p = fruitHolder.get();
d = (Apple) fruitHolder.get();
try {
Orange c = (Orange) fruitHolder.get();
} catch (Exception e) {
System.out.println(e);
}
// fruitHolder.set(new Apple());
// fruitHolder.set(new Fruit());
System.out.println(fruitHolder.equals(d));
}
}
输出:
java.lang.ClassCastException: chapter15._10._1.Apple cannot be cast to chapter15._10._1.Orange
true