15.10.1 编译器有多聪明

• 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