15.10 通配符

数组不能向上转型

package com.thinking.in.java.course.chapter15.two;
import com.thinking.in.java.course.chapter11.testArrays.App;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
class Fruit{}
class Apple extends Fruit{}
class Jonathan extends Apple{}
class Orange extends Fruit{}
public class CovarianArrays {
    public static void main(String[] args) {
        //
        Fruit[] fruits = new Apple[10];
        //fruits[0] = new Apple();
        fruits[1] = new Jonathan();
        try {
            fruits[0] = new Fruit();
        }catch (Exception e){
            System.out.println(e);
        }
        try {
            fruits[0] = new Orange();
        }catch (Exception e){
            System.out.println(e);
        }
        //泛型容器 在编译器就已经检测到异常了
        List<? extends Fruit> fruitList = new ArrayList<Fruit>();
        System.out.println(fruitList.contains(new Apple()));
        List<? extends Fruit> list = Arrays.asList(new Apple());
        //list.add(new Apple());
        Apple apple = (Apple)list.get(0);
        list = Arrays.asList(new Fruit());
        System.out.println(list.contains(apple));
        System.out.println(list.indexOf(apple));
    }
}

public class GenericAndCovariance {
    public static void main(String[] args) {
        //无法添加任何数据
        List<? extends Fruit> fruitList = new ArrayList<Fruit>();
        //fruitList.add(new Apple());
        //fruitList.add(new Fruit());
        //fruitList.add(new Orange());
        //fruitList.add(new Jonathan());
        fruitList.add(null);
         //因为contains方法的参数为Object类下 可以使用
        fruitList.contains(new Apple());
    }
}

超类通配符

List<? super ClassType> : 从类的继承关系来看的话，ClassType为上界，可以执行添加操作。注意与👆例子的区别

public class SuperTypeWilds {
    public static void main(String[] args) {
        List<? super Fruit> list = new ArrayList<>();
        System.out.println(Arrays.toString(list.getClass().getTypeParameters()));
        list.add(new Apple());
        list.add(new Jonathan());
        list.add(new Fruit());
        System.out.println(list);
        List<? extends Fruit> eList = new ArrayList<>();
        //eList.add(new Apple());
    }
}