java 泛型没有协变类型， 所以要重用extends， 但使用List<? extends Fruit> 可以是ArrayList<Fruit>()、ArrayList<Apple>()、ArrayList<Orange>(), 因此不能add元素进去

class Fruit{}
class Apple extends Fruit{}
class SubApple extends Apple{}
class Orange extends Fruit{}

class Holder<T>{
    private T value;
    public Holder(){}
    public Holder(T value){this.value = value;}
    public T getValue() {
        return value;
    }
    public void setValue(T value) {
        this.value = value;
    }
    public boolean equals (Object obj){
        return value.equals(obj);
    }
    
}
public class CovarianArrays {

    public static void main(String[] args) {
            Fruit [] fruitArray = new Apple[10];
            fruitArray[0] = new SubApple();
            //数组有协变类型，能放入SubApple
            //编译时正常。运行时错误 ArrayStoreException 数组在运行时检查类型
            //fruitArray[0] = new Fruit();  
            //fruitArray[1] = new Orange();
            
            //编译时就出错
            //List<Fruit> fruitList = new ArrayList<Apple>();
            
            List<Apple> fruitList = new ArrayList<Apple>();
            fruitList.add(new Apple());
            //只能是Apple,泛型没有协变类型
            //fruitList.add(new SubApple());
            
            List<? extends Fruit> fruitExtendList = new ArrayList<Apple>();
            //连Object都不行，只能是无意义的null
            //fruitExtendList.add(new Apple());
            //fruitExtendList.add(new Object());
            fruitExtendList.add(null);
            
            List<? extends Fruit> fruitAsList = Arrays.asList(new Apple());
            Apple a = (Apple) fruitAsList.get(0);
            //以下两个方法的参数都是Object
            fruitAsList.contains(new Apple());
            int i =fruitAsList.indexOf(new Apple());
            System.out.println(i);//-1  不存在
            
            List<? super Apple> supList = new ArrayList<Apple>();
            supList.add(new Apple());
            supList.add(new SubApple());
            //supList.add(new Fruit());
            
            Holder<Apple> holdApple = new Holder<Apple>(new Apple());
            Apple a1 = holdApple.getValue();
            System.out.println(a1);  //com.Array.Apple@10b30a7
            
            //Holder<Fruit> holdFruit = holdApple;
            Holder<? extends Fruit> holdFruit = holdApple;
            Fruit f1 = holdFruit.getValue();
            System.out.println(f1);   //com.Array.Apple@10b30a7
            Apple a2 = (Apple) holdFruit.getValue();
            System.out.println(a2);
            Orange o1 = (Orange) holdFruit.getValue();  //运行时出错ClassCastException
            System.out.println(o1);
            //不能调用setValue()
            //holdFruit.setValue(new Orange());
            
            
            
    }

}

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