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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