https://blog.csdn.net/weixin_33753003/article/details/86444289
https://blog.csdn.net/hudashi/article/details/105417555
// 不可变
List<Fruit>fruits =newArrayList<Apple>();// 编译不通过
// 协变
List<?extendsFruit>wildcardFruits =newArrayList<Apple>();
// 协变->方法的返回值，对返回类型是协变的:Fruit->Apple
Fruitfruit =wildcardFruits.get(0);
// 不可变
List<Apple>apples =newArrayList<Fruit>();// 编译不通过
// 逆变
List<?superApple>wildcardApples =newArrayList<Fruit>();
// 逆变->方法的参数，对输入类型是逆变的:Apple->Fruit
wildcardApples.add(newApple());
