background
I encountered a problem before. It failed when assigning the List of the subclass to the Lis of the parent. I checked the reason at that time and there was no record. Now I will sort it out.
＜？ extends E＞
Upper limit wildcard, used to limit the upper limit of the element type
＜？ super E ＞
Lower limit wildcard, used to limit the lower limit of the element
Code example
public class Demo {
public static void main(String[] args) {
// Person 是 Girl 父类, 但是 List<Person> 不是 List<Girl> 父类
Person person = new Girl();
List<Person> personList = new ArrayList<>();
List<Girl> girlList = new ArrayList<>();
personList = girlList; // 编译报错, List<Person> 不是 List<Girl> 父类
// 读
Person person1 = personList.get(0);
Girl girl1 = personList.get(0); // 编译报错, 父类无法赋值给子类
// 写
personList.add(new Girl());
personList.add(new Person());
// 上限通配符, 标记是Person或其子类
List<? extends Person> peopleListExtends = new ArrayList<>();
peopleListExtends = personList;
peopleListExtends = girlList;
// 读
Person personExtends = peopleListExtends.get(0);
Girl girlExtends = peopleListExtends.get(0); // 编译报错, 因为可能是Person对象, 无法赋值给子类
// 写
peopleListExtends.add(new Person()); // 编译报错, 上限是Person, 但子类可能是多个, 无法确定具体类型
peopleListExtends.add(new Girl()); // 编译报错, 上限是Person, 但子类可能是多个, 无法确定具体类型
// 下限通配符, 标记其是Girl或其父类
List<? super Girl> peopleListSuper = new ArrayList<>();
peopleListSuper = personList;
peopleListSuper = girlList;
// 读
Girl girl2 = peopleListSuper.get(0); // 编译报错, 下限是Girl, 但可能是父类, 无法确定具体类型
Person person2 = peopleListSuper.get(0); // 编译报错, 下限是Girl, 但可能是父类, 无法确定具体类型
// 写
peopleListSuper.add(new Girl());
peopleListSuper.add(new Person()); // 编译报错, 下限是Girl, 父类无法转化为子类
}
}
class Person {
}
class Girl extends Person {
}
class Boy extends Person {
}