集合泛型无法添加已定义类型的对象

Question

I have created 2 objects: Person and Men (extends Person ). Then I created a generic collection to add only the Men objects, but for some reason it's not allowing me to add the Men object.

class Person{

}

class Men extends Person{

}

public class test extends HashSet<Person>{

    public static void main(String[] args) {

        List<? extends Person> p1 = new ArrayList<Men>();
        p1.add(new Men());

    }

}

Answer 1

List<? extends Person> List<? extends Person> means "a List of some concrete type that extends Person , but we don't know what that concrete type actually is". For all we know, it could be a List<Women> ! For this reason, you can't actually add anything to this list (except null ). You likely want:

List<Men> p1 = new ArrayList<Men>();  // a list of only Men instances

See also: Wildcards

Answer 2

make it List<? super Men> p1 = new ArrayList<Men>(); List<? super Men> p1 = new ArrayList<Men>(); . You can read about Polymorphism and generics