public class User {

}

public class Child extends User {

}

import java.util.LinkedList;

public class Test {
    public static void main(String[] args) {
        LinkedList<? super Child> linkedList = new LinkedList<>();
        linkedList.add(new Child());
        linkedList.add(new User());
      /**
         * 上面添加User报错
         * The method add(capture#2-of ? super Child) in the type LinkedList<capture#2-of ? super Child> is not applicable for the arguments (User)
         */
    }
}