How can a child get stuff that is put in a parent's pocket?

This might be a stupid question ;-/, but the results from the code below really confused me. In the addit(Person p) method, it looks the Person object is added to a parent of Group g (i.e. a Person HashSet) via super.add(p). But g also gets the Persons that are added to super. I suspect my understanding of the relationship between g and super is not right. Can anybody give me some help here? Many thanks.

--- Bing

import java.util.*;

public class Group extends HashSet<Person>{
public static void main(String[] args) {
Group g = new Group();
g.addit(new Person("Hans"));
g.addit(new Person("Lotte"));
g.addit(new Person("Jane"));
g.addit(new Person("Hans"));
g.addit(new Person("Jane"));
System.out.println("Total:" + g.size() + " " + g);
}

public boolean addit(Person p) {
System.out.println("Adding:" + p + "; super size is " + super.size());
return super.add(p);
}
}

class Person {
private final String name;
public Person(String name){this.name=name;}
public String toString(){return name;}
}

The "relationship" between those two things is that there is only one thing. The Group object IS the HashSet<Person> object -- they aren't two separate objects. That's what it means for class A to extend class B, it means that every A object is also a B object.

I see. Thanks for the explanation. So I just did a quick test and changed super to this, the code gave exactly the same results.

Yes, it would. (Group doesn't override the "add" method so its "add" method is the "add" method from its parent class, which is what "super" means.) I was wondering why they did it that way -- maybe it was supposed to illustrate some point. Could you tell us where the example came from?

The original problem is from the MasterExam quiz set for SCJP 6.0, as shown below. I made two changes in the code I posted yesterday: 1) from add(Object o) to add(Person p), otherwise it won't compile; and 2) from add(...) to addit(...), which was just for the testing purpose.

I also tried commenting out the defition of add(Person p) method in the code below. By all the testing, I now understand why super is used here. Thanks again for the help, Paul.

--- Bing

public class Group extends HashSet<Person>{
public static void main(String[] args) {
Group g = new Group();
g.add(new Person("Hans"));
g.add(new Person("Lotte"));
g.add(new Person("Jane"));
g.add(new Person("Hans"));
g.add(new Person("Jane"));
System.out.println("Total:" + g.size());
}

public boolean add(Object o) {
System.out.println("Adding:" + o);
return super.add(o);
}
}

class Person {
private final String name;
public Person(String name){this.name=name;}
public String toString(){return name;}
}

Which of the following occur at least once when the code is compiled and run? (Choose all that apply)
A Adding Hans
B Adding Lotte
C Adding Jane
D Total: 3
E Total: 5
F The code does not compile.
G An exception is thrown at runntime.