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Another Generics question

neha verma
Ranch Hand

Joined: Jun 17, 2007
Posts: 37
Refer to K&B book Q no 12- of Chapter 7-
import java.util.*;
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; }
Here the answer is that code does not compile. I am not able to understand what is the problem with g.add(new Person("Hans"));

Neha Verma
Manfred Klug
Ranch Hand

Joined: Jun 04, 2007
Posts: 377
Hi neha,

compile it and look at which line the error occurs. Currently you are looking at the wrong part of the code.
ahmed yehia
Ranch Hand

Joined: Apr 22, 2006
Posts: 424
The problem is in add() method, it should be add(Person o) instead of add(Object o) since Group's super class declaration was HashSet<Person>.
Remko Strating
Ranch Hand

Joined: Dec 28, 2006
Posts: 893
Even if you've solved that the code wouldn't work correct, because the class Person doesn't have a overridden equals method.

public boolean equals(Object obj) {}

So the duplicates aren't removed from the HashSet<Person>

Remko (My website)
SCJP 1.5, SCWCD 1.4, SCDJWS 1.4, SCBCD 1.5, ITIL(Manager), Prince2(Practitioner), Reading/ gaining experience for SCEA,
ahmed yehia
Ranch Hand

Joined: Apr 22, 2006
Posts: 424
Yes and override hashCode also required, public int hashCode() {}
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subject: Another Generics question
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