Another Generics question

[neha verma]

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"));

[Manfred Klug]

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]

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]

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>

[ahmed yehia]

Yes and override hashCode also required, public int hashCode() {}