JAVA中HASHSET程序问题

import java.util.*;

public class HashSetTest {
public static void main(String[] args) {
HashSet<Student> hs = new HashSet<Student>();
hs.add(new Student(1, "zhangsan"));
hs.add(new Student(2, "lishi"));
hs.add(new Student(3, "wangwu"));
hs.add(new Student(1, "zhangsan"));
Iterator<Student> it = hs.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}

}

class Student {
private int num;
private String name;

public Student(int num, String name) {
this.num = num;
this.name = name;
}

public String toString() {
return "num :" + num + " name:" + name;
}

public int hashCode() {
return num * name.hashCode();
}

public boolean equals(Object o) {
Student s = (Student) o;
return num == s.num