Collections - Set to prevent duplicating items

Hello,
I guess my problem is "no-brainer" to some of you, but for now I fell
completely helpless. Here is an easiest example:

package test;
import java.util.*;

class Vertex {
 int number;

 public Vertex(int number) {
  this.number = number;
 }

 public String toString() {
  return number + "";
 }

 @Override
 public boolean equals(Object obj) {
  return this.number == ((Vertex) obj).number;
 }
}

public class SetTest {
 public static void main(String[] args) {
  Set vertices = new HashSet();

  Vertex a = new Vertex(2);
  Vertex b = new Vertex(3);
  Vertex c = new Vertex(3);

  System.out.println(b.equals(c));

  vertices.add(a);
  vertices.add(b);
  vertices.add(c);

  System.out.println(vertices);
 }
}

Console prints:
true (b equals c)
[3, 2, 3] (Vertex was added to set althought it equals another Vertex)

Some refertence:
http://java.sun.com/j2se/1.4.2/docs/api/java/util/HashSet.html#contains(java.lang.Object)
http://java.sun.com/j2se/1.4.2/docs/api/java/util/Set.html

Thank you in advance