Re: Collections - Set to prevent duplicating items

Eric Sosman <esosman@ieee-dot-org.invalid>
Mon, 12 Jul 2010 14:48:28 -0400
On 7/12/2010 2:38 PM, Stefan wrote:

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 + "";

  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);




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

     "I observed immediately that the malefactor had made one crucial
error in carrying out his fiendish plan: He forgot to override the
hashCode() method when overriding equals(). As any student of the art
of detection knows well, these two are inseparable: Override both, or
override neither, or invoke chaos upon yourself -- as many a resident
of Her Majesty's Gaols can testify tearfully."

     "Astounding, Holmes!"

     "Elementary, my dear Watson. I also note that this perpetrator is
a particularly clumsy example of the species, having implemented an
equals() that fails miserably if given an argument that is `null', say,
or a reference to anything other than a `Vertex' instance. Like so many
of the criminal underclass, he fails to consider the consequences of his
actions in a wider context than his immediate plot."

     "The criminal `underclass', Holmes? Surely you meant `subclass'."

     "You're starting to get on my nerves, Watson. Must I uncase my
violin again?"

     "I say, Holmes, I believe I'll go out for a bit of a stroll."

Eric Sosman

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