Re: Collections - Set to prevent duplicating items

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 12 Jul 2010 14:10:15 -0700

Message-ID:

<vYydnd8215grGabRnZ2dnUVZ_gCdnZ2d@earthlink.com>

Stefan wrote:

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);
}
}

As has already been pointed out, you have inconsistent equals and
hashCode. More specifically, you are not conforming to their contract as
described in the Object documentation at e.g.
http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/api/java/lang/Object.html

In general, when overriding a method inherited from a superclass you
should examine the superclass documentation to find the requirements for
the method. The equals documentation says "Note that it is generally
necessary to override the hashCode method whenever this method is
overridden, so as to maintain the general contract for the hashCode
method, which states that equal objects must have equal hash codes."

Patricia