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From: Stefan on 12 Jul 2010 14:38
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
From: Eric Sosman on 12 Jul 2010 14:48
On 7/12/2010 2:38 PM, 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);
> }
> }
>
> 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
esosman(a)ieee-dot-org.invalid
From: markspace on 12 Jul 2010 14:49
Stefan wrote:

> Set vertices = new HashSet();

> [3, 2, 3] (Vertex was added to set althought it equals another Vertex)


Yeah, hashes require that you override hashcode() when you override
equals(Object). So what happened was your HashSet hasted to find the
"3" that was already there, didn't find it because the hashcode was
different, and added the second "3".
From: Patricia Shanahan on 12 Jul 2010 17:10
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
From: Stefan on 12 Jul 2010 17:21
Brilliant, Holmes, brilliant! :D

Thank you :)

On 12 Lip, 20:48, Eric Sosman <esos...(a)ieee-dot-org.invalid> wrote:
> On 7/12/2010 2:38 PM, 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);
> >   }
> > }
>
> > 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
> esos...(a)ieee-dot-org.invalid

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