Collections - Set to prevent duplicating items


S

Stefan

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
 
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E

Eric Sosman

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."
 
M

markspace

Stefan said:
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".
 
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S

Stefan

Brilliant, Holmes, brilliant! :D

Thank you :)

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."
 

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