override equals and hashcode

From:

"lowenbrau" <customer@newsnet.com>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 09 Aug 2007 12:44:21 GMT

Message-ID:

<F_Dui.17597$4A1.16583@news-server.bigpond.net.au>

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Hi,

I have a class Obj. I created three objects o1,o2,03. Objects are equal =
if their id is equal. For example, o1 and o3 are equal. I tried adding =
these three objects to a HashSet, and when I print the size of the =
hashset I get "3" and not "2". Something is not right with the equals or =
hashcode method. Can someone shed some light?

code snippet:

import java.util.*;

public class Test
{
public static void main (String[] args)
{
  Set hs = new HashSet();
  Obj o1 = new Obj ("w", 10);
  Obj o2 = new Obj ("r", 20);
  Obj o3 = new Obj ("w", 30);

  hs.add(o1);
  hs.add(o2);
  hs.add(o3);

  System.out.println(hs.size());
}

public static class Obj
{
  String id;
  int length;

  Obj (String id, int length)
  {
   this.id = id;
   this.length = length;
  }

  boolean equals (Obj o)
  {

   if (o == null)
    return false;
   else if (o instanceof Obj)
    return (this.id.equals(o.id));
   else return false;
  }

  public int hashCode()
  {
   return id.hashCode();
  }
}
}
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; =
charset=iso-8859-1">
<META content="MSHTML 6.00.6000.16481" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face=Verdana size=2>Hi,</FONT></DIV>
<DIV><FONT face=Verdana size=2></FONT> </DIV>
<DIV><FONT face=Verdana size=2>I have a class Obj. I created three =
objects
o1,o2,03. Objects are equal if their id is equal. For example, o1 and o3 =
are
equal. I tried adding these three objects to a HashSet, and when I print =
the
size of the hashset I get "3" and not "2". Something is not right with =
the
equals or hashcode method. Can someone shed some light?</FONT></DIV>
<DIV><FONT face=Verdana size=2></FONT> </DIV>
<DIV><FONT face=Verdana size=2>code snippet:</FONT></DIV>
<DIV><FONT face=Verdana size=2></FONT> </DIV>
<DIV><FONT face=Courier size=2>import java.util.*;</FONT></DIV>
<DIV><FONT face=Courier></FONT> </DIV>
<DIV><FONT face=Courier size=2>public class =
Test<BR>{<BR> public static
void main (String[] args)<BR> {<BR>  Set hs = new
HashSet();<BR>  Obj o1 = new Obj ("w", =
10);<BR>  Obj o2 =
new Obj ("r", 20);<BR>  Obj o3 = new Obj ("w",
30);<BR>  <BR>  hs.add(o1);<BR>  hs.add(o2)=
;<BR>  hs.add(o3);<BR>  <BR>  System.out.pr=
intln(hs.size());<BR> }<BR> <BR> public
static class Obj<BR> {<BR>  String id;<BR>  int =

length;<BR>  <BR>  Obj (String id, int
length)<BR>  {<BR>   this.id =
id;<BR>   this.length =
length;<BR>  }<BR>  <BR>  boolean equals =
(Obj
o)<BR>  {<BR>   <BR>   if (o =
==
null)<BR>    return false;<BR>   else =
if (o
instanceof Obj)<BR>    return
(this.id.equals(o.id));<BR>   else return
false; <BR>  }<BR>  <BR>  public int
hashCode()<BR>  {<BR>   return
id.hashCode();<BR>  }<BR> }<BR>}</FONT></DIV></BODY></HTML=

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