.equals(o) and Set membership

Re: .equals(o) and Set membership

Murat Tasan wrote:[color=blue]
>
> i thought the Set code checks for membership by applying the .equals()
> method, which at runtime should bind to my overridden method, no?
>[/color]

No, that is not correct. Whenever equals() is overloaded, hashCode()
must be overloaded as well. The Collections classes depend on
hashCode() and equals(). Read the javadoc for java.lang.Objec t
carefully and implement hashCode() for your classes for which you have
overridden equals() and you will see better results.

HTH,
Ray

Re: .equals(o) and Set membership

Murat Tasan <[email protected] u.edu> wrote in message news:<Pine.SOL. 4.53.0312041520 470.18529@homer >...[color=blue]
> i have a class, in which i have override the Object.equals(O bject o)
> method... and then i try to make two equal objects (under the new
> definition) and place them into a HashSet... which should only accept one
> of them, as in the following code. the class holds two objects (a pair of
> objects) and if another member of the class contains the same two objects
> (in either place of the ordered pair), it is considered equal.
>
> Integer key1 = new Integer(1);
> Integer key2 = new Integer(2);
> HashSet x = new HashSet();
> // demonstrate that the equals() method works...
> System.out.prin tln((new MyClass(key1, key2)).equals(n ew myClass(key2,
> key1)));
> System.out.prin tln(x.add(new MyClass(key1, key2)));
> System.out.prin tln(x.add(new MyClass(key2, key1)));
>
>
> if i'm correct in reading the docs about how Set classes work, the output
> of this should be:
>
> true
> true
> false
>
> but instead i'm getting:
>
> true
> true
> true
>
> and indeed the Set x has two elements after this code.
>
> i thought the Set code checks for membership by applying the .equals()
> method, which at runtime should bind to my overridden method, no?
>
> thanks for any help,
>
> murat[/color]

My guess is: your equals() method is not equal to the Object#equals()
method in its signature. I also made the error in my Java beginner
days, numerous times may be!

Re: .equals(o) and Set membership

Murat Tasan <[email protected] u.edu> wrote in message news:<Pine.SOL. 4.53.0312041520 470.18529@homer >...[color=blue]
> i have a class, in which i have override the Object.equals(O bject o)
> method... and then i try to make two equal objects (under the new
> definition) and place them into a HashSet... which should only accept one
> of them, as in the following code. the class holds two objects (a pair of
> objects) and if another member of the class contains the same two objects
> (in either place of the ordered pair), it is considered equal.
>
> Integer key1 = new Integer(1);
> Integer key2 = new Integer(2);
> HashSet x = new HashSet();
> // demonstrate that the equals() method works...
> System.out.prin tln((new MyClass(key1, key2)).equals(n ew myClass(key2,
> key1)));
> System.out.prin tln(x.add(new MyClass(key1, key2)));
> System.out.prin tln(x.add(new MyClass(key2, key1)));
>
>
> if i'm correct in reading the docs about how Set classes work, the output
> of this should be:
>
> true
> true
> false
>
> but instead i'm getting:
>
> true
> true
> true
>
> and indeed the Set x has two elements after this code.
>
> i thought the Set code checks for membership by applying the .equals()
> method, which at runtime should bind to my overridden method, no?
>
> thanks for any help,
>
> murat[/color]

Sorry, my previous reply is wrong. Java.util.HashX xx data structure
classes check hashCode() of the added/stored objects. See javadoc for
Object#hashCode () and source files HashSet.java and HashMap.java. The
former depends on the latter in logic reuse.

Re: .equals(o) and Set membership

thanks much, i totally forgot about that!

murat

On Fri, 5 Dec 2003, Raymond DeCampo wrote:
[color=blue]
> Murat Tasan wrote:[color=green]
> >
> > i thought the Set code checks for membership by applying the .equals()
> > method, which at runtime should bind to my overridden method, no?
> >[/color]
>
> No, that is not correct. Whenever equals() is overloaded, hashCode()
> must be overloaded as well. The Collections classes depend on
> hashCode() and equals(). Read the javadoc for java.lang.Objec t
> carefully and implement hashCode() for your classes for which you have
> overridden equals() and you will see better results.
>
> HTH,
> Ray
>
>[/color]

--
Murat Tasan
[email protected] u
[email protected] .edu
murat.tasan@cwr u.edu