Re: .equals(o) and Set membership
From: hiwa (HGA03630_at_nifty.ne.jp)
Date: 12/05/03
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Date: 4 Dec 2003 22:26:04 -0800
Murat Tasan <tasan@eecs.cwru.edu> wrote in message news:<Pine.SOL.4.53.0312041520470.18529@homer>...
> i have a class, in which i have override the Object.equals(Object 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.println((new MyClass(key1, key2)).equals(new myClass(key2,
> key1)));
> System.out.println(x.add(new MyClass(key1, key2)));
> System.out.println(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
Sorry, my previous reply is wrong. Java.util.HashXxx 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.
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