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From "Mark Shead (JIRA)" <j...@apache.org>
Subject [jira] Updated: (COLLECTIONS-359) A ∩ B != B ∩ A when duplicates are present in a list
Date Mon, 05 Jul 2010 15:49:49 GMT

     [ https://issues.apache.org/jira/browse/COLLECTIONS-359?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mark Shead updated COLLECTIONS-359:
-----------------------------------

    Attachment: COLLECTIONS-359.patch

This patch fixes the problem and adds a unit test that will fail when the problem remains.

> A  ∩ B  !=  B  ∩ A when duplicates are present in a list
> --------------------------------------------------------
>
>                 Key: COLLECTIONS-359
>                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-359
>             Project: Commons Collections
>          Issue Type: Bug
>          Components: Collection
>    Affects Versions: 3.2
>            Reporter: Mark Shead
>         Attachments: COLLECTIONS-359.patch
>
>
> When duplicates are present in a list, ListUtils.intersection doesn't behave as expected.
The intersection of two lists should give the same result regardless of which list comes first.
ListUtils.intersection(A,B) should equal ListUtils.intersection(B,A).  This is not the case
when the list contains duplicates.
> Right now:
>  [a, b]  ∩  [a, a, b, b] =  [a, a, b, b]
> and
>  [a, a, b, b]  ∩  [a, b] =  [a, b]
> Expected behavior:
>  [a, a, b, b]  ∩  [a, b] =  [a, b]
>  [a, b]  ∩  [a, a, b, b] =  [a, b]
> Code demonstrating the problem.
> 		List A = new ArrayList();
> 		List B = new ArrayList();
> 		A.add("a");
> 		A.add("b");
> 		B.add("a");
> 		B.add("a");
> 		B.add("b");
> 		B.add("b");
> 		System.out.println("List A: " + A);
> 		System.out.println("List B: " + B);
> 		System.out.println("A ∩ B = " + ListUtils.intersection(A,B));
> 		System.out.println("B ∩ A = " +ListUtils.intersection(B,A));
> output:
> List A: [a, b]
> List B: [a, a, b, b]
> A ∩ B = [a, a, b, b]
> B ∩ A = [a, b]

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