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From "Thomas Neidhart (JIRA)" <j...@apache.org>
Subject [jira] [Reopened] (COLLECTIONS-359) A ∩ B != B ∩ A when duplicates are present in a list
Date Thu, 28 Feb 2013 19:37:16 GMT

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

Thomas Neidhart reopened COLLECTIONS-359:
-----------------------------------------

    
> 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
>             Fix For: 4.0
>
>         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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