[jira] [Reopened] (COLLECTIONS-359) A ∩ B != B ∩ A when duplicates are present in a list

["Thomas Neidhart (JIRA)" <ji...@apache.org>]

     [ 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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