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From "Mark Shead (JIRA)" <j...@apache.org>
Subject [jira] Created: (COLLECTIONS-359) A ∩ B != B ∩ A when duplicates are present in a list
Date Mon, 05 Jul 2010 15:01:52 GMT
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


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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