A ∩ B != B ∩ A when duplicates are present in a list
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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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