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https://issues.apache.org/jira/browse/LANG-1627?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17531800#comment-17531800
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Steve Bosman commented on LANG-1627:
------------------------------------
I've added two pull requests:
* https://github.com/apache/commons-lang/pull/890 - additional javadoc on
existing methods
* https://github.com/apache/commons-lang/pull/891 - new method
> BooleanUtils.xor not behaving as expected with any odd number of true's
> -----------------------------------------------------------------------
>
> Key: LANG-1627
> URL: https://issues.apache.org/jira/browse/LANG-1627
> Project: Commons Lang
> Issue Type: Bug
> Affects Versions: 3.11
> Reporter: Alberto Scotto
> Priority: Major
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Hi,
> I was expecting a xor function that takes a variable number of arguments to
> *return true if and only if exactly one among all of the arguments is true*,
> regardless of the number of arguments.
> This holds true given three false's:
> {code:java}
> @Test
> public void threeFalse() {
> boolean[] bools = new boolean[]{Boolean.FALSE, Boolean.FALSE, Boolean.FALSE};
> assertFalse(BooleanUtils.xor(bools));
> }{code}
>
> It also holds true given 4 true's, as well as for any even number of trues.
> {code:java}
> @Test
> public void fourTrue() {
> boolean[] bools = new boolean[]{Boolean.TRUE, Boolean.TRUE,
> Boolean.TRUE, Boolean.TRUE};
> assertFalse(BooleanUtils.xor(bools));
> }
> {code}
> The above tests pass.
> But with any odd number of true's that doesn't hold anymore:
>
> {code:java}
> @Test
> public void threeTrue() {
> boolean[] bools = new boolean[]{Boolean.TRUE, Boolean.TRUE, Boolean.TRUE};
> assertFalse(BooleanUtils.xor(bools));
> }
> {code}
> This test fails.
> That was totally unexpected to me.
> But as it turns out, even
> {noformat}
> true ^ true ^ true{noformat}
> evaluates to true. That was unexpected too to me, at a first sight.
> The thing is that xor (I mean the original boolean operator) is a binary
> operator, so if you want to make it n-ary, one simple solution is to apply it
> in two by two: ((a ^ b) ^ c) ^ d
> And that's what is done in the implementation of the method BooleanUtils#xor.
> But that brings to BooleanUtils.xor(true, true, true) == true, and at the
> same time BooleanUtils.xor(true, true, true, true) == false, which just
> doesn't sound right to me.
> Whether or not you agree with me that that is a bug of the method, please at
> least update the Javadoc, because right now it is not providing the user
> enough information. Look:
> {code:java}
> Performs an xor on a set of booleans.
> BooleanUtils.xor(true, true) = false
> BooleanUtils.xor(false, false) = false
> BooleanUtils.xor(true, false) = true
> {code}
>
> Thanks.
> Cheers
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