[jira] [Created] (LANG-1627) BooleanUtils.xor not behaving as expected with any odd number of true's

[Alberto Scotto (Jira)]

Alberto Scotto created LANG-1627:
------------------------------------

             Summary: 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


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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