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

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.11
• Fix Version/s: 3.13.0
• Assignee: Unassigned
• Reporter: Alberto Scotto
• Created: 16/Dec/20 23:18
• Updated: 05/Jul/22 19:05
• Resolved: 05/Jul/22 19:05

Description

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:

@Test
public void threeFalse() {
 boolean[] bools = new boolean[]{Boolean.FALSE, Boolean.FALSE, Boolean.FALSE};
 assertFalse(BooleanUtils.xor(bools));
}

 
It also holds true given 4 true's, as well as for any even number of trues.

    @Test
    public void fourTrue() {
        boolean[] bools = new boolean[]{Boolean.TRUE, Boolean.TRUE, Boolean.TRUE, Boolean.TRUE};
        assertFalse(BooleanUtils.xor(bools));
    }

The above tests pass.

But with any odd number of true's that doesn't hold anymore:

 

@Test
public void threeTrue() {
 boolean[] bools = new boolean[]{Boolean.TRUE, Boolean.TRUE, Boolean.TRUE};
 assertFalse(BooleanUtils.xor(bools));
}

This test fails.

That was totally unexpected to me.
But as it turns out, even

true ^ true ^ true

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:

Performs an xor on a set of booleans.
         BooleanUtils.xor(true, true)   = false
         BooleanUtils.xor(false, false) = false
         BooleanUtils.xor(true, false)  = true

 

Thanks.

Cheers