java - Is an XOR swap equivalent to a traditional swap in all cases? -

below method performs ‘in place ' string reverse i.e. black cat becomes cat black. in second swap section if traditional swap (commented out) used tests pass if xor swap used 1 test passes.

is not possible 'swap'

        (int = count; <= (end + count) / 2; i++) {             char temp = arr[i];             arr[i] = arr[end - (i - count)];             arr[end - (i - count)] = temp;         } 

to

        (int = count; <= (end + count) / 2; i++) {             arr[i] ^= arr[end - (i - count)];             arr[end - (i - count)] ^= arr[i];             arr[i] ^= arr[end - (i - count)];         } 

method

public class reversestring {      public static char[] revstring(string input) {          char[] arr = input.tochararray();         int length = arr.length;          (int = 0; < (length / 2); i++) {             arr[i] ^= arr[length - - 1];             arr[length - - 1] ^= arr[i];             arr[i] ^= arr[length - - 1];           }          int end;         int charcount;         int count = 0;         while (count < length) {              if (arr[count] != ' ') {                  charcount = 0;                               while (count + charcount < length && arr[count + charcount] != ' ') {                     charcount++;                 }                  end = count + charcount - 1;  //              (int = count; <= (end + count) / 2; i++) { //                  char temp = arr[i]; //                  arr[i] = arr[end - (i - count)]; //                  arr[end - (i - count)] = temp; //              }                  (int = count; <= (end + count) / 2; i++) {                     arr[i] ^= arr[end - (i - count)];                     arr[end - (i - count)] ^= arr[i];                     arr[i] ^= arr[end - (i - count)];                 }                  count += charcount;              } else {                 count++;             }                    }         return arr;     }    } 

test

@runwith(junitparamsrunner.class) public class reversestringtest {      @test     @parameters(method = "getstrings")     public void testrevstring(string testvalue, char[] expectedvalue) {              assertthat(reversestring.revstring(testvalue), equalto(expectedvalue));          }      private static final object[] getstrings() {         return new object[] {             new object[] {"black cat", "cat black".tochararray()},             new object[] {"left to", "to left".tochararray()}         };     }    } 

output fail

java.lang.assertionerror:  expected: ["c", "a", "t", " ", "b", "l", "a", "c", "k"] but: ["c", " 

xor swap fails when swapping value itself. here's code:

arr[i] ^= arr[end - (i - count)]; arr[end - (i - count)] ^= arr[i]; arr[i] ^= arr[end - (i - count)]; 

let's assume i == end - (i - count). then:

arr[i] ^= arr[end - (i - count)]; 

sets arr[i] 0 (since xor'd zero).

the next 2 lines nothing, xor'ing 0 has no effect, leaving arr[i] 0 , corrupting input.

whether above assumption ever true depends on length of input, noted.

because of gotcha, xor swapping dangerous. since it's hard read, , isn't performance win on modern platform, micro-optimization trick obsolete , should avoided.