The length of "abcde" is 5
First character of abcde 'a'
Fourth character of abcde 'd'
-String exception: Out of bounds: -1
+String exception: Out of bounds: 7
After "abcde"[2] = 'Z' : abZde
String exception: Out of bounds: 5
After "abcde"[5] = 'X' : abZde