code: public static int indexOfDifference(final CharSequence cs1, final CharSequence cs2) {
          if (cs1 == cs2) {
              return INDEX_NOT_FOUND;
          }
          if (cs1 == null || cs2 == null) {
              return 0;
          }
          int i;
          for (i = 0; i < cs1.length() && i < cs2.length(); ++i) {
              if (cs1.charAt(i) != cs2.charAt(i)) {
                  break;
              }
          }
          if (i < cs2.length() || i < cs1.length()) {
              return i;
          }
          return INDEX_NOT_FOUND;
      }
docs: <p>Compares two CharSequences, and returns the index at which the
  CharSequences begin to differ.</p>
  
  <p>For example,
  {@code indexOfDifference("i am a machine", "i am a robot") -> 7}</p>
  
  <pre>
  StringUtils.indexOfDifference(null, null) = -1
  StringUtils.indexOfDifference("", "") = -1
  StringUtils.indexOfDifference("", "abc") = 0
  StringUtils.indexOfDifference("abc", "") = 0
  StringUtils.indexOfDifference("abc", "abc") = -1
  StringUtils.indexOfDifference("ab", "abxyz") = 2
  StringUtils.indexOfDifference("abcde", "abxyz") = 2
  StringUtils.indexOfDifference("abcde", "xyz") = 0
  </pre>
  
  @param cs1  the first CharSequence, may be null
  @param cs2  the second CharSequence, may be null
  @return the index where cs1 and cs2 begin to differ; -1 if they are equal
  @since 2.0
  @since 3.0 Changed signature from indexOfDifference(String, String) to
  indexOfDifference(CharSequence, CharSequence)