public class test {

  public static void main(String[] args)
  {
    StringBuffer sp_string = new StringBuffer("AbCdEf");
    swapPairs(sp_string);
    System.out.println(sp_string);
  }

  public static void swapPairs(StringBuffer s)
  {
    int s_len = s.length();

    // This covers zero and 1 length strings.
    if (s_len < 2) {
      return;
    }

    char stash = ' ';

    for (int i = 0; i < s_len; i += 2) {
      if (i + 2 > s_len) {
        break;
      }

      stash = s.charAt(i);
      s.setCharAt(i, s.charAt(i+1));
      s.setCharAt(i + 1, stash);
    }

  }
}