code: public static String swapCase(String str) {
        int strLen;
        if (str == null || (strLen = str.length()) == 0) {
            return str;
        }
      StringBuilder buffer = new StringBuilder( strLen );
  
        char ch = 0;
        for (int i = 0; i < strLen; i++) {
            ch = str.charAt(i);
            if (Character.isUpperCase(ch)) {
                ch = Character.toLowerCase(ch);
            } else if (Character.isTitleCase(ch)) {
                ch = Character.toLowerCase(ch);
            } else if (Character.isLowerCase(ch)) {
                ch = Character.toUpperCase(ch);
            }
            buffer.append(ch);
        }
        return buffer.toString();
    }
docs: <p>Swaps the case of a String changing upper and title case to
  lower case, and lower case to upper case.</p>
  
  <ul>
  <li>Upper case character converts to Lower case</li>
  <li>Title case character converts to Lower case</li>
  <li>Lower case character converts to Upper case</li>
  </ul>
  
  <pre>
  GosuStringUtil.swapCase(null)                 = null
  GosuStringUtil.swapCase("")                   = ""
  GosuStringUtil.swapCase("The dog has a BONE") = "tHE DOG HAS A bone"
  </pre>
  
  <p>NOTE: This method changed in Lang version 2.0.
  It no longer performs a word based algorithm.
  If you only use ASCII, you will notice no change.
  That functionality is available in WordUtils.</p>
  
  @param str  the String to swap case, may be null
  @return the changed String, <code>null</code> if null String input