code: public static String replaceChars(String str, char searchChar, char replaceChar) {
        if (str == null) {
            return null;
        }
        return str.replace(searchChar, replaceChar);
    }
docs: <p>Replaces all occurrences of a character in a String with another.
  This is a null-safe version of {@link String#replace(char, char)}.</p>
  
  <p>A <code>null</code> string input returns <code>null</code>.
  An empty ("") string input returns an empty string.</p>
  
  <pre>
  GosuStringUtil.replaceChars(null, *, *)        = null
  GosuStringUtil.replaceChars("", *, *)          = ""
  GosuStringUtil.replaceChars("abcba", 'b', 'y') = "aycya"
  GosuStringUtil.replaceChars("abcba", 'z', 'y') = "abcba"
  </pre>
  
  @param str  String to replace characters in, may be null
  @param searchChar  the character to search for, may be null
  @param replaceChar  the character to replace, may be null
  @return modified String, <code>null</code> if null string input
  @since 2.0