code: public static String difference(String str1, String str2) {
          if (str1 == null) {
              return str2;
          }
          if (str2 == null) {
              return str1;
          }
          int at = indexOfDifference(str1, str2);
          if (at == INDEX_NOT_FOUND) {
              return EMPTY;
          }
          return str2.substring(at);
      }
docs: <p>Compares two Strings, and returns the portion where they differ.
  (More precisely, return the remainder of the second String,
  starting from where it's different from the first.)</p>
  
  <p>For example,
  <code>difference("i am a machine", "i am a robot") -> "robot"</code>.</p>
  
  <pre>
  StringUtils.difference(null, null) = null
  StringUtils.difference("", "") = ""
  StringUtils.difference("", "abc") = "abc"
  StringUtils.difference("abc", "") = ""
  StringUtils.difference("abc", "abc") = ""
  StringUtils.difference("ab", "abxyz") = "xyz"
  StringUtils.difference("abcde", "abxyz") = "xyz"
  StringUtils.difference("abcde", "xyz") = "xyz"
  </pre>
  
  @param str1  the first String, may be null
  @param str2  the second String, may be null
  @return the portion of str2 where it differs from str1; returns the
  empty String if they are equal
  @since 2.0