public class Main {

  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 == -1) {
          return "";
      }
      return str2.substring(at);
  }
 
  public static int indexOfDifference(String str1, String str2) {
      if (str1 == str2) {
          return -1;
      }
      if (str1 == null || str2 == null) {
          return 0;
      }
      int i;
      for (i = 0; i < str1.length() && i < str2.length(); ++i) {
          if (str1.charAt(i) != str2.charAt(i)) {
              break;
          }
      }
      if (i < str2.length() || i < str1.length()) {
          return i;
      }
      return -1;
  }

}

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