public static String differenceStrings(String str1, String str2) {
    if (str1 == null) return str2;
    if (str2 == null) return str1;

    int at = indexOfDifferenceStrings(str1, str2);

    if (at == INDEX_NOT_FOUND) return "";

    return str2.substring(at);
  }
@Test
  public void testDifferenceStrings() {
    assertNull(Utils.differenceStrings(null, null));
    assertEquals("abc", Utils.differenceStrings("abc", null));
    assertEquals("abc", Utils.differenceStrings(null, "abc"));
    assertEquals("", Utils.differenceStrings("abc12345", "abc"));
    assertEquals("", Utils.differenceStrings("pqrstuv345", "pqrstuv"));
    assertEquals("12345", Utils.differenceStrings("abc", "abc12345"));
  }