code: public static String substring(final String str, final int beginIndex) {
          assertStringNotNull(str);
          if (str.length() < beginIndex) {
              String msg = "The length of the string was smaller than the begin index:";
              msg = msg + " str=" + str + ", beginIndex=" + beginIndex;
              throw new StringIndexOutOfBoundsException(msg);
          }
          return str.substring(beginIndex);
      }
docs: Extract sub-string by begin index. (skip front string)
  <pre>
  substring("flute", 2)
  returns "ute"
  </pre>
  @param str The target string. (NotNull)
  @param beginIndex The from-index.
  @return The part of string. (NotNull)